Neurogenetics User Manual 2007/2008 Page 1 of 24
User Manual 2007-2008
Neurogenetics User Manual 2007/2008 Page 2 of 24
1. Neurogenetics Unit 3-6
Service provision/ DNA tests available
Target response times
2. Genetic tests in Parkinson’s disease and Parkinsonism 7
3. Genetic tests in inherited ataxias 8
4. Genetic tests in Huntington’s disease and choreiform disorders
5. Genetic tests in dystonia 13
6. Genetic tests in inherited neuropathies 14
7. Genetic tests in muscle disease and Ion channel disease
Neurogenetics User Manual 2007/2008 Page 3 of 24
1. NEUROGENETICS UNIT: USER MANUAL
The Neurogenetics Unit is situated within the Department of Molecular Neuroscience at the
Institute of Neurology. Included in the unit is the Neurogenetics Laboratory of the National
Hospital for Neurology and Neurosurgery/University College London Hospital. The laboratory is
CPA accredited and provides a regional, national and international diagnostic service for
inherited neurological disorders. This manual is designed to give guidance to users for currently
available DNA service tests and is correct as of January 2008. It is updated annually.
Professor N.W. Wood, PhD, FRCP, FMedSci
Professor M.G. Hanna, MD, FRCP
Dr. M.M. Reilly, MD, FRCPI
Dr. S. Tabrizi, PhD, MRCP
Dr H Houlden PhD, MRCP
Clinical Nurse Specialists
Ms. R. Taylor, RGN MSc
Mr Colm Treacy RGN
Dr. M.B. Davis, PhD, FRCPath
Ms. A Haworth, MSc, FRCPath
Ms. M.G. Sweeney, BSc, DipRCPath
Dr. W. Wakeling, PhD, DipRCPath
Dr. V. Gibbons, PhD
Dr. R. Labrum, PhD
Ms. C. Woodward, BSc
Dr. S. Pemble, PhD
Dr R Sud PhD
Mr J Polke BSc
Ms. E. Mudanohwo
Mr J. Hehir
Mr B Phillimore
Ms. N. Boateng
Service provision-Laboratory contact number 0845 155 5000 ext 724250
Diagnostic, predictive and prenatal genetic tests are carried out when clinically appropriate.
Informed consent is required.
Diagnostic tests will be carried out for outside hospitals when appropriate clinical details and
evidence of consent accompany the sample.
Predictive and prenatal tests will only be carried out for individuals who have been seen in a
genetic clinic either at the NHNN or at a regional clinical genetics centre. Enquires concerning
the clinical indications for genetic testing should be addressed to Professor Wood (movement
disorders including Parkinson’s disease and cerebellar ataxias), Professor Hanna (muscle
disease, mitochondrial, ion channel), Drs M Reilly/H Houlden (peripheral nerve) or Dr Tabrizi
Enquires concerning the range of tests are available, type of sample required, analytical
procedures and result interpretation should be addressed to Dr MB Davis (ext 724282), Ms MG
Sweeney or Ms A Haworth (ext 724266).
Information and consent forms are available from Ms N Boateng (ext 724250). Result
enquiries should be addressed to ext 724250. If this extension is unmanned a message
should be left on the answer phone and enquiries will be dealt with as soon as possible.
Autosomal dominant cerebellar ataxias (SCA1, SCA2, SCA3, SCA6, SCA7, SCA12, SCA17).
Andersen Syndrome (KCNJ2)
Dentatorubropallidoluysian Atrophy (DRPLA).
Dopa Responsive Dystonia (GCH1).
Episodic ataxia type 1 (KCNA1)
Neurogenetics User Manual 2007/2008 Page 4 of 24
Episodic ataxia type 2 (CACNA1A)
Familial Amyloid Polyneuropathy (TTR).
Finnish Type Amyloidosis (Gelsolin)
Familial British Dementia (BRI).
Familial Hemiplegic Migraine (CACNA1A).
Friedreich's Ataxia (FRDA).
Frontal lobe temporal dementia (MAPT, TAU). Known family mutations only.
Hereditary Motor and Sensory Neuropathy (chromosome 17p11.2 duplication, CX32, MPZ,
Hereditary Neuropathy with Liability to Pressure Palsy (chromosome 17p11.2 deletion,
Hereditary Sensory Neuropathy (SPTLC1)
Huntington's disease (HD).
Leber's Hereditary Optic Neuropathy (LHON).
Mitochondrial Myopathies (MM).
Myotonia Congenita (CLCN1).
Paramyotonia Congenita (SCN4A).
Parkinson’s disease (PARK2, PARKIN, LRRK2 G2019S)
Periodic paralysis (SCN4A, CACNA1S).
Primary Torsion Dystonia (DYT1).
X linked Bulbospinalneuronopathy (XLBSN).
The laboratory is also willing to confirm research results for other disorders in individual
cases. Testing will then be available to other family members.
For availability of other tests in UK genetic labs please see
DNA from individuals with disorders for which tests are not currently available may be sent for
storage, pending future analysis. Details of current developments are available from the
For genetic analysis, we request at least two mls blood, in two PLASTIC EDTA tubes. Clinical
details, family history and where possible a detailed pedigree should be supplied.
Samples from wards and clinics at the NHNN should be accompanied by a yellow analysis
request form. Blood and DNA samples from outside hospitals can be sent by first class mail.
Tissue samples e.g. muscle, should be sent frozen, on dry ice, by courier. Please advise the
laboratory of the arrival of these samples in advance. CVS samples are not accepted except by
Consent is required for all genetic tests. For samples originating within the NHNN, clear
indication of consent, for diagnostic and/or research purposes must be completed on the request
form. Samples referred from other laboratories must be sent with written consent or a clear
indication that consent has been obtained. Samples received into the laboratory without this
information will be stored without analysis until consent is obtained.
Target report times:
Tests available within the Neurogenetics laboratory:
All predictive and prenatal tests: 2 weeks
Diagnostic tests with acute clinical need: 2 weeks
Non urgent referrals
Autosomal dominant cerebellar ataxias (SCA1, SCA2, SCA3, SCA6, SCA7, SCA12, SCA17):
Andersen Syndrome (KCNJ2): 4 months
Dentatorubropallidoluysian Atrophy (DRPLA): 8 weeks
Dopa Responsive Dystonia (GCH1): 4 months
Episodic ataxia type 1 (KCNA1): 4 months
Neurogenetics User Manual 2007/2008 Page 5 of 24
Epsiodic ataxia type 2 (CACNA1A): 4 months
Familial Amyloid Polyneuropathy (TTR): 4 months
Finnish Type Amyloidosis (Gelsolin) :8 weeks
Familial British Dementia (BRI): 8 weeks
Familial Hemiplegic Migraine (CACNA1A): 4 months
Friedreich's Ataxia (FRDA): 8 weeks
Frontal lobe temporal dementia (MAPT, TAU). Known family mutations only: 8 weeks
Hereditary Motor and Sensory Neuropathy: chromosome 17p
HMSN point mutation analysis: Connexin-32 (GJB3), GDAP1, MPZ, MFN2: 4 months
Hereditary Liability to Pressure Palsy: 8 weeks
Hereditary Sensory Neuropathy (SPTLC1): 4 months
Huntington's disease (HD): 8 weeks
Leber's Hereditary Optic Neuropathy (LHON): 8 weeks
Mitochondrial Myopathies (MM): 8 weeks/4 months depending on analysis required
Myotonia Congenita (CLCN1): 4 months
Paramyotonia Congenita (SCN4A): 4 months
Parkinson’s disease (PARK2, PARKIN, PARK8, LRRK2:G2019S): 4 months
Periodic paralysis (SCN4A, CACNA1S): 4 months
Primary Torsion Dystonia (DYT1): 8 weeks
X linked Bulbospinalneuronopathy (XLBSN): 8 weeks
Where a report will not be available within the stated time, the referring clinician will be
contacted as soon as possible.
Report times are determined by the receiving laboratory: the Neurogenetics laboratory will
forward the report to the referring clinician as soon as it is available. Report enquiries may be
made to the Neurogenetics Unit.
Charges for provider to provider and private referrals are available from the Neurogenetics
2. NHNN GUIDE TO GENETIC TESTING FOR PARKINSON'S DISEASE AND
Professor Nicholas Wood
Parkinson's disease and parkinsonism is a commonly occurring akinetic rigid syndrome. The
last few years have seen a dramatic change in our understanding of the genetic bases of
Parkinson's disease although classically thought of as a non-genetic disease, it is now clear
that a significant sub-group of patients do have primary mendelian mutation. Currently there
are 5 genes which are widely accepted as being associated with a phenotype compatible with
classical Parkinson's disease. Two of these, alpha-synuclein and LRRK2, are inherited in
autosomal dominant fashion. Three, Parkin, DJ1 and PINK1 are autosomal recessively
inherited. These bare facts relating to the mode of inheritance however, do not tell the full
story. For instance, the emerging story of LRRK2 is now associated with reduced
penetrance, thereby gene carriers may be unaffected and a clear autosomal dominant family
history may not be obvious. There is also accumulating but unproven evidence that single hit
mutations in the recessive genes may well contribute to an individual’s parkinsonian
syndrome. Therefore, giving guidance on gene testing is complicated.
It is generally too early to give precise estimates of the relative burdens of these mutations.
However a few facts can be stated:
• Alpha- synuclein (SCNA or Park1)
Mutations in alpha-synuclein are incredibly rare and this is not currently being offered as
• LRRK2 (aka Dardarin or Park8)
Neurogenetics User Manual 2007/2008 Page 6 of 24
This is the most recently identified gene and a single base pair mis-sense mutation
(G2019S) has been shown to be found commonly in both familial and apparently
• DJ1 (Park7)
This is a rare autosomal recessive condition and this is not yet available in service.
• PINK1 (Park6)
Mutations in PINK1 appear to be more common than DJ1 but still relatively rare, this is
not yet in service.
• Parkin (Park2)
Numerically, worldwide mutations in the parkin gene have been found most frequently of
all the Parkinson’s genes. The range of mutations is wide and includes mis-sense,
nonsense and a number of gene rearrangements. The genetic analysis is complex and
we are currently able to offer direct sequencing of exons in this gene and dosage analysis
for rearrangements. It should be noted, however, that most patients in outbred
populations such as the UK are compound heterozygotes, i.e. they have a different
mutation on each allele.
Complicated parkinsonism such as MSA and PSP generally do not appear to have a high
recurrence risk and no gene tests are indicated in clinically typical cases. However, in the
case of familial or unusual syndromes, which may include earlier age of onset, consideration
for sequencing of the TAU gene in cases of PSP as well as complicated fronto-temporal
dementia parkinsonism is suggested.
DOPA RESPONSIVE DYSTONIA
This may sometimes present as young onset parkinsonian syndrome with or without dystonia.
For details of genetic testing please see section on dystonia.
SPINO CEREBELLAR ATAXIA
Some of the complicated dominant ataxias, particularly SCAs 1, 2 and 3 may be complicated
by extra-pyramidal syndromes including parkinsonism. In selected cases, such as those with
a family history, or parkinsonism complicated by ataxia, it may be worthwhile considering SCA
gene mutation. For details, please see section on ataxias.
There are a number of other rare neurodegenerative disease which may present with
parkinsonian features (e.g. Pank2, Wilson’s disease). If a genetic analysis is being considered
please contact the laboratory for advice as to the availability of such tests.
3. NHNN GENETIC GUIDE TO INHERITED ATAXIAS
Professor Nicholas Wood and Dr Paola Giunti
Ataxia may be found in a variety of clinical situations. There are a large number of genetic
causes, but a few basic rules can be recognised. If a degenerative ataxia is being
considered, the age of onset is important. Generally speaking, onset below the age of 20
years is very likely to be autosomal recessive. Onset over the age of 25 conversely, is very
likely to be autosomal dominant. X-linked forms of ataxia are very rare and when present are
usually extremely complicated and other clues such as aetiology is given by the clinical
presentation and other investigations. Mitochondrial DNA mutation may also give rise to
ataxia, but again this is usually a complicated picture, including such features as pigmentary
retinopathy, epilepsy, etc. These are discussed under the section on mitochondrial diseases.
Autosomal recessive ataxias
The commonest of these is Friedreich's ataxia. This has a distinctive clinical phenotype
encompassing progressive gait and limb ataxia, pyramidal signs and a neuropathy
predominantly of sensory type. Genetic testing for this is a relatively straightforward matter
Neurogenetics User Manual 2007/2008 Page 7 of 24
with a very high sensitivity and specificity. The analysis looks for the presence of an
expansion in intron 1 of the Frataxin gene. The vast majority of patients have an abnormal
expansion on both alleles but approximately 2-4% has a single expansion and a point
mutation. The analysis performed in the NHNN diagnostic laboratory will only pick up the
expansion. Therefore, if no expansion is found, this effectively excludes the diagnosis of
Friedreich's ataxia as to date; there are still no reports of two point mutations being found in a
single individual. If, however, a single expansion is found, this has 2 possible explanation:
that it is the cause of the patient’s ataxia and the second allele carries a point mutation
(detectable by sending the sample to an outside laboratory); or it represents the carrier
frequency in the general population of about 1 in 120. Therefore a clinical decision must be
made and if requested we can send the DNA away for sequencing to look for the point
There are a number of other rare ataxic syndromes including ataxia telengiectasia and ataxia
associated with ocular motor apraxia (AOA1 & 2). These can be also assessed in
appropriately selected cases by Professor Malcolm Taylor Cancer Research Institute
University of Birmingham.
Although rare, perhaps the most important ataxia not to be missed in this group is an ataxia
associated with Vitamin E deficiency. This is due to mutations in the alpha tocopherol
transporting protein. This is not offered as a genetic service but as the diagnosis is readily
reached by measuring Vitamin E this does not hamper diagnostic accuracy. Although rare, it
is potentially treatable or at least modifiable by Vitamin E supplementation and therefore
should be considered in appropriate cases.
There are a number of other rarer syndromes which may present with ataxia including:
A variant of Tay Sachs due to hexosaminidase A deficiency, this is usually complicated by a
predominantly motor neuronopathy.
A newly described tremor ataxia syndrome associated with presence of the permutation of the
fragile X gene (FXTAS).
Rarely Niemann Pick type C can present with an early onset complicated ataxia picture.
Vertical gaze abnormalities are usually present.
Autosomal dominant ataxias
Slightly confusingly there are 2 classification systems which can be viewed in parallel. The
clinical classification based on Anita Harding’s work in the early 1980s, describes three sub-
groups of autosomal dominant cerebellar ataxia (ADCA). Type 1 was complicated by a
variety of additional feature including some but not necessarily all of the following:
neuropathy, dementia, extrapyramidal features, and optic atrophy. Type 2 was complicated
by a maculopathy and Type 3 was relatively pure. This is still clinically useful and can guide
the relevant genetic analysis.
The genetic classification for the SpinoCerebellar Ataxias (SCAs) is based on the order in
which the chromosomal location of the causative gene was identified. At the time of writing,
the number of SCAs is approximately 30. In not all of these has the gene been identified and
even when the gene has been identified, not all are available as a diagnostic service.
Routine diagnostic services at NHNN involves genes SCAs 1,2,3,6,7, 12 and 17.
SCA7 counts for virtually all the cases of the Type 2 form of ADCA, namely that associated
with a maculopathy. This is the rarest of the gene tests.
SCAs 1, 2 and 3 account for approximately 50% of ADCA Type 1 and SCA6 accounts for
approximately 50% of the pure type. This is generally speaking a later onset disease and
more slowly progressive than the others. Therefore it can be seen that in 50% or perhaps a
little over, cases of ataxia with a dominant family history SCA testing on the currently
available list will identify the mutation.
We can also offer on special request SCA12 and 17 which are much rarer. As clinical clues,
SCA12 is associated with prominent tremor and SCA17 may present with a complicated
ataxia with chorea and can even look a bit like Huntington's disease. We are not currently
offering the other SCAs at this time.
SCA14 is currently being offered on a research basis by colleagues in Oxford (Dr Kevin
Talbot) and in selected cases we can arrange for samples to be sent for analysis to Dr Talbot.
However, as with all research-based results, unless these are then validated in a CPA
approved laboratory we advise most strongly against communicating these to patients and
4. NHNN GENETIC GUIDE TO GENETIC TESTING FOR CHOREIFORM DISORDERS
Dr Sarah Tabrizi
Neurogenetics User Manual 2007/2008 Page 8 of 24
Huntington’s Disease (autosomal dominant inheritance) is the commonest cause of inherited
chorea in the UK, therefore with a choreiform patient where the cause is clearly not acquired -
this should be the first genetic test. There is not always a positive family history although this
should always be asked for.
The implications for other family members of a positive diagnostic test for HD should also
always be discussed and please refer for advice if any questions regarding this.
Clinical features of HD
Common motor abnormalities such as chorea are seen in 90% of adult onset cases of HD.
Also think of Huntington’s Disease if patients present with a combination of dystonia,
parkinsonism, bradykinesia - as HD may present with an akinetic rigid form particularly in
young adults or juvenile cases. In a young person (<20y) presenting with a parkinsonian
syndrome, juvenile Huntington’s Disease should be tested for.
Patients with Huntington’s Disease commonly have oculomotor disturbance characterised by
delayed initiation of saccades, slowing saccades and head thrusting to initiate saccades.
Pursuit is impaired and there may be evidence of gaze impersistence.
Patients have impairment of voluntary motor function, dysarthria, dysphagia and cognitive and
psychiatric problems are common.
Huntington’s Disease may also present with multiple tics, predominant psychiatric symptoms
or predominant cognitive symptoms in a frontal/subcortical pattern.
DRPLA (autosomal dominant) is rare in Europe and the United States; however, it does
present with a number of clinical phenotypes for which the gene may be tested for. It can
present with a Huntington’s Disease phenocopy with an identical picture to Huntington’s
In young onset patients, particularly under the age of 30y, it tends to present with a
progressive myoclonic epilepsy-picture with more marked dementia than is seen in patients
with Huntington’s Disease.
In patients with later onset typically above the age of 30y, DRPLA typically presents with a
milder phenotype with ataxia, mild chorea and milder cognitive impairment.
Gene Testing for Huntington’s Disease phenocopies
A number of diseases can present with clinical features very similar to Huntington’s Disease,
and these may be tested for in the Neurogenetics laboratory, if the Huntington’s Disease and
DRPLA gene tests are negative.
Other inherited causes of HD phenocopies are:
Inherited prion diseases which may be tested for by analysing the open reading frame of the
prion gene. This is undertaken by Professor John Collinge’s team, and this is done as a free
Spinocerebellar ataxia (SCA) types1, 2, 3 and 17 may all present like Huntington’s Disease.
Neuro-acanthocytosis (AD or AR) and neuro-ferritinopathy may present like Huntington’s
Disease. However, genetic testing for these disorders is expensive and not easily available.
HDL-2 (due to mutations in the Junctophillin 3 gene) although this has only been described in
people of North African origin to date.
Other causes of inherited chorea that may present with features similar to Huntington’s
Disease, but with additional neurological symptoms are Neuro-acanthocytosis as mentioned
above, but repeated blood films are a useful test for this; Macleod syndrome which is X-linked
with mutations in the XK gene (these patients typically have acanthocytes on peripheral blood
film, a high CK and their red blood cells have weak expression of Kell system antigens due to
no expression of XK protein); and benign hereditary chorea. There are genes available for
these, but are not currently testable on a service basis in the UK.
Mitochondrial disorders may mimic many neurodegenerative disorders, and these can be
tested via the Neurogenetics laboratory.
Other inherited choreiform disorders
Neurogenetics User Manual 2007/2008 Page 9 of 24
Other diseases that may present with chorea with additional extrapyramidal features that we
can test for in the Neurogenetics laboratory are Wilson’s disease (autosomal recessive due to
mutations in a copper transport ATP’ase), Friedreich’s ataxia (autosomal recessive due to
intronic GAA repeat mutation in the FRDA gene), Neuro-degeneration with brain iron
accumulation (autosomal recessive due to PANK2 mutations), Ataxia Telangectasia
(autosomal recessive disorder due to mutations in the AT mutation gene), and
Neuroferritinopathy (autosomal dominant due to mutations in the ferritin light chain).
In summary and practical advice on what tests to ask for:
Of the above gene tests summarised the ones that are important to be aware of and
may be tested for in the Neurogenetics lab without discussion are Huntington’s
Disease, DRPLA, and SCA 17 (in that order) in patients presenting with Huntington’s
disease-like/choreiform clinical phenotypes.
Patients with SCA 1,2 and 3 may present with this phenotype but testing for these
mutations in this group of patients should be discussed with the Neurogenetics
All the other genetics tests mentioned should only be asked for and will only be
undertaken after discussion with the Neurogenetics team for patients that present
with a choreiform disorder.
HD TESTING FLOW DIAGRAM
Neurogenetics User Manual 2007/2008 Page 10 of
5. NHNN Genetic Guide to genetic testing for dystonia
Dr Sarah Tabrizi
Dystonias are a heterogeneous group of disorders which are known to have a strong inherited
basis. Genetic testing is available for DYT1 (primary torsion dystonia) or DRD (dopa
DYT1 associated dystonia:
Mutations in the DYT1 gene have been associated with autosomal dominant early onset
idiopathic torsion dystonia. This typically develops first in the arm or leg in middle to late
childhood and progresses to generalised dystonia within about 5 years. Patients are
intellectually normal and have no other neurological abnormalities. Although onset is more
typical in childhood or early adulthood, it has been described in middle life. There is marked
clinical variability in DYT1-associated dystonia ranging from children who are profoundly
disabled, to adults who carry this autosomal dominantly inherited gene and have no
symptoms. This indicates that the gene is transmitted with reduced penetrance. The DYT1
mutation is caused by a GAG deletion in the protein encoding torsin A. This can be tested
for in patients with the appropriate dystonic phenotype in the Neurogenetics laboratory
Dopa Responsive Dystonia:
Dopa Responsive Dystonia (DRD) is characterised by dystonia, concurrent or subsequent
parkinsonism, diurnal worsening of symptoms in 75% of cases and a dramatic therapeutic
response to Levodopa. Women are affected 2-4 times more frequently than men. Dystonia is
the most common presentation and maybe the only feature, however, in childhood DRD may
present with a phenotype resembling spastic paraparesis, whereas in adulthood it can
present with a phenotype similar to spastic paraparesis or a parkinsonian syndrome.
In view of the wide spectrum of symptoms and age of onset, the diagnosis of DRD may be
missed and it is widely recommended that all patients with an extrapyramidal motor disorder
or individual symptoms within the spectrum of DRD be given a trial of Levodopa. DRD is
usually inherited as an autosomal dominant trait with incomplete penetrance. It is due to
mutations in the gene encoding GTP-cyclohydrolase 1.
There are many different mutations that are found in GCH 1; these can be tested for in the
Neurogenetics laboratory. However, before sending off for testing for the DRD gene, it is
recommended that the patient has been given and had a good response to Levodopa; a
phenylalanine loading test, and CSF examination which may reveal low concentrations of
dopamine and serotonin metabolites, neopterin, biopterin and GTP-CH1 activity, all of which
can be tested in the metabolic laboratory in the National Hospital for Neurology and
Other dystonic syndromes:
There are currently 15 loci associated with inherited dystonia. An important one to be aware
of is the myoclonus/dystonia syndrome which is characterised by a myoclonic/dystonic
syndrome which is alcohol-responsive in some patients. This is autosomal dominant with
incomplete penetrance and caused by mutations in the gene encoding epsilon-sarcoglycan.
Dystonia is also associated with many other syndromes, mainly the heredo-degenerative
dystonias, and these will be covered under the sections encompassing choreiform disorders
and parkinsonian disorders.
6. NHNN GUIDE TO DNA-BASED DIAGNOSIS IN PATIENTS WITH SUSPECTED
Dr Mary M. Reilly
The hereditary neuropathies are a clinically and genetically a heterogeneous group of
disorders. Although there are many genetic diseases in which the peripheral nervous system
is involved, by far the most important of these is Charcot-Marie-Tooth disease (CMT), which
affects 1 in 2,500 of the population. This disease is characterised clinically by distal wasting
and weakness, distal sensory loss, hyporeflexia and a variable amount of foot deformity. The
Neurogenetics User Manual 2007/2008 Page 11 of
simplest classification system is neurophysiological and divides CMT into type 1 (median
motor conduction velocity (MCV) < 38 m/s) and type 2 (median MCV > 38 m/s) although an
intermediate form (median MCV 25 – 45 m/s) is increasingly recognised. Clinically it is often
difficult to distinguish some forms of CMT2 from some forms of the Hereditary Sensory and
Autonomic Neuropathies (HSAN) as will be discussed below. It is not uncommon for patients
with CMT to have no sensory symptoms and occasionally no sensory signs which can make
the distinction from distal Hereditary Motor Neuronopathy (dHMN) difficult clinically but clear
using neurophysiology. Therefore it is necessary to consider CMT, HSAN and dHMN together
in determining an approach to the diagnosis of these conditions. Hereditary neuropathy with
liability to pressure palsies (HNPP) is genetically related to CMT1A and also needs to be
considered under the genetic neuropathies.
There are now at least 28 genes that cause CMT (Table 1), 5 genes that cause dHMN (Table
2) and 5 genes that cause HSAN (Table 3). Despite the major advances in identifying the
causative genes for these genetic neuropathies only a few of these are available as routine
tests (see below) and if these are not present it is usually appropriate that the patient is seen
in the peripheral nerve genetic clinic for further evaluation. Patients should always have a
detailed clinical evaluation including family history done before genetic testing is considered.
In most situations as detailed below, it is appropriate to evaluate the patients
neurophysiologically (NCS) before proceeding to genetic testing. In specific circumstances
(detailed below) nerve biopsy is indicated.
All other inherited neuropathies are rare and appropriate testing should either be discussed
with Dr. Reilly or the patient should be referred to her peripheral nerve genetic clinic.
Availability of genetic testing in peripheral nerve disease varies in different parts of the UK
and this guide will outline what is available in the NHNN with reference to some tests which
can be done elsewhere in the UK.
General approach to inherited neuropathies (CMT, HNPP, HSAN, dHMN)
The first step is to assess if the neuropathy is hereditary. This is crucial as inflammatory
neuropathies such as chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) are
an important differential as they are treatable. The hereditary nature of the neuropathy may
be obvious from the family history but in sporadic cases, a long history, the presence of foot
deformity, the lack of positive sensory symptoms or in the case of CMT1, the uniform slowing
of the MCV may point to the neuropathy being hereditary.
If the neuropathy is hereditary, neurophysiology will help both distinguish CMT from dHMN
and also differentiate CMT1 from CMT2. It may be possible to work out the inheritance
pattern in clinic but the families are often small making this difficult.
The general steps to diagnosing a hereditary neuropathy are:
1. Establish using family history +/- clinical pattern whether the neuropathy is likely to be
hereditary. Attempt clinically to differentiate between CMT, HNPP, HSAN and dHMN.
2. Use neurophysiology to distinguish CMT from dHMN.
3. In the case of CMT use neurophysiology to classify CMT into types 1, 2 or
4. Use neurophysiology to support clinical diagnosis of HNPP.
5. Occasionally in young children with CMT clinically it is justifiable to screen the
Chromosome 17 duplication before neurophysiology is undertaken.
6. Occasionally in family members where an index case has a genetic diagnosis, it is
reasonable to proceed to genetic testing without doing neurophysiology.
7. If there is any doubt about the neuropathy being genetic and an inflammatory basis is
being considered, a nerve biopsy or referral to the peripheral nerve clinic may be
8. Occasionally in young severely affected patients nerve biopsy may be considered
early specifically to rule out a treatable cause of the neuropathy.
Charcot-Marie-Tooth disease (CMT)
Neurogenetics User Manual 2007/2008 Page 12 of
Using neurophysiology and a family history it is usually possible to classify CMT into type 1 or
type 2 and to attempt to define the inheritance pattern.
For CMT1, whether it is autosomal dominant (AD) or sporadic, it is always appropriate to
screen the chromosome 17 duplication (containing the peripheral myelin protein 22 gene
(PMP-22)) first as it accounts for 70% of CMT1 in the UK. Some of the apparent sporadic
cases of CMT1 have an early onset severe phenotype and would have been called Dejerine
Sottas disease (DSD) or Congenital Hypomyelinating Neuropathy (CHN) previously. It is now
known that most cases of DSD and CHN are due to de novo mutations in the causative genes
for AD CMT1.
In AD or sporadic CMT1, if the chr. 17 duplication is negative and there is no definite
evidence of male to male transmission, connexin 32 (CX32) should be screened next. If CX32
is negative or there is male to male transmission then myelin protein zero (P0) should be
screened. If this is negative PMP-22 can be screened but this is not done at NHNN. In
appropriate circumstances after discussion with Dr. Reilly, PMP-22 can be sent elsewhere in
the UK to be screened.
All other causes of AD CMT1 are very rare and not available routinely. At this stage it is
appropriate to refer patients to the peripheral nerve genetic clinic (PN genetic clinic).
Genetic testing in AD or sporadic CMT1:
1. Chr. 17 duplication.
2. If Chr. 17 negative and there is no ♂ to ♂ transmission screen CX32.
3. If CX32 negative or there is ♂ to ♂ transmission screen P0.
4. If P0 negative, discuss PMP-22 screening with Dr. Reilly.
5. If PMP-22 negative consider referring to the PN genetic clinic.
AR CMT1 may be difficult to diagnose clinically unless there is consanguinity or affected
siblings. It is often useful to examine the parents clinically and electrophysiologically.
Currently the only tests routinely available are screening of P0 and PMP-22. Mutations in both
of these genes can cause AD, AR and de novo CMT1. If these are negative it is appropriate
to consider referral to the PN genetic clinic.
Genetic testing in AR CMT1:
1. Screen P0.
2. If P0 is negative discuss PMP-22 sequencing with Dr. Reilly.
3. If P0 and PMP-22 are negative consider referral to the PN genetic clinic.
Most cases of CMT2 present with a clear history of AD inheritance or as apparent sporadic
cases. It is important in apparent sporadic cases to consider acquired neuropathies as
neurophysiology (which is reasonably good at differentiating genetic causes of a
demyelinating neuropathy from acquired causes) is not good at differentiating genetic causes
of an axonal neuropathy from acquired causes.
In AD or sporadic CMT2, the first gene which should be screened is Mitofusin 2 (MFN2) which
accounts for approximately 20% of cases of AD CMT2. MFN2 mutations can also occur de
novo so consider testing this gene is sporadic cases as well. If MFN2 screening is negative,
P0 should be screened as P0 mutations can cause CMT2 as well as CMT1. These are the
only genes routinely screened in CMT2 and if they are negative referral to the PN genetic
clinic should be considered although there are a few useful phenotype pointers to individual
genes that may help the clinician including including a predominant sensory neuropathy with
sensory complications seen in CMT2B associated with RAB7 mutations and much greater
upper limb (hand) than lower limb wasting and weakness in CMT2D associated with GARS
mutations (Table 1.)
Neurogenetics User Manual 2007/2008 Page 13 of
Genetic testing in AD or sporadic CMT2:
1. Screen MFN2.
2. If MFN 2 negative, screen P0.
3. If P0 negative consider referral to the PN genetic clinic.
There are no genes routinely screened for AR CMT2 so referral to the PN genetic clinic
should be considered in these patients.
This term has been recently in favour to describe patients or families with intermediate
conduction velocities. It is particularly useful in recognising patients with x-linked CMT due to
CX32 mutations. In a family if the males have a demyelinating neuropathy and the females an
axonal neuropathy CX32 should be very high on the list of causes. Also in an individual
patient if the MCVs in some nerves are in the demyelinating range and in other nerves in the
axonal range CX32 should also be considered. This is important as occasionally these
patients have been diagnosed erroneously with CIDP.
Hereditary Neuropathy with Liability to Pressure Palsies (HNPP)
Patients with HNPP often present with a typical history of recurrent pressure palsies but this
condition is underdiagnosed and the most important part of the diagnosis is considering the
condition. The crucial clinical point is that all patients with HNPP have widespread NCS
abnormalities even if they only have one nerve involved clinically. It is therefore only
appropriate to screen these patients genetically after NCS have shown widespread
conduction abnormalities. The condition is usually AD but occasionally occurs de novo and is
caused by a deletion of the same 1.4 megabase section of chromosome 17 as is duplicated in
CMT1A. When suspected the Chr. 17 deletion should be initially screened followed by PMP-
22 sequencing (after discussion with Dr. Reilly) as PMP-22 mutations can rarely cause
Genetic testing in HNPP:
1. Screen the chr. 17 deletion.
2. If chr. 17 deletion negative discuss PMP-22 sequencing with Dr. Reilly.
Distal Hereditary Motor Neuronopathy / Spinal Muscle Atrophy (dHMN / dSMA)
Distal Hereditary Motor Neuronopathy also known as distal Spinal Muscle Atrophy (dHMN /
dSMA) can be very difficult to distinguish clinically from CMT2 and used to be called spinal
CMT. Neurophysiology is essential to distinguish dHMN from CMT2 as there are no sensory
abnormalities in dHMN and there will always be at least electrical evidence of sensory
involvement in CMT2. There are no genes routinely available for screening for dHMN but as
can be seen in tables 1 and 2, many of the recently described causative genes for dHMN
have also been described to cause rare forms of CMT2 further confusing the issue. It is
appropriate to consider referring these patients to the PN genetic clinic.
Genetic testing in dHMN:
1. Consider referral to the PN genetic clinic
Hereditary Sensory and Autonomic Neuropathy (HSAN) / Hereditary Sensory
As some of the HSANs do not have significant autonomic involvement, these particular
variants should be more appropriately called HSNs. The cardinal clinical feature is marked
sensory involvement with sensory complications such as skin ulcerations, Charcot joints,
osteomyelitis, amputations etc. Some of the more rare AR forms present with congenital
insensitivity to pain (Table 3.). The commonest form in the UK is HSAN I / HSN I which is an
AD mainly sensory neuropathy although there can be significant motor involvement with time.
The causative gene for this is serine palmitoyl transferase long chain base subunit 1
(SPTLC1) and all UK patients to date have been found to have one mutation (C133W) with a
common founder. This mutation is available as a routine test and as part of the screen exons
5 and 6 are also screened but complete SPTLC1 screening is not available. The clinical
presentation of patients with HSAN I due to SPTLC1 mutations is very similar to the patients
presenting with CMT2B and RAB7 mutations described above except unusually for a genetic
neuropathy there is a high incidence of spontaneous neuropathic pain with SPTLC1
Neurogenetics User Manual 2007/2008 Page 14 of
mutations. RAB7 is not available for routine screening. In patients with the phenotype of
HSAN 1 and a family history SPTLC1 should be screened.
Genetic testing in HSAN / HSN:
1. In AD HSAN I / HSN I screen SPTLC1.
2. If SPTLC1 is negative and in all other forms of HSAN / HSN consider referral to the
PN genetic clinic.
Familial Amyloid Polyneuropathy (FAP)
Of all the other hereditary neuropathies referred to above the only one for which routine
testing is available in NHNN is familial amyloid polyneuropathy (FAP). The most common
form of FAP is AD and is due to mutations in transthyretin (TTR). This is an extremely
important diagnosis to make as liver transplantation is currently considered in all patients and
in appropriate patients can be a very useful treatment. The clinical picture is of a sensory
motor neuropathy often initially involving the small sensory fibers and presenting with pain, an
autonomic neuropathy, cardiac involvement and a variable involvement of other organs such
as the eye with vitreous deposits. This diagnosis should especially be considered in patients
of Irish or Portuguese descent but can occur in any ethnic group.
Genetic testing in FAP:
1. Screen TTR.
2. If negative consider referral to the PN genetic clinic.
Neurogenetics User Manual 2007/2008 Page 15 of
TABLE 1: CLASSIFICATION OF CHARCOT-MARIE-TOOTH DISEASE
Clinical type Inheritance Locus / Gene
1. Demyelinating (CMT 1)
CMT 1A AD Duplication 17p11.2-12 /
17p11.2-12 / Point mutation PMP-22
CMT 1B AD 1q22-q23 / Point mutation
CMT 1C AD 16p13.1 - p12.3 / SIMPLE /
CMT 1D AD 10q21-q22 / Point mutation
Charcot-Marie-Tooth type 1 x-linked (CMT 1X)
CMT 1X X-linked Xq13.1 / Point
Dejerine-Sottas disease (HMSN III)
DSD A AD (AR) 17p11.2-12 / Point mutation
DSD B AD (AR) 1q22-q23 / Point mutation
DSD C AD 10q21-q22 / Point mutation EGR2
Congenital hypomyelinating neuropathy (CHN)
CHN A AD 17p11.2-12 / Point mutation PMP-22
CHN B AD 1q22-q23 / Point mutation Po
CHN C AD (AR) 10q21-q22 / Point mutation
Hereditary neuropathy with liability to pressure palsies (HNPP)
HNPP A AD Deletion 17p11.2 / PMP-22
17p11.2-12 / Point mutation PMP-22
Charcot-Marie-Tooth type 1 autosomal recessive (CMT1 AR)
CMT1 ARA (CMT4A) AR 8q13 - 21.1/ GDAP1
CMT1 ARB1 (CMT4B1) AR 11q22 / MTMR2
CMT1 ARB2 (CMT4B2) AR 11p15 / MTMR13
CMT1 ARC (CMT 4C) AR 5q23-q33 / KIAA1985
CMT1 ARD (CMT4D / HMSNL) AR 8q24 / NDRG1
CMT1 ARE (CCFDN) AR 18q
CMT1 ARF (CMT4F) AR 19q13.1-13.3 / Periaxin
CMT1 ARG (HMSNR) AR 10q22-q23
2. Axonal (CMT 2)
Neurogenetics User Manual 2007/2008 Page 16 of
Charcot-Marie-Tooth type 2 autosomal dominant (CMT 2 / HMSN II)
CMT 2A AD 1p35 – p36 / KIF1Bβ
AD / GTPase mitofusin 2
CMT 2B AD 3q13 – q22 / RAB7
CMT 2C AD 12q23 – q24
CMT 2D AD 7p14 / GARS
CMT 2E AD 8p21 / NF-L
CMT 2F AD 7q11-q21 / HSP 27
CMT 2G AD 12q12-q13.3
CMT 2L AD 12q24 / HSP 22
CMT 2 AD 1q22-q23 / Point mutation Po
CMT 2 (HMSNP) AD 3q13.1
Charcot-Marie-Tooth type 2 x-linked (CMT 2X)
CMT 2X X-linked Xq24 – q26
Charcot-Marie-Tooth type 2 autosomal recessive (CMT2 AR)
CMT2 AR AR 1q21.2 – 21.3 / LMNA
CMT2 AR AR 19q13.1
CMT2 AR AR 8q21 / GDAP1
TABLE 2: DISTAL HEREDITARY MOTOR NEURONOPATHY (DISTAL HMN / DISTAL
HMN I unknown
HMN II 12q24 / HSP 22
HMN II / CMT2F 7q11-q21 / HSP 27
HMN III unknown
HMN IV unknown
HMN V / CMT 2D 7p / GARS
HMN V / Silver syndrome 11q12-q14 / BSCL2
HMN VI SMARD1 / IGHMBP2
HMN VII 2q14
Neurogenetics User Manual 2007/2008 Page 17 of
TABLE 3. HEREDITARY SENSORY AND AUTONOMIC NEUROPATHIES
Inheritance Locus / Gene
HSAN I /
HSAN III AR 9q31-q33 /
HSAN IV AR 1q21-q22 /
Neurogenetics User Manual 2007/2008 Page 18 of
7. NHNN GENETIC GUIDE TO DNA-BASED DIAGNOSIS IN PATIENTS WITH
SUSPECTED GENETIC MUSCLE AND CHANNEL DISEASES
Dr Michael Hanna
Approximately 50% of patients with muscle disease have a genetic cause. New genes for
muscle diseases are being found on a regular basis and there is a bewildering array of
muscle disease now shown to have a clear single gene genetic basis. Although genetic
testing is increasingly useful in achieving a precise diagnosis, there are situations in which a
genetic test should not be the first investigation selected. Availability of genetic testing in
muscle disease varies in different parts of the UK and this guide will outline what is available
at NHNN. An approach to achieve efficient diagnosis in this patient group is outlined below.
In those situations where genetic testing should be considered there is often a hierarchy of
test selection based on likelihood of positivity. In patients with suspected genetic muscle
disease, DNA tests are only ever one part of a thorough diagnostic assessment which is
always based upon detailed clinical assessment, careful family history and the informed
selection of additional tests including muscle biopsy, specialised neurophysiology and
metabolic testing. Informed written consent is essential for all genetic tests. Furthermore, the
requesting clinician should consider in advance how and by whom, genetic counseling will be
give if the genetic test is positive.
At the first consultation with a patient with suspected genetic muscle disease it should always
be possible to devise an efficient sequence of investigations that will usually lead to an
accurate diagnosis. In those muscle diseases where a genetic test should be the first test
selected, it is usually not necessary to simultaneously arrange an EMG and muscle biopsy. It
is generally better to await the genetic result, since if it is positive, these tests will be avoided.
An important principle when assessing any patient with muscle disease is always to consider
whether it is possible that the patient may have an inflammatory muscle disease. If there is a
realistic possibility of such a disease early muscle biopsy is very important.
The investigational approach to the major genetic muscle diseases and the ion channel
diseases is outlined in the text and tables below.
Myotonic dystrophy is the commonest form of muscle dystrophy seen in the adult population.
It is a dominant multisystem disease affecting skeletal muscle, cardiac muscle, brain and the
endocrine system. Distal muscle weakness, myotonia, cataracts and a myopathic facies
usually raise suspicion of the diagnosis in typical cases. However, atypical presentations with
cognitive impairment, weight loss and cardiac arrhythmias are recognised. Genetic
anticipation is a prominent feature in many myotonic dystrophy families.
If the diagnosis is suspected the first test is a genetic test for the MD-1 gene. [chr 19]
If this is positive the diagnosis is confirmed and further diagnostic tests are not required.
If EMG is readily available this may be done as part of the diagnostic testing to confirm
myotonia, but if myotonia evident clinically EMG is not essential and the diagnosis can rely on
genetic testing for DM1.
If DM1 testing is negative consideration should be given to Proximal Myotonic Myopathy –
PROMM. Although there are many clinical similarities between PROMM and MD eg the
myopathic facies and cataracts there are important distinguishing features. PROMM patients
have proximal weakness and myotonia [the myotonia is therefore often difficult to detect
clinically] also prominent muscular pain is common in PROMM.
Genetic testing is now available for PROMM-the DM2 gene on Chr 3
If DM1 and DM2 are negative it is usually necessary to consider more detailed
neurophysiological tests to assess the possibility of a skeletal muscle channelopathy and to
proceed to a muscle biopsy
Neurogenetics User Manual 2007/2008 Page 19 of
A systematic programme for MD complications screening exists in the muscle clinic and
referral should be considered.
Fascioscapulohumoral Muscular Dystrophy-FSHD
FSHD is dominant dystrophy with 95% penetrance at age 25y and a negligible new mutation
rate. The typical phenotype is usually easy to diagnose clinically and includes myopathic
weakness of facial, periscapular, peri-humeral and anterior tibial musculature.
Muscle weakness is often asymmetric and in some cases may be very mild-eg asymmetric
mild scapula winging. In a small proportion of cases the facials muscles may be spared. The
gene for FSHD has not been identified. A truncation of a repeat non-coding DNA sequence
on chromosome 4q35 has been shown to associate with the disease in around 95% of cases.
If this truncation is identified it is generally regarded as confirming a diagnosis of FSHD. It is
important to note that if this test is negative it does not necessarily exclude the diagnosis
since patients with 4q35 linked disease may not have the truncation and also because there
is likely to be a second FSHD gene not yet discovered. The genetic test should be the first
test requested. In the majority of cases it is appropriate to await the results of the genetic test
and only if negative to proceed with a muscle biopsy and emg.
A prominent inflammatory infiltrate is a recognised feature in biopsies from patients with
genetically confirmed FSHD. Occasionally patient present with a very high ck >1000. This
may cause difficulty in distinguishing from polymyositis. If there is significant weakness and a
high CK it is sensible practice to proceed to muscle biopsy and even to commence a course
of steroids while waiting for the genetic test result.
Emery Dreifuss Muscular Dystrophy
Patients have a scapulo-humero-peroneal distribution of muscle weakness. Prominent
contracture is an early clinical feature and is typical of EDMD. For this reason EDMD is often
classified as one of the “contractural muscle phenotypes”. The contractures are usually an
early feature of the muscle disease often when there is only very minimal muscle weakness.
The contractures typically affect the cervical extensor muscles [difficulty flexing neck], the
biceps tendon [difficulty extending elbow] and the long finger flexor muscle tendons [difficulty
extending fingers and wrists]. Cardiac involvement is important in EDMD usually manifesting
as hear block and full cardiac evaluation is essential. The other contractural muscle disease
to consider if genetic tests for EDMD are negative is Bethlem myopathy [caused by mutations
in the collagen 6 gene-but no genetic test currently available]. Cardiac involvement is not a
feature of Bethlem myopathy.
EDMD is known to associate with mutations in one of two muscle nuclear envelope proteins
emerin [X-linked EDMD] and lamin A/C [AD or AR EDMD].
The X-liked from of EDMD associates with mutations in emerin gene. Emerin gene analysis is
not easily available therefore in a male with EDMD the first test is a muscle biopsy with
emerin antibody immunostaining which is available at NHNN. If emerin staining is reduced
emerin gene analysis can be pursued. If emerin immuno-staining is normal then lamina/C
gene analysis should be undertaken.
In a female or in a family exhibiting clear dominant inheritance Lamin A/C gene analysis
should be the first test. If all above prove negative Bethlem myopathy should be considered
and referral to muscle clinic appropriate.
Limb Girdle muscular dystrophy
In adult males the commonest form of a LGMD presentation is Becker’s muscular dystrophy
caused by mutations in the Dystrophin gene on the X-chromosome. In a male presenting with
LGMD the first test should be dystrophin gene analysis. If this is negative muscle biopsy for
immuno-staining should be undertaken.
Non-dystrophin LGMD may be recessive or dominant. The dominant form is extremely rare
and it is almost always the case that adult patients with LGMD will have a recessive from of
The number of genes discovered to cause LGMD continues to increase at a steady rate.
There are currently 8 genes and one linked locus for the recessive form and 3 genes and 3
linked loci for the dominant form.
Most of the known genes responsible for LGMD are very large and gene testing is not easily
available and is generally not the first test that should be requested. Generally, once
dystrophin gene testing has been completed and if it is negative all LGMD patients should
proceed to muscle biopsy for immunostaining which will direct any subsequent genetic
Neurogenetics User Manual 2007/2008 Page 20 of
If a dominant pedigree is identified it is worth considering the gene test for Lamin A/C which
can occasionally cause dominant LGMD [as well as EDMD above]. At the same time it is
reasonable to proceed to a muscle biopsy/emg to confirm a dystrophic process. It should be
noted that immunostaining in a dominant muscle disease is generally not useful since it is
unusual that the responsible protein will be reduced in quantity in the muscle [in contrast to
recessive muscular dystrophies].
Sporadic or recessive pedigrees of LGMD are much more common. It should be noted that
although several genes have now been identified in LGMD most of these genes seem to be
more relevant to a paediatric age range presentations. The exception to this are LGMD2I
caused by mutations in the FKRP gene and LGMD 2B caused by mutations in the Dysferlin
gene. FKRP gene testing is more easily available and can be requested early ie before
muscle biopsy but dysferlin gene analysis is not easily available and muscle biopsy is the first
test. Suspect dysferlin if the ck is very high in adult usually .>5000
In a male Becker’s commonest-first test is Dystrophin then biopsy
Dominant LGMD extremely rare-first test Lamin A/C gene and biopsy
Recessive LGMD-most of the genes discovered relevant to paediatric not adult practice –
exceptions are FKRP and dysferlin. Gene test is available for FKRP but not for Dysferlin
where muscle biopsy is the first test.
Oculopharyngeal muscular dystrophy
This is a rare dominant muscular dystrophin but with a characteristic phenotype and an
available gene test. Occasional recessive families are described. It enters into the differential
diagnosis of mitochondrial CPEO and ocular pharyngeal predominant myasthenia. It is
typically a late onset >45 years disease and often begins with dysphagia followed by ptosis
and eventually ophthalmoplegia. Tongue atrophy may develop. There may be neck flexor and
extensor muscle weakness. Later in the disease there may be proximal limb weakness
usually mild to moderate.
Dominant and recessive OPMD is caused by a stable short (GCG)7-13 on chromosme 14.
Gene tests is the first investigation-if negative muscle biopsy and EMG. However, because of
concerns about myasthenia early EMG is often appropriate.
Neurogenetics User Manual 2007/2008 Page 21 of
First test Second test Third test
Myotonic dystrophy DM1 DM2
FSHD 4q35 rearang FKRP Biopsy
Male Muscle biopsy/emg Emerin gene Lamin A/C
Female Lamin A/C geneMuscle biopsy/emg Muscle clin
Limb girdle MD
Dominant Lamin A/C geneMuscle biopsy/emg Muscle clin
Recessive FKRP gene Muscle biopsy/emg Muscle clin
X-linked/Spor Dystrophin gene Muscle biopsy/emg Muscle clin
OPMD PABP2 gene Muscle biopsy/emg Muscle clin
Metabolic Muscle Disease
Mitochondrial Respiratory Chain Diseases
A wide range of clinical phenotypes are recognised to occur in association with mitochondrial
respiratory chain dysfunction. Initially the majority of genetically defined phenotypes had
mutations in mitochondrial DNA, but now it is increasingly recognised that mutations in
nuclear encoded genes can cause respiratory chain dysfunction resulting in neurological
disease. It is now evident that all forms of inheritance [Dominant, recessive, X-linked,
maternal and sporadic] are possible in mitochondrial respiratory chain disease depending on
the genetic basis.
In practice it remains the case that the available genetic tests for mitochondrial diseases are
mtDNA tests. It should be emphasised that the routinely available mtDNA tests are relatively
limited. Although there are 16696 nucleotides that make up the mitochondrial genome
routinely available service tests only check a handful of these positions. The service tests that
are available have been selected on the basis of the frequency with which they cause
neurological disease. At present there are no routinely available tests to check for any of the
discovered nuclear gene mutations.
A mitochondrial respiratory chain disease may be suspected in a number of clinical settings
and it is generally helpful to consider splitting up defined phenotypes although in practice
there is considerable clinical overlap. Typical phenotypes which are well described in
standard reviews are MELAS, MERRF, LHON, myopathy, CPEO and KSS. In practice the
diagnosis needs to be considered when their may only be fragments of full clinical syndromes
or the patients have elements of a number of different syndromes.
Useful pointers to consider are the presence of myopathy, neuropathy or deafness in the
context of a complex CNS phenotype.
In practice if a mitochondrial respiratory chain disease is suspected it is sensible to select the
available mtDNA point mutations to be analysed in a blood sample. These are mutations at
positions 3243, 8344 and 8993. In a patient under the age of 30 blood mtDNA deletion
analysis can be requested. The hit rate from this limited analysis of blood mtDNA is likely to
be very low however, if positive a secure diagnosis of mitochondria disease is achieved
without the need for further investigations. In patients with LHON the hit rate from blood
analysis is much better. The majority of LHON cases in the UK will have one of the three
common point mutations at positions 11778, 14484 or 3460.
Neurogenetics User Manual 2007/2008 Page 22 of
In a non-LHON patient if no mutations are detected in blood the gold standard investigations
include muscle biopsy and respiratory chain enzymology. The majority of patients with a
mitochondrial DNA mutation will have ragged red fibres and/or cytochrome c oxidase negative
fibres and/or a measurable defect on mitochondrial respiratory chain enzymology.
Furthermore, muscle tissue is a much better tissue to analyse for mtDNA mutations. In
particular mitochondrial DNA deletions are much more reliably detectable in muscle
compared to blood.
If above assessments do not produce a positive result and mitochondrial disease is
suspected referral to the muscle clinic should be considered from where access to research
based analysis can be considered.
Glycogen Storage Diseases Fatty Acid Metabolism Disorders
Genetic tests are not the first step when evaluating metabolic muscle diseases associated
with disturbed glycogen or fatty acid metabolism.
The first tests should always be detailed biochemical evaluation eg ischaemic lactate test,
urinary organic acids and acylcarnitine profiles. Muscle biopsy and fibroblast biopsies are
commonly needed. Referral to the muscle and or metabolic clinic is suggested
Metabolic muscle diseases
First test Second test Third test
Myopathy mtDNA muscle biopsy Muscle clinic
CPEO blood mtDNA muscle
MELAS screen resp chain
Glycogen Muscle biopsy Muscle/metab clinic
Stor disease Metabol tests
Eg Mcardle ischaem lact
Fatty acid Muscle biopsy Muscle/metab clinic
Disorders Skin biopsy
Urine organ acids
Skeletal Muscle Channelopathies
The muscle clinic at NHNN is the UK national referral centre for all patients suspected of
having a skeletal muscle channelopathy. This includes clinical assessment, DNA diagnosis,
molecular expression proof of pathogenicity for new mutations, and genotype specific
treatment selection and supervision.
Genetic tests have been developed over recent years and in the main now do represent the
first test to select in patients suspected of having one of the main skeletal muscle
In practice a same day electrophysiology service allows supporting evidence to be obtained to
optimize test selection. Muscle biopsy and provocative tests are now rarely required.
If gene tests are negative in suspected cases referral to the muscle clinic is suggested.
Neurogenetics User Manual 2007/2008 Page 23 of
Skeletal Muscle channelopathies
First test Second test Third test
Periodic paralysis SCN4A Specialised EMG Muscle clinic
Cardiac arrhthm Muscle clinic
Myotonia congentia CLCN1 Specialised emg Muscle clinic
Paramyotonia Cong SCN4A specialised emg Muscle clinic
Cong Myaesthenia Muscle clinic
An increasing number of single gene brain paroxysmal neurological disorders have now been
shown to be caused by mutations in genes coding for ion channels.
Routine DNA-based diagnostic tests for these disorders remains very limited but expansion of
this service is anticipated in the future.
The episodic ataxias were the first brain channelopathies to be genetically characterised and
are uncommon disorders inherited in an autosomal dominant fashion
Episodic ataxia type 1
To date only 10 families have been reported or identified in the UK and around 30 world-wide.
Patients experience movement or stress induced momentary episodes of profound cerebellar
dysfunction. Episodes last no more than a few minutes and may happen multiple times per
day. Attacks may cluster. In-between attacks there is persistent neuromyotonia caused by
peripheral nerve hyperexcitability. The neuromyotonia may be clinically evident or only
detectable by EMG. Some patients are described with isolated neuromyotonia as the only
manifestation of mutations in the brain potassium channel KCNA1 [chrom 12] that causes
EA1.Gene testing of the brain potassium channel can be arranged by discussion with Dr
Hanna and/or referral to the muscle-channel clinic may be considered.
Episodic ataxia type 2
EA2 is more common that EA1-there are hundreds of families described worldwide. Attacks
are precipitated by emotion or intercurrent illness and are characterized by generalised
cerebellar ataxia lasting hours to days. Prominent migraine like headache occurs in 50% of
attacks. Over time it is usual that patients develop a fixed cerebellar syndrome which can
become significant. Point mutations in the brain calcium channel CACNA1A are causative-
There are a large number of mutations distributed throughout this large 48 exon gene.
At present only SCA-6 expansion analysis is available as a service. The SCA-6 expansion
has occasionally been associated with the EA2 phenotype but more commonly associates
with a late onset progressive ataxia.
Familial Hemiplegic Migraine
There are now three genes reported to cause familial hemiplegic migraine-an ATPase gene, a
brain sodium channel and a brain calcium channel. Of these, only calcium channel gene
analysis is available as a service test. The commonest point mutation in CACNA1A [T666M]
that causes FHM is a routine service test.
Neurogenetics User Manual 2007/2008 Page 24 of
Episodic Ataxias and Familial Hemiplegic migraine
First test Second test Third test
EA2 SCA6 Muscle/channel-clinic
EA1 emg Muscle/channel- clinic
T666M Muscle/channel clinic