Mitochondrial DNA encodes 13 proteins for ATP production

MITOCHONDRIAL
DNA AND DISEASE
Anthony Schapira
ABSTRACT
Mitochondria are essential for providing energy for cellular metabolism. Human
mitochondria contain a small remnant of functional DNA that codes for 13 proteins,
all of which are part of the oxidative phosphorylation system for adenosine
triphosphate production. More than 100 different mutations of mitochondrial DNA
have been linked to human disease, and some consider that these are among the
most common inherited neurologic disorders. They encompass the pure myopathies,
encephalomyopathies, and a range of manifestations such as diabetes, sensorineural
deafness, young-onset stroke, and epilepsy. Presentation may be at any age from
infancy to late adulthood. Diagnosis rests upon careful clinical evaluation and
investigations, which may include peripheral blood DNA analysis, imaging, and/or
skeletal muscle biopsy. Mitochondrial DNA is inherited through the female line, and
many patients have a family history compatible with this pattern, although sporadic
cases are also common. Some nuclear gene mutations cause secondary abnormalities
of mitochondrial DNA such as multiple deletions or depletion. Genetic counseling is
possible, but complex, and is easier for those patients with an underlying
mitochondrial DNA deletion. Treatment is mainly supportive and directed at specific
complications, eg, epilepsy and diabetes, although recent advances provide hope for
more specific therapies in the future.
Continuum Lifelong Learning Neurol 2008;14(2):133–148.
INTRODUCTION
Mitochondria are present in all human
cells, and one of their important func-
tions is to provide adenosine triphos-
phate (ATP) by oxidative phosphoryla-
tion (OXPHOS). The mitochondrion
also hosts other biochemical pathways,
including fatty acid b-oxidation, Krebs
citric acid cycle, parts of the urea cycle,
and others. The role of mitochondria in
human disease has been identified only
recently but has now become an impor-
tant area of human pathology, particu-
larly given that many of the diseases are
caused by mutations of mitochondrial
DNA (mtDNA). However, mitochondrial
disorders may also be a consequence
of inherited defects of the nuclear
genome or, alternatively, may be due to
endogenous or exogenous environmen-
tal toxins (Schapira, 2006).
MtDNA STRUCTURE, FUNCTION,
AND INHERITANCE
MtDNA is a small, circular, double-
stranded molecule 16,493 bases long
encoding two ribosomal RNAs, 22 trans-
fer RNAs, and 13 proteins (Figure 8-1).
These proteins are all part of the
OXPHOS system (Table 8-1). MtDNA
remains dependent on the nucleus for
the production of proteins involved in
its transcription, translation, replica-
tion, and repair, all of which take place
133
KEY POINT:
A Mitochondrial
DNA encodes
13 proteins, all
of which are part
of the oxidative
phosphorylation
system for the
production of
adenosine
triphosphate
by aerobic
metabolism.
Relationship Disclosure: Dr Schapira has received personal compensation for serving in an editorial capacity from the European
Journal of Neurology.
Unlabeled Use of Products/Investigational Use Disclosure: Dr Schapira has nothing to disclose.
Copyright @ American Academy of Neurology. Unauthorized reproduction of this article is prohibited.

in the mitochondrion. Thus, mitochon-
dria remain dependent upon the nu-
cleus for the function of mtDNA and
also for the production of all mito-
chondrial proteins other than the 13
OXPHOS proteins encoded by mtDNA.
MtDNA-encoded proteins are trans-
lated on mitochondrial ribosomes and
incorporated directly into complexes
I, III, IV, and V on the inner mem-
brane. Nuclear-encoded proteins are
translated on cytosolic ribosomes and
134
FIGURE 8-1 Human mtDNA. Virtually all the sequence is coding (exonic), with only very small intronic segments.
The location of a number of the more common mtDNA point mutations is indicated.
Reprinted with permission from Leonard JV, Schapira AH. Mitochondrial respiratory chain disorders I: mitochondrial DNA defects.
Lancet 2000;355(9200):299–304. Copyright # 2000, Elsevier.
TABLE 8-1 The Respiratory Chain and Oxidative
Phosphorylation System
Complex Enzyme Activity
Number of
Subunits
Mitochondrial
DNA-Encoded
Subunits
Complex I NADH ubiquinone reductase 43 7
Complex II Succinate ubiquinone reductase 4 —
Complex III Ubiquinol cytochrome c reductase 11 1
Complex IV Cytochrome c oxidase 13 3
Complex V ATP synthase 14 2
ATP = adenosine triphosphate; NADH = nicotinamide adenine dinucleotide (reduced form).
"MITOCHONDRIAL DNA AND DISEASE

are imported into the mitochondrion
through a complex receptor and trans-
port system. For many proteins, this
involves an N-terminal amino tar-
geting sequence that interacts with
membrane receptors and signals the
carrier protein’s import into the mito-
chondrion. The signaling sequence is
then cleaved and the protein traf-
ficked to the appropriate mitochon-
drial compartment. Some proteins
are incorporated into the outer mi-
tochondrial membrane and others
directly into the intermembranous
space. Diseases due to defects of
the mitochondrial import system are
rare, but mutation of a gene involved
in mitochondrial import has been
demonstrated to be the cause of the
Mohr-Tranebjaerg syndrome (sensori-
neural deafness, dystonia, dysphagia,
cortical blindness, and paranoia)
(Ezquerra et al, 2005; Tranebjaerg
et al, 1995) or pure myopathy (Schapira
et al, 1990).
The OXPHOS system comprises
the four respiratory chain complexes
(I to IV) and adenosine triphosphatase
(ATPase) (complex V). These proteins
are embedded in the inner mitochon-
drial membrane and consist of a total
of approximately 85 subunits, 72 being
encoded by nuclear DNA and 13 by
mtDNA. The OXPHOS system is not
only responsible for ATP production
but is also an important source of su-
peroxide radicals (Thomas et al, 1993).
Defects of the OXPHOS system have
the potential not only to cause a failure
of energy metabolism, but also in-
creased free radical mediated damage.
MtDNA continuously replicates, and
this function is, therefore, indepen-
dent of cell cycle phases. MtDNA
replication involves mtDNA polymer-
ase gamma (POLG), thymidine kinase
2, and deoxyguanosine kinase. There
are important, but subtle, differences
in the translation code that are spe-
cific for mtDNA and prevent the
translation of nuclear messenger RNA
(mRNA) within the mitochondrion and
vice versa.
MtDNA is highly polymorphic, with
numerous differences in sequence
among individuals of the same ethnic
group, but more so among members
of different ethnic groups. MtDNA hap-
lotypes are based on specific patterns
of polymorphisms and appear to have
some influence on the prevalence of
certain diseases and the expression of
certain mitochondrial mutations, eg,
Parkinson disease, senescence, and the
expression of Leber hereditary optic
neuropathy (LHON) mutations.
MtDNA is inherited through the fe-
male line, although paternal inheri-
tance of an mtDNA microdeletion in a
complex I gene has been reported
(Schwartz and Vissing, 2002). Paternal
mtDNA from the sperm is either not
incorporated into the fertilized ovum,
or alternatively, its replication is sup-
pressed and the paternal mtDNA de-
graded. In effect, the fertilized ovum
develops using maternal mtDNA only,
and this explains the exclusively female
inheritance pattern of primary mtDNA
mutations, ie, the mutation is trans-
mitted from mother to all offspring
and subsequently by her daughters.
MtDNA mutations are often present
in heteroplasmic form, ie, coexisting
with normal wild-type mtDNA. The
proportion of mutant to wild type
may vary between individuals and be-
tween tissues of the same individual.
There is a relationship between the
mutant load and the degree of bio-
chemical defect, although this is both
tissue dependent and recessive. Based
on studies undertaken so far, 5% to
20% of wild-type mtDNA can compen-
sate at the biochemical level for cer-
tain mutations.
Mitochondria are randomly segre-
gated at cell division, including oogen-
esis, and during this process, random
segregation or partitioning of wild-type
135
KEY POINTS:
A Mitochondrial
DNA is inherited
through the
female line.
A Mitochondrial
DNA mutations
often coexist with
normal wild-type
mitochondrial
DNA in the
tissues of
patients.
Proportions
of each vary
between patients
and between
the tissues of
each patient.

and mutant mtDNA occurs. This
results in a varying proportion of
wild-type and mutant molecules in
daughter cells. When this process oc-
curs through oogenesis, it does so
through a ‘‘tight bottleneck,’’ whereby
only a small proportion of the mtDNA
population is transmitted. A high pro-
portion of mutant load in the oocyte
population would result in a high
proportion of offspring at risk of devel-
oping disease.
MITOCHONDRIAL DISEASES
The clinical expression of disorders
related to defects of the mitochondrial
respiratory chain is very broad and may
include virtually any system or tissue.
Those areas most commonly affected are
the brain and muscle, as these tissues are
probably the most dependent on oxida-
tive phosphorylation. Common symp-
toms include proximal myopathy and
external ophthalmoplegia; CNS features
may include seizures, dementia, ataxia,
deafness, retinitis pigmentosa, and dia-
betes mellitus. The relationship between
genotype and phenotype in mitochon-
drial diseases is not exact. For instance,
mtDNA deletions most frequently result
in chronic progressive external ophthal-
moplegia (CPEO) or Kearns-Sayre syn-
drome (KSS). These same mutations,
however, may result in isolated proximal
myopathy; encephalopathy; a combina-
tion of features that might, for instance,
fall within other mitochondrial pheno-
types including MELAS (myopathy en-
cephalopathy, lactic acidosis, and stroke-
like episodes); or isolated syndromes
such as cardiomyopathy or diabetes mel-
litus. Similarly, the A3243G mutation
in the transfer RNA for lysine, which is
most commonly associated with MELAS,
can also result in CPEO, pure myopathy,
isolated diabetes, or isolated deafness.
Despite these difficulties, the clinician
can identify those patients who are likely
to have a mitochondrial disorder and
even make an educated guess regarding
the underlying mutation in many of
these. However, genetic counseling must
be informed by a molecular diagnosis.
PRIMARY MITOCHONDRIAL
DNA MUTATIONS
Epidemiology
Accurate figures on the epidemiology
of mtDNA diseases is complicated by
the wide spectrum of clinical presen-
tation, the diverse range of mutations,
and the high carrier rate, all of which
will lead to underestimates of preva-
lence. A population-based study of the
A3243G mutation in northern Finland
estimated prevalence at 16.3 per
100,000 (Majamaa et al, 1998). The
mutation was found in 14.0% of
patients with hypertrophic cardiomy-
opathy, 13.0% of patients with oph-
thalmoplegia, 7.4% of patients with
maternally inherited deafness, and
6.9% of patients with occipital stroke.
In northern England, mitochondrial
diseases or the risk of their develop-
ment was estimated to have a preva-
lence of 12.48 per 100,000 (Chinnery
et al, 2000). Two studies estimated
the prevalence of mitochondrial res-
piratory chain diseases in children as
4.7 per 100,000 (8.9 per 100,000 in
preschool-aged children) (Darin et al,
2001) or 5.0 per 100,000 (Skladal et al,
2003). These latter figures will include
mutations in nuclear-encoded as well
as mtDNA genes.
Chronic Progressive External
Ophthalmoplegia and
Kearns-Sayre Syndrome
CPEO may manifest any time from
adolescence to late adulthood, al-
though usually before the age of 30.
Patients develop symmetric or asym-
metric, slowly progressive, usually
nonfatigable ptosis in association with
136
KEY POINTS:
A Mutant
mitochondrial
DNA has a high
asymptomatic or
oligosymptomatic
carrier rate.
A Many patients
have no family
history of a
mitochondrial
disorder; others
have a history
of maternal
inheritance of
features that
may include any
part of the
mitochondrial
disease spectrum,
eg, diabetes,
deafness, or
seizures.
A Exact correlation
does not exist
between
genotype and
phenotype in
mitochondrial
disease.

external ophthalmoplegia. Diplopia
does occur but is uncommon. An
associated pigmentary retinopathy of
the salt and pepper type and/or a
proximal myopathy may be present.
Additional complications are relatively
uncommon, and most patients are
never significantly disabled and usually
have a normal life span. CPEO may
have onset in late adulthood.
Creatine kinase is usually normal;
EMG may be normal or show some
mild nonspecific myopathic features.
Serum lactate may be elevated at rest,
and exercise may induce a significant
and sustained increase. Definitive di-
agnosis requires a muscle biopsy, and
this will show the morphologic fea-
tures characteristic of the mitochon-
drial myopathies. These include the
presence of ragged red fibers on the
Gomori trichrome stain. This pattern
reflects the subsarcolemmal accumula-
tion of mitochondria, often associated
with increased lipid and glycogen. The
ragged red fibers usually stain negative
for cytochrome oxidase (COX) and
positive for succinate dehydrogenase
(SDH) (Figure 8-2). At the ultrastruc-
tural level, the mitochondria are often
enlarged with abnormal cristae. The
matrix may be vacuolated, and para-
crystalline inclusions may be present in
the intermembranous space. Although
characteristic of mitochondrial myopa-
thies, these light and electron micro-
scopic findings are not specific. For
instance, ragged red, COX-negative,
SDH-positive fibers may be present in
inflammatory myopathies and also ac-
cumulate with age. Approximately 10%
of patients with mtDNA mutations may
have a normal biopsy. In such cases,
and if the clinical suspicion remains
high, biochemical assay of respiratory
chain function may demonstrate an
abnormality and an mtDNA mutation
may be found on sequencing.
The most common mutation found
in patients with CPEO is the mtDNA
deletion, being found in 70% of those
with CPEO (Morgan-Hughes et al,
1995). The deletions are single, ie, af-
fecting the same segment of the
mtDNA molecule in all tissues, but
the proportion of deleted molecules
varies from one to another. For in-
stance, it is extremely rare to find the
deletions in blood. DNA confirmation
is therefore dependent on providing
a tissue sample, ie, skeletal muscle.
It appears that the proportion of
deleted mtDNA is stable over time.
One particular mtDNA deletion is the
4977 base pair deletion, referred to as
the ‘‘common deletion,’’ which spans
the region from the ATPase 8 gene
to the ND5 gene. This occurs in ap-
proximately 30% of patients with CPEO
or KSS.
KSS is defined by CPEO and pig-
mentary retinopathy together with
either complete heart block, a CSF
protein level greater than 1 g/L, or
ataxia. Onset is before the age of 20,
although later-onset cases may occa-
sionally occur. Furthermore, proximal
limb weakness may also be present.
Considering both KSS and CPEO,
ptosis may be asymmetric in 58% or
137
FIGURE 8-2 Skeletal muscle section stained for succinate
dehydrogenase (SDH). The figure shows
several SDH-positive, ‘‘ragged red’’
equivalent fibers, eg, two fibers marked with asterisks.
These show heavy mitochondrial proliferation at the
subsarcolemma or distributed throughout the fiber.
KEY POINTS:
A Patients with
chronic
progressive
external
ophthalmoplegia
(CPEO) and
Kearns-Sayre
syndrome (KSS)
are usually
sporadic.
A CPEO and KSS
are usually
caused by
deletions of
mitochondrial
DNA. These are
not present in
blood.

unilateral in 8% of patients. Dysconju-
gate eye movements occur in 35%,
with transient or persistent diplopia in
36%. The ophthalmoplegia may be
severe, and in 62% of patients, gaze
is limited to less than 10% of normal in
any direction (Petty et al, 1986).
MtDNA deletions are also the most
common cause of KSS, occurring in
80% of patients. MtDNA deletions can
also cause Pearson syndrome, which is
an infantile-onset disorder character-
ized by sideroblastic anemia and pan-
creatic insufficiency. In contrast to
CPEO and KSS, mtDNA deletions can
be found in the blood in patients with
Pearson syndrome.
Most patients with CPEO or KSS
have no family history. This is thought
to be due to the mtDNA deletions
arising de novo in maternal oocytes.
Rarely, CPEO may be maternally in-
herited and result from a point mu-
tation of mtDNA rather than a
rearrangement. Interestingly, muscle
biopsies in these patients show the
typical CPEO pattern of COX-negative
fibers rather than the usual COX-
positive fibers associated with transfer
RNA (tRNA) mutations.
It is important to note that CPEO
may also exhibit autosomal dominant
or recessive inheritance. Onset is often
in early adulthood with ptosis, dysar-
thria, dysphasia, facial and proximal
limb weakness with cataracts, and early
death (Servidei et al, 1991). Additional
features such as cardiomyopathy, en-
docrinal abnormalities, ataxia, rhabdo-
myolysis, and peripheral neuropathy
have also been described. Patients of-
ten exhibit lactic acidosis, and muscle
biopsy may show the usual features of
ragged red fibers (Case 8-1). Autoso-
mal inheritance of CPEO may be due
to mutations in a variety of genes, in-
cluding those for the adenine nuc-
leotide translocator, twinkle, mtDNA
POLG, and thymidine phosphorylase.
Mutations of the latter are also found in
138
Case 8-1
A 20-year-old woman presents with a 2-year history of fatigue. Her family
and friends have remarked that they thought her eyes had become droopy,
and past photographs confirm this. There is no diurnal variation to the
ptosis and no diplopia. Examination shows symmetric nonfatigable partial
ptosis, complete ophthalmoplegia, and a pigmentary retinopathy of the
salt and pepper type. Mild proximal limb weakness, hyporeflexia, and
fatigue are present. Investigations include a normal creatine kinase, a
myopathic EMG, and no evidence of a neuromuscular junction defect.
Acetylcholine receptor antibody is negative. Lactic acid levels are normal at
rest but increase on exercise. Muscle biopsy shows a high proportion of
ragged red fibers. Molecular analysis confirms the presence of a single
4.9 kilobase deletion in high proportion compared with wild-type mtDNA.
Comment. This is a typical history for many patients with CPEO. Onset
may be from adolescence to old age. Differential diagnosis includes
myasthenia gravis and other myasthenic syndromes. Investigation may
show a normal lactic acid level, but this may rise and remain elevated after
exercise in some patients, providing a clue to a mitochondrial defect.
Skeletal biopsy is diagnostic, but molecular analysis is required for
counseling. For patients with a single mtDNA deletion, it is possible to
reassure them that their prognosis is generally favorable, with a low risk
for transmitting the disease to children.
KEY POINTS:
A Diagnosis of
CPEO and KSS
depends upon
muscle biopsy
and molecular
analysis.
A Prognosis for
CPEO is generally
good but for
KSS is poor.

patients with myoneuro-gastrointestinal
encephalopathy (MNGIE) (see below).
MELAS
This phenotype is characterized by
growth retardation, focal and general-
ized seizures, and recurrent strokelike
episodes, particularly hemianopia and
hemiplegia. Additional features may
include diabetes, deafness, dementia,
ataxia, and myopathy. Pigmentary ret-
inopathy is relatively uncommon. On-
set is usually in adolescence, although
patients may manifest later, and MELAS
remains an important differential diag-
nosis in patients with stroke under the
age of 40. Clinical progression is usual,
but at a variable rate. Many patients
develop features of migraine with aura
and recurrent episodes of lactic acido-
sis associated with nausea and vomit-
ing. The strokelike episodes may be
transitory or result in permanent neu-
rologic deficit. Imaging in patients in
MELAS usually reveals abnormal areas
affecting gray and white matter that
typically do not conform to vascular
territories and most often affect the
parietal occipital regions. Elevation of
CSF lactate is invariable in patients
with encephalopathy.
Skeletal muscle morphology in
patients with MELAS usually demon-
strates ragged red and SDH-positive
fibers. In contrast to those with
mtDNA deletions, the COX fibers are
usually strongly positive. Histochemi-
cal analysis of cerebral vessels also
demonstrates strong SDH reactivity,
suggesting that angiopathy is a signif-
icant component of the pathogenesis
of MELAS-associated strokes. Basal
ganglia calcification is also a common
radiologic feature of MELAS. This most
commonly affects the globus pallidus
but can also include the striatum,
thalamus, and internal capsules.
Eighty percent of patients meeting
the clinical criteria for MELAS are posi-
tive for the A3243G mutation in the
tRNA for leucine. Other mutations
have been associated with the MELAS
phenotype, including four different
mutations within the tRNA leucine
(UUR) gene, and in numerous other
tRNAs (Case 8-2).
Myoclonic Epilepsy and
Ragged Red Fibers
The core features of myoclonic epilep-
sy and ragged red fibers (MERRF)
are myoclonus, ataxia, and seizures.
Patients often manifest with myoclonic
epilepsy, which may be stimulation
sensitive. Additional seizure types may
also occur, including drop attacks,
tonic-clonic seizures, and focal seizures.
The myopathy is usually relatively mild
and involves proximal upper- and
lower-limb weakness. Additional clinical
features may include hearing loss,
neuropathy, dementia, and growth
retardation (Silvestri et al, 1993). Some
patients have been recorded with
ophthalmoplegia, ptosis, optic atrophy,
and cervical lipomas. The presence of
lipomas is an interesting phenomenon
in patients with MERRF and usually
comprises multiple symmetric cervical
lipomas (Larsson et al, 1995). In some
patients, this may be the only manifes-
tation of MERRF.
Phenotypic variation, as suggested
by the above range of clinical features,
can be substantial, even within the
same family. For instance, in one family
some patients presented with Leigh
syndrome (see below), others with
spinocerebellar degeneration or a form
of motor and sensory neuropathy
(Howell et al, 1996).
The EEG may be abnormal in
patients with MERRF (So et al, 1989),
but the changes are not specific.
Plasma and CSF lactate levels may be
elevated. Muscle biopsy shows ragged
red fibers, which are SDH positive and
COX negative.
139

The most common mutation de-
tected in MERRF is the A8344G muta-
tion, which is present in 80% of patients
(Shoffner et al, 1990). The tRNA lysine
gene may also harbor other mutations
that can result in the MERRF pheno-
type. Likewise, single-base changes in
other tRNAs for leucine or serine may
also produce a phenotype with features
of MERRF. Some have suggested that
the mutant load in the muscle or blood,
the age of onset, and certain clinical
parameters may give some indication
for prognosis (Hammans et al, 1993).
Neurogenic Muscle
Weakness, Ataxia, and
Retinitis Pigmentosa
The key features of the neurogenic
muscle weakness, ataxia, and retini-
tis pigmentosa (NARP) syndrome are
myopathy, peripheral neuropathy,
ataxia, seizures, dementia, and retinitis
pigmentosa (Holt et al, 1990) but may
also include migraine and mental
retardation.
Ragged red fibers and other mor-
phologic abnormalities are usually
absent in the muscle biopsies from
patients with NARP. Biochemical anal-
ysis may also be normal unless ATPase
activity is assessed specifically. Blood
levels of citrulline have been found to
be low in some patients with NARP,
and this may be a surrogate marker of
this phenotype (Parfait et al, 1999).
The most common cause of this
phenotype is the T8893G mutation
in the ATPase 6 gene. This mutation
is heteroplasmic and, when present
140
Case 8-2
A 38-year-old woman with a 3-year history of non–insulin-dependent
diabetes mellitus (NIDDM) presents to the emergency department within
2 hours of onset of hemisensory disturbance and loss of vision to one
side. Her mother had NIDDM; her sister was deaf; and a brother had
had ‘‘cerebral palsy’’ and died in childhood from status epilepticus.
Examination shows her to be of small of stature and confirms the presence
of hemianesthesia and hemianopsia. Imaging confirms a parieto-occipital
stroke. Lactic acid levels are elevated. She has no vascular risk factors
for early stroke. An A3243G mutation of mtDNA is subsequently confirmed.
The patient completely recovers from her stroke but goes on to develop
generalized seizures, weakness, fatigue, dementia, and ataxia. She dies
at the age of 41 years. Her 12-year-old son develops NIDDM.
Comment. This patient exhibited features typical of MELAS and
harbored the most common mtDNA mutation that causes this phenotype.
The clinical course is variable, and MELAS is an important part of the
differential diagnosis for young stroke. The family history is characteristic
of maternal inheritance. Epilepsy in mitochondrial disease usually responds
to routine anticonvulsants, although sodium valproate should be
avoided because of its effect on mitochondrial function. A diagnosis
of cerebral palsy has been applied inappropriately to some patients with
mitochondrial disease in the past. The prognosis for the patient’s son
remains unclear; he could continue with only NIDDM, or alternatively
develop additional features of MELAS. Confirmation that he carried the
A3243G mutation would be required to diagnose a mitochondrial disease.
A high mutation load in blood (eg, greater than 80%) might suggest that
clinical evolution to MELAS was more likely. No prophylactic medication
has shown benefit.
KEY POINTS:
A The encephalo-
myopathies (eg,
MELAS, MERRF)
manifest with
a variable range
of clinical
features that
evolve over
time.
A MELAS and
MERRF can be
diagnosed by
identification of
a mutation in
peripheral blood.
The clinical
phenotype is
helpful in
directing the
search for
specific
mutations.

at around 70%, is associated with a
NARP phenotype, and when present
at greater than 90%, is associated
with maternally inherited Leigh syn-
drome. The T8993C mutation has also
been associated with NARP (de Vries
et al, 1993).
ADDITIONAL PHENOTYPES
Approximately 1% to 2% of all NIDDM
is thought to be related to the
presence of the A3243G mutation
(Gerbitz et al, 1995). This may occur
as the only clinical manifestation of
this mutation in some patients or be
present with associated sensory neural
deafness. Sensorineural hearing loss
may occur as an isolated phenomenon
with mutations in the tRNA for serine
(Guan et al, 1998).
Cardiomyopathy that may be either
dilated or hypertrophic has been
reported with a number of mtDNA
mutations including rRNA genes
(Santorelli et al, 1999). A number of
cases have also been described with
pure myopathy (Hudgson et al, 1972;
Kamieniecka, 1977). These patients
usually have fatigue and exercise
intolerance (Petty et al, 1986), and
some patients with recurrent myo-
globinuria and exercise intolerance
have mtDNA deletions (Andreau et al,
1999; Ohno et al, 1991). COX gene
mutations have also been associated
with myopathy and myoglobinuria
(Keightley et al, 1996; Rahman et al,
1999). Mutations in mtDNA COX genes
have been associated with a variety of
clinical presentations, including motor
neuron disease (ALS) and sideroblastic
anemia (Gattermann et al, 1997).
Leber Hereditary
Optic Neuropathy
LHON is a maternally inherited disor-
der characterized by acute or subacute
bilateral sequential painless visual fail-
ure. LHON is considered the most
common disease caused by mtDNA
mutations, with a prevalence esti-
mated at 11.82 per 100,000 population
(Riordan-Eva et al, 1995). The mean
age of onset is in the early 20s, and
90% of patients are affected by age 45;
85% are men. In most cases, the visual
loss is severe and permanent, al-
though this is dependent on the
underlying mutation.
Three mtDNA mutations are con-
sidered primary, and all are located
within mtDNA complex I genes. These
are the G11778A mutation in ND4,
which is found in 50% to 70% of
patients with LHON, the G3460A
mutation in 15% to 25% of patients,
and the T14484C mutation in ND6.
The latter is relatively uncommon but
is associated with visual recovery in
70% (Wallace et al, 1988).
The mutations are detectable in
blood and are usually homoplasmic.
There is evidence for a complex I
defect in muscle and platelets of
patients (Smith et al, 1994). Only
15% of women who carry one of the
primary mutations are clinically affect-
ed. Some families exhibiting features
of LHON may have additional features
such as dystonia or striatal degenera-
tion; these have also been found to be
related to mutations in mtDNA com-
plex I genes.
Myoneuro-Gastrointestinal
Encephalopathy
The diagnostic criteria for the MNGIE
phenotype include peripheral neurop-
athy, CPEO, and gastrointestinal dys-
motility. Skeletal muscle has the
typical morphologic features of mito-
chondrial myopathy. The gastrointes-
tinal disease usually includes nausea,
vomiting, and diarrhea. MRI may
demonstrate a leukodystrophy. The
peripheral neuropathy is sensorimotor
in type (Ginsberg et al, 2007). The
average age of onset is usually in
adolescence, and most patients are
dead by the age of 40 years.
141
KEY POINTS:
A Muscle biopsy is
useful if clinical
diagnosis remains
high and
mutation screen
is negative, as not
all mutations may
be included in
routine analysis.
A Leber hereditary
optic neuropathy
is a common
cause of
blindness,
especially in
otherwise healthy
young men.
A The diagnosis of
Leber hereditary
optic neuropathy
may be made by
DNA analysis of
blood for the
three common
mitochondrial
DNA mutations.
A The female
carrier rate for
Leber hereditary
optic neuropathy
mutations is high.
A Mitochondrial
diseases may be
caused by nuclear
gene mutations
(eg, CPEO,
MNGIE, Leigh
syndrome).

Southern blotting demonstrates
multiple mtDNA deletions and deple-
tion of mtDNA. Inheritance is autoso-
mal recessive, and mutations in
thymidine phosphorylase that map to
chromosome 22 have been identified
in those patients. Thymidine phos-
phorylase activity is severely reduced
in patients, and plasma thymidine
levels are high.
Leigh Syndrome
This is a subacute necrotizing ence-
phalomyelopathy. The clinical features
are varied and include psychomotor
retardation, respiratory abnormalities,
oculomotor disturbance, optic atro-
phy, seizures, and lactic acidosis.
Onset is usually in the first few months
of life or during childhood. This
syndrome might be caused by a variety
of biochemical abnormalities, includ-
ing respiratory chain defects, pyruvate
dehydrogenate deficiency, or biotini-
dase deficiency. Respiratory chain
defects include those patients with
mutations of nuclear genes encoding
subunits of complexes I or II. Defi-
ciency of complex IV in patients with
Leigh syndrome may be associated
with mutations of the SURF gene that
is responsible for assembly and main-
tenance of complex IV. As indicated
above, approximately 20% of patients
with Leigh syndrome have an ATPase
6 mutation with mutant loads of 90%
or greater. The MELAS or MERRF mu-
tations have also been associated with
a Leigh syndrome phenotype.
MtDNA Mutations Secondary to
Nuclear Gene Mutations
As indicated above, mtDNA is depen-
dent on nuclear DNA for its mainte-
nance and replication. Two types of
mtDNA abnormality due to nuclear
gene mutations have been identified.
These comprise multiple mtDNA de-
letions and depletion of mtDNA. Mu-
tations in the genes for adenine
nucleotide translocator 1 and TWINKLE
(a DNA helicase) can result in auto-
somal dominant inheritance of CPEO
and multiple mtDNA deletions or mod-
erate depletion of mtDNA. POLG muta-
tions may result in autosomal dominant
or recessive CPEO, Alpers syndrome
(an autosomal recessive disorder with
epilepsy, cortical blindness, and liver
failure), and have been found in pa-
tients who may present with progres-
sive external ophthalmoplegia and go
on in later life to develop Parkinson
disease (Luoma et al, 2004; Naviaux and
Nguyen, 2004). POLG mutations may
also cause multiple deletions or deple-
tion of mtDNA.
Severe depletion of mtDNA pres-
ents in the infantile period with liver
failure, lactic acidosis, and myopathy
and may be due to mutation of the
deoxyguanosine kinase gene. Late-
onset myopathic forms of mtDNA
depletion may be due to thymidine
kinase mutations. Those infants born
with severe mtDNA depletion usually
die within the first few days of life,
while some patients with milder de-
pletion and a pure myopathic form
may recover and survive.
NUCLEAR GENE MUTATIONS OF
RESPIRATORY CHAIN SUBUNITS
Although 72 of the 85 subunits of the
OXPHOS system are encoded by nu-
clear DNA, mutations of these genes
have only rarely been described. To
some extent this may be a reflection
of the deleterious nature of such mu-
tations, as affected fetuses may have
been aborted at an early stage of
development. Those mutations that
have been described generally mani-
fest in the neonatal period or early
infancy, although occasional late-onset
patients have been identified. These
patients manifest with a range of
phenotypes, including infantile Leigh
syndrome, cardiomyopathy, seizures,
and liver failure. The association with
142
KEY POINTS:
A Skeletal muscle
biopsy in
autosomal cases
may exhibit
typical features
such as ragged
red fibers.
A The presence of
multiple mtDNA
deletions in a
patient indicates
an underlying
nuclear gene
defect.

paragangliomas and pheochromocyto-
mas is very rare. The late-onset cases
have included progressive ataxia.
Toxin-Induced MtDNA
Abnormalities
A recently recognized exogenous
cause of mtDNA abnormalities is HIV
infection and the use of antiretroviral
therapy. Studies have now demon-
strated that untreated patients with
HIV have reduced mtDNA content,
abnormalities of OXPHOS function,
and oxidative damage (Miró et al,
2004). Antiretroviral treatment inhibits
mtDNA POLG and induces a secondary
mtDNA depletion that can be associ-
ated with toxicity, including myopathy,
lipodystrophy, lactic acidosis, and he-
patic failure.
TREATMENT OF
MITOCHONDRIAL DISEASES
Treatment for diseases due to muta-
tions of mtDNA remains unsatisfactory
and confined to supportive measures
and supplementation with coenzyme
Q10, although no large-scale studies
have been done to determine the
effectiveness of coenzyme Q10. Sup-
portive treatments include eye props
or ptosis surgery for patients with
CPEO. Such features as deafness and
diabetes require specific treatment, as
appropriate.
Defects of the respiratory chain re-
sult in the increased production of free
radicals, and so the administration of
antioxidants has some scientific basis.
N-acetylcysteine and coenzyme Q10,
both antioxidants, improved OXPHOS
function and reduced free radical
production in cybrid cells carrying the
T8993G mutation that causes NARP or
maternally inherited Leigh syndrome
(Mattiazzi et al, 2004). However, the
use of antioxidants in mtDNA disease
has yet to be tested in a clinical trial.
Various strategies are being evalu-
ated to modify the mtDNA mutant
load in cells and tissues in patients. An
obvious target would be the preferen-
tial expansion of wild-type mtDNA or
the suppression of mutant mtDNA
expansion (Smith et al, 2004). It
appears that the recruitment of skele-
tal muscle satellite cell expansion can
shift heteroplasmy in favor of wild
type, as mutant mtDNA is absent or at
a low level in these cells (Clark et al,
1997). Satellite cells can be provoked
to expand by vigorous exercise regi-
mens or toxic damage, although for
obvious reasons both have some
practical limitation in patients. Manip-
ulating mtDNA replication by the
import into mitochondria of endonu-
cleases that might selectively destroy a
specific mutant sequence has been
possible in vitro, but considerable
barriers must be overcome before in
vivo application (Tanaka et al, 2002).
An alternative mechanism for salvag-
ing OXPHOS function in cells with
tRNA mutations of mtDNA is the
import of normal tRNAs from the
cytosol to mitochondria. The import
of nuclear-encoded RNAs into the
mitochondrial matrix has been dem-
onstrated (Entelis et al, 2001). The
import of normal tRNAlys
from cytosol
to mitochondria improved OXPHOS
function in cybrid cells bearing the
tRNAlys
A8344G mutation that causes
MERRF (Kolesnikova et al, 2004).
MNGIE may be treated in the short-
term with platelet infusions, as these
contain thymidine phosphorylase. De-
finitive treatment may be possible with
bone marrow transplantation.
GENETIC COUNSELING
The inheritance of mtDNA mutations
will be maternal, ie, from mother to all
offspring, with subsequent passage by
the daughters alone. Although there is
one report of paternal inheritance,
143
KEY POINTS:
A Genetic
counseling is
dependent on
a molecular
diagnosis.
A Males with a
confirmed
mtDNA mutation
will not transmit
the mitochondrial
disease to
children.
A The transmission
rate of single
mtDNA deletions
(eg, in CPEO) is
generally less
than 5%.

males who carry an mtDNA mutation
can be reassured that they almost
certainly will not transmit the muta-
tion. Mitochondrial diseases caused by
nuclear gene mutations will be trans-
mitted by mendelian inheritance.
Mothers with CPEO have a low risk
of transmission (4%), and the risk of
more than one sibling being affected is
likewise low. Practical advice can in-
clude the possibility of in vitro fertil-
ization using a donor egg, but another
intriguing possibility for the future is
of nuclear transfer from a maternal
egg and fertilization in a donor cyto-
plasm using paternal sperm.
CONCLUSION
Much has been learned about the
potential for mtDNA mutations to
cause human disease, but much
remains to be discovered about the
molecular pathogenesis of these muta-
tions. Attention has recently been
focused on nuclear gene mutations
causing secondary mtDNA abnormali-
ties, and again, the phenotypic expres-
sion of these appears to be quite
variable. Not discussed in detail in this
article are the nuclear-encoded res-
piratory chain gene mutations that
are relatively rare or the nuclear
gene mutation affecting neither the
OXPHOS system nor mtDNA directly,
eg, the PINK1 mutation in Parkinson
disease. Thus, mitochondria are be-
coming an increasing focus for the
biomedical sciences, and research will
no doubt provide interest not only
into their association with human
disease but also into normal mito-
chondrial function and physiology.
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148

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