Genetics of mitochondrial diseases

1. Chairperson: Dr.Gayathri.N.
Dr.Bindu. P.S.
Presenter: Dr.Govindaraju.C.
Date.02.08.2011 4:30 PM

2. Topic Overview
 Introduction
 Mitochondrial genetics – Basics
 Respiratory chain
 Classification
 Mitochondrial diseases – nDNA mutations
- mtDNA mutations
• Genetic counselling
• Prenatal testing
• MITOMAP
• Conclusions

3. Introduction
 Mitochondria are cytoplasmic organelles whose major
function is to generate ATP.
 Mediated by the respiratory electron transport chain
(ETC) and the two electron carriers, coenzyme Q
(CoQ) and cytochrome-c.

4. Introduction
• The basic and peculiar molecular characteristics of the
mitochondrial genetic system were discovered at the
beginning of the 1980s.(Anderson et al Nature 1981, Montoya et
al Nature 1981)
• In 1988 the first mutations associated with diseases
were found. (Wallace et al, Science 1988. Zeviani et al, Neurology
1988)
• One of the peculiarity of mitochondria is that it has
its own genetic system.
 It has all the machinery necessary for their expression.

5. Introduction
• MtDNA is a small, circular extra-nuclear chromosome
encoding essential components of the respiratory
chain.
• MtDNA molecule consists of 16,569-nucleotide
sequence.
• Contains a total of 37 genes
ETC - 13
tRNA- 22
rRNA- 2

6. Human mitochondrial genome map
(Fauci et. alHarrisson’s principles of internal medicine 17th edition)
MELAS
MERRF NARP

7. Introduction
 The integrated activity of several hundred proteins is
required for normal mitochondrial biogenesis,
function and integrity.
 Most encoded by nuclear genes.
 Nuclear-encoded proteins are synthesized in the cell
cytoplasm and imported to their location of activity in
mitochondria through a complex biochemical process.

8. Introduction
 The mitochondria have their own genome consisting
of numerous copies (polyploidy) per mitochondrion of
mtDNA.
 This dual genetic control of mitochondrial function
can result in fascinating patterns of inheritance.

10. Introduction
• Mitochondrial cytopathies, a heterogeneous group of
multisystem disorders preferentially affecting the
skeletal muscle and nervous systems.
• Caused either by mutations in the maternally inherited
mitochondrial genome (Nature 1988) or by nuclear
DNA-mutations (Nature 1989. Br J Hosp Med 1996.)

11. Dual genetic control and multiple organ system
manifestations of mitochondrial disease

12. Introduction
• Till date approximately 200 different disease causing
mutations of mitochondrial DNA (mtDNA) are
known. (Schmiedel J et al, J Neurol 2003.)
• Due to the increased knowledge about nuclear
genetics during the last few years, several nuclear
mutations have been described.

13. Mitochondrial genetics: the basics
• Replication and transcription mechanisms of mtDNA
differ from mechanisms in the nuclear genome.
• Greatly reduced stringency of proofreading and
replication error correction lead to a much greater
degree of sequence variation.

14. Mitochondrial genetics: the basics
 Each mitochondrion contains many copies of mtDNA
and number of mitochondria per cell can vary,
mtDNA copy number per mitochondrion and per cell
can also vary within the lifetime of a cell.

15. Mitochondrial genetics: the basics
• With respect to transcription, initiation can occur on
both strands and proceeds through the production of
an intronless polycistronic precursor RNA.
• Precursor RNA processed to produce the 13 mRNA,
22 tRNA and 2 rRNA products.
• The 37 mtDNA genes comprise 93% of the 16,569
nucleotides of the mtDNA in coding region.

16. Mitochondrial genetics: the basics
• The control region(Non coding)
 ~1.1 kilobases (kb).
Major role in replication and transcription initiation.
D loop- containing two hypervariable regions (HVR-I and
HVR-II)
 Large interindividual variability within the human population.
• The mutation rate is considerably higher in the control region.

17. Multiple Copy Number (Polyploidy)
• Each aerobic cell in the body has multiple
mitochondria, often numbering many hundreds or
more.
• The number of copies of mtDNA within each
mitochondrion varies from 1 000 and 10 000 copies,
ranging from a few hundred in spermatozoids up to
100 000 in the oocyte.

18. Mitochondrial genetics: the basics
• In the case of somatic cells, newly acquired somatic
mutation is likely to be very small in terms of total
cellular or organ system function.
• Higher mutation rate during mtDNA replication,
numerous different mutations may accumulate with
the aging.

19. Mitochondrial genetics: the basics
 Total cumulative burden of acquired somatic mtDNA
mutations with age may result in perturbation of
mitochondrial function.
 Such acquired somatic mtDNA mutations contributes
Aging
Metabolic syndrome
Diabetes
Cancer
Neurodegenerative
Cardiovascular disease
 Somatic mutations in mtDNA are not carried forward
to the next generation.

20. Lack of Recombination
• Nuclear genome is characterized by homologous pairs
of chromosomes of biparental origin.
• Homologous pairs undergo meiotic recombination
during gametogenesis.
• In contrast, mtDNA molecules do not undergo
recombination, such that mutational events represent
the only source of mtDNA genetic diversification.

21. Maternal Inheritance
 The mtDNA is inherited maternally with a vertical
non-Mendelian pattern.
 The mother transmits her mitochondrial genome to all
her children.
 But only the daughters will pass it on to all the
members of the next generation and so on.(Sutovsky P et
al Nature 1999)

22. Maternal inheritance of mtDNA
disorders and heritable traits

23. Maternal inheritance
 Evidence of paternal transmission can almost certainly
be rule out an mtDNA genetic origin of phenotypic
variation or disease.
 Conversely, a disease affecting both sexes without
evidence of paternal transmission strongly suggests a
heritable mtDNA disorder.

24. Maternal inheritance
 One interesting consequence of uniparental
inheritance and lack of recombination is the utility of
mtDNA marker and sequence analysis in tracing
matrilineal ancestry in phylogenetic research.

25. Homoplasmy and heteroplasmy
 The polyploid nature of the mitochondrial genome
gives rise to an important feature of mitochondrial
genetics, homoplasmy and heteroplasmy.
 Homoplasmy is when all copies of the mitochondrial
genome are identical.
 Heteroplasmy is when there is a mixture of two or
more mitochondrial genotypes.

26. Mitotic segregation
 Mitotic segregation refers to the unequal distribution
of wild-type and mutant versions of the mtDNA
molecules during cell divisions.
 Occur during prenatal development and subsequently
throughout the lifetime of an individual.

27. Threshold" effect
 “Threshold" effect, wherein the actual expression of
disease depends upon the relative percentage of
mitochondria whose function is disrupted by mtDNA
mutations.
 There is tremendous heterogeneity in disease
penetrance and severity, as well as complexity of
organ system involvement among the offspring.

28. Threshold" effect
 This heterogeneity arises from differences in the
degree of heteroplasmy among oocytes.
 This may create difficulty in recognizing a maternal
pattern of inheritance and making the diagnosis of an
mtDNA genetic cause of disease.

29. Genetic drift
 During the course of human evolution, certain
heteroplasmic mtDNA sequence variants may drift to
a state of homoplasmy, wherein all of the mtDNA
molecules in the organism contain the new sequence
variant.
 This arises due to a "bottleneck" effect followed by
genetic drift during the process of oogenesis itself.

31. Genetic drift
 In other words, during certain stages of oogenesis, the
mtDNA copy number becomes substantially reduced,
such that the particular mtDNA species bearing the
novel or derived sequence variant may become
increasingly predominant, and eventually exclusive
version of the mtDNA for that particular nucleotide
site.

32. Genetic drift
 The offspring of a woman bearing an mtDNA
sequence variant or mutation that has become
homoplasmic will also be homoplasmic for that
variant.
 Female offspring will transmit it forward in
subsequent generations.
 This process establishes a new mtDNA haplotype in
the human population.

33. Respiratory chain
 The respiratory chain consists of four multi subunit
complexes (Complexes I-IV) which, together with
complex V (ATP synthase), form the respiratory
chain/oxidative phosphorylation system.
 The respiratory chain is unique, in that it is under the
control of two separate genomes: mtDNA and nDNA.

34. The subunits of the respiratory chain encoded by
nuclear DNA (nDNA)
Luft’s dis

35. Respiratory chain
• The coordination of the signals between the nucleus
and the mitochondrion are poorly understood.
• Disorders associated with nDNA follow the traditional
mendelian patterns of inheritance.
• Pathogenic mutations have been identified thus far
only in Complexes I, II, III and IV.

36. Genetic Classification of Human
Mitochondrial Disorders

38. Nuclear DNA Mutations
 Mutations in structural subunits.
 Mutations in assembly factors.
 Mutations in translation factors.
 Multiple mtDNA deletions or mtDNA depletions.

39. Nuclear DNA Mutations
Nuclear genetic disorders of the mitochondrial respiratory
chain, mutations in structural subunits
 Leigh syndrome with complex I deficiency (NDUFS1,
NDUFS4, NDUFS7, NDUFS8, NDUFV1)
Cardiomyopathy and encephalopathy (complex I deficiency)
(NDUFS2)
Leigh syndrome with complex II deficiency (SDHA)
 Leukodystrophy with complex II deficiency (SDHAF1)
Optic atrophy and ataxia (complex II deficiency) (SDHA)
 Hypokalemia and lactic acidosis (complex III deficiency)
(UQCRB)

40. Nuclear DNA Mutations
Nuclear genetic disorders of the mitochondrial respiratory
chain, mutations in assembly factors
 Leigh syndrome (SURF1, LRPPRC)
 Hepatopathy and ketoacidosis (SCO1)
 Cardiomyopathy and encephalopathy (SCO2)
 Leukodystrophy and renal tubulopathy (COX10)
 Hypertrophic cardiomyopathy (COX15)
 Encephalopathy, liver failure, renal tubulopathy (with complex
III deficiency) (BCS1L)
 Encephalopathy (with complex V deficiency) (ATPAF2)

41. Nuclear DNA Mutations
Nuclear genetic disorders of the mitochondrial
respiratory chain, mutations in translation factors
 Leigh syndrome, liver failure, and lactic acidosis (GFM1)
Lactic acidosis, developmental failure, and dysmorphism
(MRPS16)
 Myopathy and sideroblastic anemia (PUS1)
Leukodystrophy and polymicrogyria (TUFM)
 Leigh syndrome and optic atrophy with COX deficiency
(TACO1)

42. Nuclear DNA Mutations
 Nuclear genetic disorders associated with multiple mtDNA deletions or
mtDNA depletion
 Autosomal progressive external ophthalmoplegia (POLG, C10orf2, SLC25A4)
 Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE)
(thymidine phosphorylase deficiency) (TYMP)
 Alpers-Huttenlocher syndrome (POLG)
 Infantile myopathy / spinal muscular atrophy (TK2)
 Encephalomyopathy and liver failure (DGUOK)
 Hypotonia, movement disorder, and/or Leigh syndrome with methylmalonic
aciduria (SUCLA2)
 Hypotonia, encephalopathy, renal tubulopathy, lactic acidosis (RRM2B)
 Mitochondrial encephalomyopathy with combined RC deficiency (AIF1)

43. Nuclear DNA Mutations
Others
Coenzyme Q10 deficiency (COQ2, COQ9, CABC1,
ETFDH)
Barth syndrome (TAZ)
Cardiomyopathy and lactic acidosis (mitochondrial
phosphate carrier deficiency) (SLC25A3)

44. Nuclear-encoded gene mutations
associated with mitochondrial disease
Leigh’s,
GRACILE
BCS1L

47. Complex I disorders
• Reduces NADH and shuttles electrons to Coenzyme
Q10 (CoQ10).
• It is the largest enzyme complex of the respiratory
chain and is comprised of at least 42 subunits, of
which 7 are encoded by the mitochondrial genome .
• Isolated Complex I deficiency appears to be one of the
most common causes of mitochondrial
encephalomyopathies.(Morris et al,Ann Neurol, 1996)

48. Complex I disorders
 The most common clinical presentation is Leigh
syndrome (LS), with 40-50% of these cases having
associated cardiomyopathy.(Rahman et al, Ann Neurol 1996)
 Fatal neonatal lactic acidosis is also common.
 All nDNA-encoded Complex I deficiencies described
to date have been inherited as recessive traits.

49. Complex II disorders
 Oxidizes succinate to fumarate (in the citric acid cycle)
and transfers electrons from FADH2 to CoQ10.
 Complex II is the only respiratory chain complex that is
encoded entirely by the nuclear genome.

50. Complex II disorders
 Wide clinical spectrum of diseases associated with
Complex II deficiency include
Kearns-Sayre syndrome
Muscle weakness
Hypertrophic cardiomyopathy
Leigh syndrome
Optic atrophy
Cerebellar ataxia
Hereditary paraganglioma .

51. Complex III disorders
 In Complex III (cytochrome bc1 complex) two
electrons are removed from QH2 at the QO site and
sequentially transferred to two molecules of
cytochrome c.
 It is a severe, multisystem disorder that includes
features such as lactic acidosis, hypotonia,
hypoglycemia, failure to thrive, encephalopathy, and
delayed psychomotor development.
 It is generally caused by mutations in nuclear DNA in
the BCS1L, UQCRB and UQCRQ genes and
inherited in an autosomal recessive manner.

52. Complex IV disorders
• Transfers electrons from cytochrome c to molecular
oxygen and pumps protons across the inner
mitochondrial membrane.
• It is comprised of thirteen subunits: the 3 largest are
encoded by mtDNA and the other 10 by nDNA.
• Isolated COX deficiency due to mutations in mtDNA-
encoded genes has been associated with myopathies
(Keightley JA et al, Nat Genet 1996) and multisystemic
disease.
• No pathogenic mutations in the nuclear encoded
subunits of COX have been found. (Jaksch M et al, J Med
Genet 1998)

53. Complex V disorders
 One candidate disorder is Luft disease, which might
be due to defects in Complex V.
 Luft disease is a rare condition that presents in
adolescence with fever, heat intolerance, profuse
sweating, polyphagia, polydipsia, tachycardia, and
mild to moderate weakness. (DiMauro S, J Neuro Sci 1976)

54. Coenzyme Q10 (CoQ10)
• Coenzyme Q10 (CoQ10) is a lipophilic quinone that
accepts electrons from Complex I and Complex II and
transfers them to Complex III.
• Partial defects (20-30%) of CoQ10 have been reported
in association with KSS and a number of undefined
myopathies (Zierz S et al, J Neurol 1989)
• To date, no known mutations responsible for defective
CoQ10 activity have been identified.

55. Leigh disease
 Leigh syndrome (LS) is an inherited, progressive,
metabolic disease of infancy and childhood.
 Leigh syndrome is the most common clinical
phenotype of mitochondrial disorders in childhood.
 Begins late in first year, rapid decline in function
occurs, marked by seizures, encephalopathy,
dementia, ventilatory failure.

56. Leigh disease
 The diagnostic criteria are
(1) progressive neurological disease with motor and
intellectual developmental delay.
(2) signs and symptoms of brainstem and/or basal
ganglia disease.
 (3) raised lactate levels in blood and/or cerebrospinal
fluid (CSF).
(4) characteristic symmetric necrotic lesions in the
basal ganglia and/or brainstem. (Rahman et al Ann Neurol
1996)

57. Leigh disease
 8993T>G, 8344A>G, pyruvate carboxylase
deficiency, pyruvate dehydrogenase deficiency,
complex I - IV deficiency, SURF 1 deficiency.
 Causative genes exist in both nuclear and
mitochondrial genomes.
 Observations suggested an autosomal recessive
inheritance, autosomal dominant, X-linked, and
maternally inherited .

58. Leigh disease
 The most characteristic neuroradiological findings in
Leigh syndrome are bilateral, symmetric focal
hyperintensities in the basal ganglia, thalamus,
substantia nigra, and brainstem nuclei.

59. Leigh's disease
7 mo old child: Regression,
EOM abnormalities,abnormal
respiration
MRI:
Putamen,thalami,SN,Periaqued
uctal gray matter affected
15 mo:Leigh's
phenotype
MRI:
Putamen,Subthalamic
nucleus,SN,white
matter affected

60. Polymerase γ(POLG)
 Mutations in the POLG gene have emerged as one of
the most common causes of inherited mitochondrial
disease in children and adults.
 MtDNA is replicated by DNA polymerase gamma
(POLG) encoded by the nuclear POLG gene.

61.  Alpers-Huttenlocher syndrome (AHS)
 Childhood myocerebrohepatopathy spectrum
(MCHS)
 Myoclonic epilepsy myopathy sensory ataxia
(MEMSA)
 The ataxia neuropathy spectrum (ANS) includes
mitochondrial recessive ataxia syndrome (MIRAS)
and sensory ataxia neuropathy dysarthria and
ophthalmoplegia (SANDO).
POLG-Related Disorders

62. POLG-Related Disorders
 Autosomal recessive progressive external
ophthalmoplegia (arPEO)
 Autosomal dominant progressive external
ophthalmoplegia (adPEO)

63. Alpers syndrome
 Alpers syndrome is a developmental mitochondrial
DNA depletion syndrome leading to fatal brain and
liver disease in children and young adults.
 Mutations in the gene for the mitochondrial DNA
polymerase (POLG) have recently been shown to
cause this disorder.

64. Alpers syndrome
 The most common Alpers-causing mutation was the
A467T substitution, located in the linker region of the
pol gamma protein.
 Accounted for about 40% of the alleles and was
present in 65% of the patients.
 All patients with POLG mutations had either the
A467T or the W748S substitution in the linker region.
(Nguyen KV, J Hepatol. 2006)

65. Mitochondrial neurogastrointestinal
encephalopathy (MNGIE)
 The diagnosis of MNGIE disease is based on the presence
of the following clinical findings
Severe gastrointestinal (GI) dysmotility
Cachexia
Ptosis
External ophthalmoplegia
Sensorimotor neuropathy (usually mixed axonal and
demyelinating)
Asymptomatic leukoencephalopathy manifest as diffusely
abnormal brain white matter on brain MRI. (Hirano et al
1994, Nishino et al 1999, Nishino et al 2000)

66. MNGIE
 Family history consistent with autosomal recessive
inheritance.
 Molecular genetic testing of TYMP, the gene encoding
thymidine phosphorylase, detects mutations in
approximately 100% of affected individuals.

67. mtDNA depletion syndromes(DPSs)
 Early-onset, age-specific syndromes and are
phenotypically quite heterogeneous.
 DPSs have been linked to mutations in nine genes
(POLG1, PEO1 (twinkle), thymidine-kinase (TK2),
DGUOK, SUCLA2, SUCLG1, MPV17, RRM2B,
TYMP).
 Three main clinical presentations
Myopathic(TK2 or RRM2B genes)
Encephalo-myopathic(SUCLA2 or SUCLG1 genes.)
 Hepato-cerebral form(PEO1, POLG1, DGUOK or
MPV17 genes).

68. Mitochondrial DNA Mutations
(mt DNA)
 Rearrangements (deletions and duplications)
 Point mutations
tRNA genes
rRNA genes

69. Mitochondrial DNA Mutations
(mt DNA)
Rearrangements (deletions and duplications)
• Chronic progressive external ophthalmoplegia
• Kearns-Sayre syndrome
• Diabetes and deafness
Point mutations
• Protein-encoding genes
 Leber hereditary optic neuropathy (LHON) (m.11778G>A,
m.14484T>C, m.3460G>A)
 Neurogenic weakness with ataxia and retinitis
pigmentosa(m.8993T>G) / Leigh syndrome (m.8993T>C)
(Arpa et al, Muscle Nerve. 2003

70. Mitochondrial DNA Mutations
(mt DNA)
 tRNA genes
• MELAS (m.3243A>G, m.3271T>C, m.3251A>G)
• MERRF (m.8344A>G, m.8356T>C)
• Chronic progressive external ophthalmoplegia (m.3243A>G,
m.4274T>C)
• Myopathy (m.14709T>C, m.12320A>G)
• Cardiomyopathy (m.3243A>G, m.4269A>G)
• Diabetes and deafness (m.3243A>G, m.12258C>A)
• Encephalomyopathy (m.1606G>A, m.10010T>C)
 rRNA genes
• Nonsyndromic sensorineural deafness (m.7445A>G)
• Aminoglycoside-induced nonsyndromic deafness
(m.155A>G)
(Arpa et al, Muscle Nerve 2003. )

71. Mitochondrial DNA Disease
 Rough estimates suggest that heteroplasmic germ-line
pathogenic mtDNA mutations may affect up to
approximately 1 in 5000 individuals.

72. Mitochondrial Diseases Due to mtDNA Point Mutations and Large-
Scale Rearrangements

73.  Both the nuclear as well as mitochondrial genomic
background modify disease penetrance. Thus, for example,
LHON has a greater penetrance and severity in men than in
women, pointing to an epistatic interaction with the
nuclear genome. Moreover, disease susceptibility for a
given mutation is modulated by mtDNA haplotype
background, with certain haplotypes being protective. Of
interest, patients with this syndrome are often
homoplasmic for the disease-causing mutation. The
somewhat later onset in young adulthood and modifying
effect of genetic background may have enabled
homoplasmic pathogenic mutations to have escaped
evolutionary censoring.

74. Leber hereditary optic neuropathy
(LHON)
 Leber hereditary optic neuropathy (LHON) is a
common cause of maternally inherited visual failure.
 LHON
Young adulthood
Subacute painless loss of vision
Cerebellar ataxia
Peripheral neuropathy
Cardiac conduction defects

75. LHON
 In >95% of cases, LHON is due to one of three point
mutations of mtDNA that affect genes encoding
different subunits of complex I of the mitochondrial
ETC.

76. Mitochondrial encephalomyopathy, lactic
acidosis, and stroke-like episodes(MELAS)
 MELAS is probably the most common mtDNA
disease.
Progressive encephalomyopathy characterized by repeated
stroke-like events
Recurrent migraine-like headache
Vomiting
Exercise intolerance
Seizures
Short stature
Lactic acidosis

77. MELAS
 Brain lesions do not follow the distribution of
vascular territories.
 The most commonly described pathogenic point
mutations are A3243G and T3271C in the gene
encoding the leucine tRNA.

78. Myoclonic epilepsy with ragged red
fibers(MERRF)
 Multisystem disorder characterized by
Myoclonus
Seizures
Ataxia
Myopathy with ragged red fibers
Hearing loss
Exercise intolerance
Neuropathy
Short stature

79. MERRF
 Almost all MERRF patients have mutation in the
mtDNA tRNAlys gene and the A8344G mutation is
responsible for 80–90% of MERRF cases.

80. Neurogenic weakness, ataxia, and
retinitis pigmentosa (NARP)
 Neurogenic weakness, ataxia, and retinitis pigmentosa
(NARP) is characterized by moderate diffuse cerebral
and cerebellar atrophy and symmetric lesions of the
basal ganglia on MRI.
 A heteroplasmic T8993G mutation in the gene ATPase
6 subunit gene has been identified as causative.
 Ragged red fibers are not observed in muscle biopsy.

81. NARP
 When >95% of mtDNA molecules are mutant(mutant
load), a more severe clinical, neuroradiologic and
neuropathologic picture (Leigh's syndrome) emerges.
 Point mutations in the mtDNA gene encoding the 12S
rRNA result in heritable nonsyndromic hearing loss.
 One such mutation causes heritable ototoxic
susceptibility to aminoglycoside antibiotics, which
opens a pathway for a simple pharmacogenetic test in
the appropriate clinical settings.

82. Large-scale mtDNA rearrangements
 Kearns-Sayre syndrome (KSS), sporadic progressive
external ophthalmoplegia (PEO) and Pearson
syndrome are three disease phenotypes caused by
Large-scale mtDNA rearrangements.

83. Kearns-Sayre syndrome(KSS)
 KSS is characterized by the triad of onset before age
20, chronic progressive external ophthalmoplegia, and
pigmentary retinopathy.
 Cerebellar syndrome, heart block, increased
cerebrospinal fluid protein, diabetes and short stature
are also part of the syndrome.
 Single deletions/duplication can also result in milder
phenotypes such as PEO, proximal myopathy and
exercise intolerance.
 In both KSS and PEO, diabetes mellitus and hearing
loss are frequent accompaniments.

84. CPEO
 PEO is characterised by bilateral ptosis and
ophthalmoplegia.
 Frequently associated with muscle weakness and
exercise-intolerance.
 Occasionally, with ataxia, cataract, retinitis
pigmentosa, hearing loss or cardiomyopathy.
 Associated with single mtDNA deletions

85. Pearson syndrome
 Pearson syndrome is characterized by diabetes
mellitus from pancreatic insufficiency, together with
pancytopenia and lactic acidosis.
 Caused by the large-scale sporadic deletion of several
mtDNA genes.

86. Secondary mitochondrial dysfunction
 Mitochondrial dysfunction is seen in a number of
different genetic disorders
Ethylmalonic aciduria (caused by mutation of
ETHE1.(Tiranti et al 2009)
Friedreich ataxia (FXN). (Rötig et al 1997)
 Hereditary spastic paraplegia 7 (SPG7).(Casari et al 1998 )
Wilson disease (ATP7B). (Lutsenko & Cooper 1998)
Part of the aging process.

87. Testing algorithm for molecular diagnosis of
patients with suspected mitochondrial disease

88. Contd…

89. Genetic counseling
 Genetic counseling is the process of providing
individuals and families with information on the
nature, inheritance, and implications of genetic
disorders to help them make informed medical and
personal decisions.

90. Contd…
 Since mitochondrial diseases lead frequently to severe
phenotypes and are often hereditary, there is a need
for genetic counselling of the affected families.
 The provision of accurate genetic counseling and
reproductive options to these families is complicated
by the unique genetic features of mtDNA.
 Include maternal inheritance, heteroplasmy, the
threshold effect, tissue variation, and selection.

91. Contd…
 MtDNA defects are transmitted by maternal
inheritance (Thorburn & Dahl 2001).
 Nuclear gene defects may be inherited in an
autosomal recessive manner or an autosomal
dominant manner.

92. Risk to other family members-
Mitochondrial DNA

93. Parents of a proband
 Single mtDNA deletions
 Mitochondrial DNA deletions generally occur de
novo and thus affect only one family member, with
no significant risk to other family members.
 When single mtDNA deletions are transmitted,
inheritance is from the mother.

94. Parents of a proband
 Mitochondrial DNA point mutations and
duplications
 Mitochondrial DNA point mutations and
duplications may be transmitted through the
maternal line.
 The father of a proband is not at risk of having the
disease-causing mtDNA mutation.
 The mother of a proband (usually) has the
mitochondrial mutation and may or may not have
symptoms.

95. Sibs of a proband
 The risk to the sibs depends on the genetic status of
the mother.
 If the mother has the mtDNA mutation, all sibs are at
risk of inheriting it.
 When a proband has a single mtDNA deletion, the
current best estimate of the recurrence risk to sibs is
1/24 (Chinnery et al 2004).

96. Offspring of a proband
 Offspring of males with a mtDNA mutation are not at
risk.
 All offspring of females with a mtDNA mutation are
at risk of inheriting the mutation.
A female harboring a heteroplasmic mtDNA point
mutation may transmit a variable amount of mutant
mtDNA to her offspring, resulting in considerable
clinical variability among sibs within the same
nuclear family (Poulton & Turnbull 2000).

97. Contd…
 For the m.8993T>G, m.8993T>C, m.3243A>G,
m.8344A>G, and m.11778G>A mtDNA mutations,
the risk of having clinically affected offspring appears
to be related to the percentage level of mutant mtDNA
in the mother's blood (Chinnery et al 1998, White et al 1999,
Chinnery et al 2001).
 However, these data were obtained retrospectively and
should not be directly used for genetic counseling.

98. Risk to other family members
 The risk to other family members depends on the
genetic status of the Proband’s mother.
 If she has a mtDNA mutation, her siblings and
mother are also at risk.

99. Prenatal testing
 Mitochondrial DNA mutations.
Prenatal genetic testing and interpretation for mtDNA
disorders is difficult because of mtDNA heteroplasmy.
 The percentage level of mutant mtDNA in a
chorionic villus sampling (CVS)may not reflect the
percentage level of mutant mtDNA in other fetal
tissues.

100. Contd…
Percentage level may change during development and
throughout life (Poulton et al 1998).
The interpretation of a CVS result is difficult.
Prenatal diagnosis is not recommended for most
heteroplasmic mtDNA mutations.

101. Prenatal testing
 m.8993T>G and m.8993T>C mutations show a more
even tissue distribution and the percentage level of
these two mutations does not appear to change
significantly over time. (White et al 1999)
 Successful prenatal molecular diagnosis has been
carried out for these two mutations (Harding et al 1992,
White et al 1999) using DNA extracted from fetal cells
obtained by amniocentesis or CVS.

102. Contd…
 Empirical risks were recently provided for
MELAS,MERRF and LHON.
 In MELAS and MERFF, higher levels of mutant
mtDNA in the mothers' blood were associated with an
increased frequency of affected offspring.
 CPEO and KSS are in general sporadic disorders
without increased recurrence risks in the offspring.
 As Leigh syndrome is found with maternal,
autosomal recessive or X chromosomal transmission,
the definition of the molecular defect is crucial for
genetic counselling.

103. Contd…
 Current reproductive options that may be considered
for prevention of transmission of mtDNA mutations
Use of donor oocytes
Prenatal diagnosis
Preimplantation genetic diagnosis
Nuclear transfer
Cytoplasmic transfer

104. MITOMAP
 MITOMAP: a human mitochondrial genome database.
 Grown rapidly in data content over the past several years.
 MITOMAP (http://www.mitomap.org/) is a comprehensive
database of human mitochondrial DNA (mtDNA) variation and
its relationship with human evolution and disease.
 In MITOMAP, the location of each gene and regulatory-
functional element is defined by its beginning and ending
nucleotide positions.
 MITOMAP also maintains a compendium of all known
pathogenic mtDNA mutations.

105. Conclusions
 It is nearly 25 years since human mitochondrial
genome has been sequenced.
 Significant progress in the mitochondrial field
continues to be made.
 Understanding of the pathogenesis of mtDNA disease
will greatly improve by studying the basic processes.

106. Conclusions
 Unequivocally, mtDNA mutations are an important
cause of genetic disease.
 The clinical variability of these disorders makes the
recognition of patients with mtDNA disease a real
challenge.
 Clinicians must be aware of its impact; accurate
diagnosis requires a combination of different studies
and should be carried out in specialist centres.

107. Conclusions
 Most disappointing area has been the lack of treatment
for patients with mtDNA disease.
 Several new experimental approaches are currently
under investigation.
 It is crucial that further work and ideas are
forthcoming to realistically treat or prevent the
transmission of mtDNA disease to future generations.

108. Thank you