A mitochondrion (singular of mitochondria) is part of every cell in the body that contains genetic material. Mitochondria are responsible for processing oxygen and converting substances from the foods we eat into energy for essential cell functions. The mitochondria of the zygote come from the oocyte, that is, from the mother and almost never from the sperm, form of transmission is called maternal inheritance Which mitochondrial gene is mutated. The extent of replicative segregation of the mutant mitochondrial genome during the early stages of embryonic development. The abundance of the mutant mitochondrial gene in a particular tissue. The threshold level of mutant mitochondrial DNA required in a tissue before an abnormality is evident clinically Mitochondrial disease affects tissues most highly dependent on ATP production *Nerves *Muscles Endocrine Kidney Low energy-requiring tissues are rarely directly affected, but may be secondarily Lung Connective tissue Symptoms can be intermittent Increased energy demand (illness, exercise) Decreased energy supply (fasting) Common feature myoclonus epilepsy, deafness, blindness, anemia, diabetes, seizures and loss of cerebral blood supply (stroke). Myoclonic epilepsy and ragged-red fiber disease (MERRF) MERRF is a member of a group of disorders called mitochondrial encephalomyopathies that feature mitochondrial defects with altered brain and muscle functions. The term “ragged red fibers” refers to large clumps of abnormal mitochondria that accumulate mostly in muscle cells and are stained red by a dye that is specific for complex II of the electron transport chain. rare, maternally inherited, heteroplasmic, (point mutation in tRNA lysine gene) Mutation is MTTK*MERRF8344G. MT means mitochondrial gene is mutated T means transfer RNA gene K means the single-letter amino acid designation for lysine MERRF means the clinical features 8344G means the mutant nucleotide is guanine (G) at nucleotide position 8344 If 90% of the mitochondria in nerve and muscle cells carry the MTTK*MERRF8344G mutation, then the defining symptoms of MERRF are present. Maternally inherited mitochondrial disease The MTTL1*MELAS3243G mutation accounts for more than 80% of the cases of MELAS. This base substitution is in one of the two mitochondrial transfer RNALeu genes. the A3243G mutation occurs in thetRNALeu(UUR) gene When this mutation is present in ≥90% of the mitochondrial DNA of muscle tissue, there is an increased likelihood of recurrent strokes, dementia, epilepsy, and ataxia. When heteroplasmy for the A3243G mutation is ~40% to 50%, chronic progressive external ophthalmoplegia (CPEO), myopathy, and deafness are likely to occur. Other MELAS mutations occur at sites 3252, 3271, and 3291 within the tRNALeu(UUR) gene and in the mitochondrial tRNAVal (MTTV) and COX III (MTCO3) genes. Reduced activities in Complexes I and IV are established

1. Molecular Genetics of
Mitochondrial Disorders
Prepared by
Amr Said
Assistant research, Biotechnology Research
Lab. Horticulture Research Institute.
Amr_amz_9@yahoo.com

2. Mitochondria and Oxidative Phosphorylation

3. A mitochondrion (singular of mitochondria) is part of
every cell in the body that contains genetic material.
Mitochondria are responsible for processing oxygen and
converting substances from the foods we eat into energy
for essential cell functions.
Mitochondria produce energy in the form of ATP, which is
then transported to the cytoplasm of a cell for use in
numerous cell functions.
Mitochondria

7. mtDNA
• 16.6 kb
• 37 genes
– 13 proteins
22 t-RNAs
– 2 r-RNAs

10. Figure 12.2 Schematic representation of oxidative phosphorylation (OXPHOS). Electrons (e-) produced by
oxidation of reduced nicotinamide adenine dinucleotide (NADH) to nicotinamide adenine dinucleotide (NAD)
by complex I, of succinate to fumarate by complex II, and of reduced flavin adenine nucleotide (FADH2) to
flavin adenine nucleotide (FAD) by electron transport factors (ETF) are passed to coenzyme Q (CoQ) and
then to complex III and complex IV. Complex IV catalyzes the production of H2O. Protons (H+) are pumped
into the intermembrane space by complexes I, III, and IV and are used by complex V to synthesize ATP.
Adenine nucleotide translocator (ANT) facilitates the passage of ADP into and ATP out of the matrix. Each
box in the upper left corner of the representations of complexes I to V contains the number of mitochondrial
(m.p.) and nuclear (n.p.) subunits encoded by mitochondrial and nuclear genes, respectively.

11. Mitochondrial Genetics

13. Bottleneck and Segregation
• The mitochondria of the zygote come from the oocyte, that is, from the mother
and almost never from the sperm, form of transmission is called maternal
inheritance
• A human oocyte has approximately 100,000 mitochondria During the first few
days of embryonic cell division, mitochondrial propagation builds up the number
of mitochondria per cell to about 10,000 or more.
• The process of reducing the number of mitochondria from 100,000 to less than
100 has been called a genetic bottleneck.

14. The term homoplasmy describes the situation in which all the mitochondria of a
cell or tissue have the same genome.
Heteroplasmy denotes a cell or tissue containing both mutant and wild-type
mitochondrial genomes.
If a mitochondrial gene mutation reduces the production of ATP, then cells with
a high energy demand that are homoplasmic for the mutant mitochondrial DNA
will be seriously damaged.
homoplasmy for mutant mitochondrial DNA would have little impact on cells
with low energy requirements.
The proportion of mutant to wild-type mitochondrial DNA determines whether
an energy shortage occurs.
The threshold at which the deleterious effects of a mitochondrial gene mutation
become apparent depends on the energy needs of a particular cell or tissue.
The brain, skeletal muscles, heart, and liver, all of which have considerable
energy requirements, are highly susceptible to mitochondrial gene mutations.

15. Mitochondrial Disorders

16. • Which mitochondrial gene is mutated.
• The extent of replicative segregation of the mutant
mitochondrial genome during the early stages of embryonic
development.
• The abundance of the mutant mitochondrial gene in a
particular tissue.
• The threshold level of mutant mitochondrial DNA required in
a tissue before an abnormality is evident clinically.
The combination of defects associated with a particular
mitochondrial disorder depends on:

17. 1. Mitochondrial disease affects tissues most highly
dependent on ATP production
1. *Nerves
2. *Muscles
3. Endocrine
4. Kidney
2. Low energy-requiring tissues are rarely directly
affected, but may be secondarily
1. Lung
2. Connective tissue
3. Symptoms can be intermittent
1. Increased energy demand (illness, exercise)
2. Decreased energy supply (fasting)
4. Common feature
1. myoclonus epilepsy, deafness, blindness, anemia, diabetes, seizures
and loss of cerebral blood supply (stroke).

18. 1- Myoclonic epilepsy and ragged-red fiber disease (MERRF)
 MERRF is a member of a group of disorders called mitochondrial
encephalomyopathies that feature mitochondrial defects with altered brain
and muscle functions.
 The term “ragged red fibers” refers to large clumps of abnormal
mitochondria that accumulate mostly in muscle cells and are stained red by
a dye that is specific for complex II of the electron transport chain.
 rare, maternally inherited, heteroplasmic, (point mutation in tRNA lysine
gene)
 Mutation is MTTK*MERRF8344G.
 MT means mitochondrial gene is mutated
 T means transfer RNA gene
 K means the single-letter amino acid designation for lysine
 MERRF means the clinical features
 8344G means the mutant nucleotide is guanine (G) at nucleotide position
8344
 If 90% of the mitochondria in nerve and muscle cells carry the
MTTK*MERRF8344G mutation, then the defining symptoms of MERRF are
present.

21. 2- Mitochondrial myopathy, Encephalomyopathy, Lactic Acidosis, and Stroke-
like episodes (MELAS)
Maternally inherited mitochondrial disease
The MTTL1*MELAS3243G mutation accounts for more than 80% of the cases of MELAS.
This base substitution is in one of the two mitochondrial transfer RNALeu genes.
• the A3243G mutation occurs in thetRNALeu(UUR) gene
• When this mutation is present in ≥90% of the mitochondrial DNA of muscle tissue,
there is an increased likelihood of recurrent strokes, dementia, epilepsy, and ataxia.
• When heteroplasmy for the A3243G mutation
• is ~40% to 50%, chronic progressive external ophthalmoplegia (CPEO), myopathy,
and deafness are likely to occur.
• Other MELAS mutations occur at sites 3252, 3271, and 3291 within the
tRNALeu(UUR) gene and in the mitochondrial tRNAVal (MTTV) and COX III
(MTCO3) genes.
Reduced activities in Complexes I and IV are established.

22. The A3243G mutation, the only phenotypic features are diabetes
mellitus (DM) and deafness (DF).
Myopathy predominates with mutations at sites 3250, 3251, 3302, 3303,
and 3260.
Cardiomyopathy is the main defect associated with base substitutions
at sites 3260 and 3303.
In one patient, a deletion of the thymidine residue at 3271 was
responsible for deafness by the age of 5, followed by degenerative eye
disease, renal failure, seizures, cerebral damage, and, ultimately, death.
One patient with a C3256T mutation had both MELAS and MERRF
ymptoms.

24. 3- Leber Hereditary Optic Neuropathy (LHON)
rare mitochondrial disorder of the eye.
the first sign of LHON is a blurring of vision, followed within a few months
by a painless, complete, or near-complete, loss of sight, Often, both eyes are
affected simultaneously.
Degeneration of the optic nerve and neurons of the retina are the principal
pathological features of LHON.
The onset of LHON is usually when patients are in their mid-20s, but can
range from childhood to adults older than 70 years.
There is generally a sex bias, with approximately five times more males than
females showing the disorder.
At least 18 different missense mutations in nine genes (ND1, ND2, CO1,
ATP6, CO3, ND4, ND5, ND6, CYTB) that encode mitochondrial proteins are
responsible directly or indirectly for the LHON phenotype.

25. Three mutations (MTND1*LHON3460A, MTND4*LHON11778A,
MTND6*LHON14484C) account for more than 90% of all cases
and the 11778A mutation is found in 50% to 70% of these patients.
The 11778A mutation lowers the rate of oxidation of NAD-linked
substrates,
3460A reduces the activity of complex I by about 80%,
14484C also may decrease the activity of complex

27. 4- Neuropathy, Ataxia, and Retinitis Pigmentosa (NARP)
rare heteroplasmic mitochondrial disorder
Common feature: delayed development, muscle weakness, dementia, seizures,
retinitis pigmentosa, and diminished sensory functions.
Mutations at site 8993 in the mitochondrial ATP6 gene (MTATP6*NARP8993G and
MTATP6*NARP8993C) when the level of heteroplasmy of the mutant gene ranges
from ~70% to 90%.
When heteroplasmy for either 8993 mutation is greater than 90%, a fatal infancy
disorder called Leigh syndrome (LS) occurs.
A moderately benign form of LS, called familial bilateral striatal necrosis (FBSN), is
the result of mutations at sites 8851 (MTATP6*FBSN8851C) and 9176
(MTATP6*FBSN9176C) of the ATP6 gene.

29. 5- Kearns–Sayre Syndrome
The common clinical features are progressive external ophthalmoplegia (PEO) and
degeneration of the pigment layer of the retina.
The KSS phenotype includes cardiac conduction abnormalities, ataxia, and, to a
lesser extent, deafness, dementia, and diabetes mellitus.
Onset begins before 20 years of age, and most patients die a few years after
diagnosis.
Analysis of muscle mitochondria DNA of KSS patients has revealed DNA structural
changes including large deletions (>1000bp) and duplications.
About one-third of the cases of KSS are associated with a 4977-bp deletion
(“common” deletion) that has breakpoints in the ATP8 and ND5 genes with the loss
of the intervening structural and tRNA genes
When deleted and/or duplicated mitochondrial genomes are abundant in blood-
forming cells, Pearson syndrome (PS) feature the inability of blood cells to use iron
for hemoglobin synthesis, causing sideroblastic anemia.

31. Nuclear-Encoded Mitochondrial
Disorders
Each nuclear-encoded mitochondrial protein usually has a sequence of 15 to 30 amino
acids (target sequence) at its N-terminus.
The target sequence binds to a receptor on the surface of the mitochondrial outer
membrane.
Mitochondrial receptors facilitate the entry of proteins through the outer membrane
into the inte rmembrane space or through a contact site into the matrix.
the nuclear-encoded proteins transport molecules between the intermembrane space
and the matrix, metabolize substrates, produce ATP, uptake of iron, control
mitochondrial DNA replication, and maintain the structural integrity of mitochondrial
DNA.
Mutations of nuclear genes that impair mitochondrial functions are generally rare, the
catalog of such defects is increasing.

32. Mitochondrial Protein Importation Defects
Pyruvate dehydrogenase complex (PDHC) is a mitochondrial matrix multienzyme
complex with pyruvate decarboxylase (E1), dihydrolipoyl transacetylase (E2), and
dihydrolipoyl dehydrogenase (E3) activities.
The E1 component of PDHC is a tetramer with two E1 α subunits and two E1β
subunits
The PHDA1 gene encodes the E1 α subunit and is located at Xp22.1.
Mutational analysis of one patient with low levels of pyruvate dehydrogenase activity
revealed a base substitution in both alleles of the PHDA1 gene that it prevented the
importation of E1α into the mitochondrial matrix.
Ornithine aminotransferase (OAT) is a monomeric mitochondrial matrix enzyme
that catalyzes the conversion of ornithine to glutamic g-semialdehyde and vice versa.
A deficiency in OAT activity is associated with blindness in young adults as the
result of degeneration of the choroid and retina.

33. Substrate Transport Defects
The oxidation of fatty acids in mitochondria is important
for energy production, especially in heart and skeletal
muscle during exercise
Genetic defects have been identified in a number of the
genes that encode the fatty acid oxidation genes.
The phenotypes for these mutations are similar and include
abnormalities of the liver and both heart and skeletal
muscles.
Multisystem abnormalities are evident and death occurs
within days or a few months after birth.

34. Substrate Utilization Defects
Mutations of the CPT2 gene at chromosome 1p32 that encodes carnitine
palmitoyl transferase produce two phenotypes. Muscle weakness and
soreness after strenuous activity or fasting are common features of the
adult-onset form.
The infant-onset form, which is severe and usually fatal, affects the liver,
heart muscles, and skeletal muscles. In both of these cases, oxidation of
long-chain fatty acids by CPTII, which is bound to the inner
mitochondrial membrane,
is impaired.
Mutations of genes that encode enzymes of the Krebs cycle are very rare.
However, a deficiency of mitochondrial fumarase (fumarate hydratase,
FH) causes infant death with many of the symptoms of a mitochondrial
encephalomyopathy.This condition is inherited as an autosomal recessive
trait and is the result of mutations in the FH gene located at chromosome
1q42.1.

35. Iron Transport Defect
Friedreich ataxia (FRDA) is a fatal trinucleotide repeat expansion disorder
inherited as an autosomal recessive trait
Clinical features include progressive failure of limb movement and, to a
lesser extent, blindness, deafness, and diabetes mellitus.
Studies of the FRDA gene product, frataxin, in mice and the frataxin
homolog in yeast localized this protein to the inner mitochondrial
membrane or the mitochondrial matrix.
Biochemical examination of some FRDA patients showed that the uptake of
iron-sulfur (Fe-S) clusters that form part of the electron transport function
of OXPHOS complexes I, II, and III and aconitase, which is a Krebs ycle
enzyme, is not properly regulated when the mutant form of frataxin is
present.
excess iron accumulates in the mitochondria, which either directly or
indirectly disrupts mitochondrial functions.

36. Mitochondrial DNA Defects
Multiple mitochondrial DNA deletions have also been found
in a rare autosomal recessive disorder called mitochondrial
neurogastrointestina encephalomyopathy (MNGIE) that has
PEO, other physical abnormalities indicative of a
mitochondrial disorder, and OXPHOS deficiency as features
of a multisystem phenotype.
The loss of all of the DNA of the mitochondria of specific
tissues is a major feature of a group of rare, fatal autosomal
recessive disorders that have been designated as
mitochondrial DNA-depletion syndromes (MDSs).

38. POSSIBLE SYMPTOMS of MD
• Brain
* Developmental delays
* Mental retardation
* Dementia
* Seizures
* Neuro-psychiatric disturbances
* Migraines
* Atypical Cerebral Palsy
* Strokes
* Autistic featues

39. Eyes & Ears
* Visual loss/blindness
* Ptosis ( droopy eyelids)
* Ophthalmoplegia
* Optic Atrophy
* Hearing Loss/deafness
* Acquired strabismus (squint)
* Retinitis pigmentosa
Muscles
• Weakness
• Muscle pain
• Pseudo-obstruction
• Irritable Bowel Syndrome
• Cramping
• Diarrhea or Constipation
• Dysmotility
• Gastroesophogeal reflux
• Hypotonia
•Gastrointestinal problems

40. mtDNA
Mutation
Clinical disease process
A8344G MERRF, MELAS, Leigh’s syndrome, Multiple
Symmetric Lipomatosis, Parkinson with Neuropathy
and Myopathy
T8356C MERRF, MELAS
G8363A MERRF, Cardiomyopathy
G8342A PEO with Myoclonus
A8296G Deafness and Diabetes
G8313A MELAS, MNGIE
A2343G MELAS, Cardiomyopathy, PEO, Diabetes,
Rhabdomyolysis
C3256T MERRF with diabetes/optic atrophy/retinopathy,
Diabetes, SIDS
T3250C MELAS, riboflavin sensitive myopathy
T5824C MNGIE, PEO, MELAS, myopathy

43. Thank you