Mitochondrial Diseases Flashcards
mitochondrial proteome
encompasses ~1130 proteins
- majority encoded by nuclear DNA and imported into mitochondria following their cytosolic translation
- 13 polypeptides encoded by mitochondrial DNA
mitochondrial genome
- 5-10 copies of mtDNA per mitochondrion
- thousands of copied of mtDNA per cell
- required for translfating the transcripts of mitochondria encoded polypeptides
- completely maternally inherited
- > 200 different rearrangements and missense variants in mtDNA causing human disease
size of mitochondrial genome
16 569 bp
structure of mitochondrial genome
double stranded closed circle
encodes 37 genes
- 13 subunits of enzymes involved in ox phosphorylation
- two rRNA and 22 tRNA genes
> necessary RNA components for intra-mitochondrial protein synthesis
OH and OL are the original heavy- and light-strand mtDNA replication
replicative segregation
each cell mitochondria replicate and sort randomly
mitochondria are distributed randomly between two daughter cells
significant variability in manifestations of mitochondrial disorders among different tissues and/or patients
homoplasmy and heteroplasmy
random distribution of mutant and normal mtDNA into daughter cells and organelles
different proportion of wild-type and mutant mitochondrial genomes
mixture of mutant and wild-type mtDNA in organelles called heteroplasmy
if pure normal or pure mutated mtDNA = homoplasmy
mitochondrial genome bottleneck
- number of mtDNA molecules within developing oocytes is reduced before being amplified to the huge total seen in mature oocytes
- the restriction and subsequent amplification of mtDNA during oogenesis = bottleneck
- mothers with high proportion of mutant mtDNA = more likely to produce eggs with higher proportion of mutant mtDNA = more likely affected offspring
T or F. Primary mitochondrial diseases can affect any organ and any system
T! but mostly bain and muscles
or usually multi-organ affected
which organs are affected the most with primary mitochondrial diseases
high energy organs = brain and muscles
pattern of inheritance if multiple family members affected
mitochondrial inheritance
autosomal recessive inheritance
autosomal dominant
X-linked inheritance
this is helpful in muscle biopsies but cannot differentiate between primary and secondary mt disorders
histochemical stains
functions of the mitochondria
ATP synthesis via ox phosphorylation system
contribution to regulation of phospholipid biosynthesis,
reactive O2 species (ROS) signalling, Ca2+ handling, iron-sulfur cluster homeostasis
serv as signalling platforms for several pathways:
- innate immune response
- programmed cell death through apoptosis
- cell differentiation
mitochondrial DNA synthesis
- continuous event
- not regulated by cell cycle
- require balanced supply of intra-mitochondrial nucleotides imported from cytosol via transporters
- balanced quantities of enzymes through mitochondrial fission and fusion
- all proteins required for synthesis encoded by nuclear genes
- pathogenic variants in those nuclar genes impair mtDNA synthesis
> quant defects = mtDNA depletion
> qual defects = mtDNA deletions
mitochondrial DNA maintenance
- essential to functioning of mitochondria
- meeting energy needs of all cells
- requires proteins essential for mtDNA synthesis, maintenance of mitochondrial nucleotide pool, mediating mitochondrial fusion
cell and tissue dysfunction results when the fraction of mitochondria carrying a mutation exceeds a _______ level
threshold
replicative segregation of a heteroplasmic mitochondrial mutation
T or F. Females heteroplasmic for missense variants or duplications will pass those to all of their children
T! BUT severity can vary in their children
- depends on fraction of mutant variant in their mother
- due to random chance operating small numbers of mitochondria per cell at oocyte bottleneck
T or F. Heteroplasmic deletions, duplications and missense variants are heritable
F! deletions are not heritable
what are primary mitochondrial diseases due to?
nuclear or mitochondrial genome variants
inheritance patterns of primary mitochindrial diseases
mitochondrial
aut recessive (most common)
autosomal dom
maternal
X-linked
sporadic
nuclear DNA genes
more than 1000 (compared to 37 mtDNA genes)
if only one family member is affected or simplex case
aut dom (de novo or decreased penetrance)
aut recessive, X-linked, maternally inherited
acquired (non-genetic)
childhood onset of encephalomyopathy and adults with PEO or KSS represent these
simplex cases
multiple affected family members
mitochondrial inheritance
aut recessive inheritance
aut dom inheritance
X-linked inheritance
molecular genetic investigations for mitochondrial diseases
- mit disorders multigene panels for specific phenotypes/neuroimaging feats
- exome sequencing
- genome sequencing
- mit genome sequencing
> mtDNA pathogenic variants are heteroplasmic, proportion of mutated mtDNA may be undetectable in blood
> use skeletal muscle, urinary epithelium, etc.
if no specific features in mit disorders
exome sequencing
some common mtDNA pathogenic variants (eg. large-scale deletions cause CPEO) may only be detected here
skeletal muscles
supportive investigations for mitochondrial diseases
plasma/CSF lactate/ pyruvate measurements
cardiac assessments
neuroimaging (brain CT/MRI)
magnetic resonance spectroscopy
tissue biopsy (muscle, skin, liver) to investigate ragged fibers, ox phosphorylation enzyme activity measurements, muscle histopathology/chemistry, muscle electron microscopy
neurophysiological studies (EEG, EMG, NCV)
how can we histologically asses mitochondrial function?
H&E
modified Gomori trichrome = ragged red fibres (RRF) due to abnormal subsarcolemmal proliferation of mitochondria
muscle fiber diameter (hypertrophic fibers)
presence of any abnormal inclusions or central nuclei
lacking histocytochemical cytochrome c oxidase (COX) activity = COX deficient fibers
T or F. Histochemistry for muscle biopsy needs to be requested
F! It’s routine
T or F. Histochemistry can distinguish between 1ry and 2ry mt disorders
F!
why is prenatal molecular genetic testing and interpretation for mtDNA disorders difficult?
mtDNA heteroplasmy
- percentage level of mutated mtDNA in a chorionic villus sampling biopsy may not reflect percentage level of mutated mtDNA in other fetal tissues
- percentage level of mutated mtDNA may change during development and throughout life
successful prenatal molecular diagnosis has been carried out for these pathogenic variants using DNA extracted from fetal cells via amniocentesis /CVS
m.8993T>G
m.8993T>C
- more ven tissue distribution
- percentage level of these 2 variants does not appear to change significantly overtime
- 100% homoplasmy
SMD vs PMD
SMD can be inherited or acquired
PMD (primary) = only inherited
chronic progressive ophthalmoplegia
- ptosis, external opthalmoplegia
- single large-scale mtDNA deletion (1.1-10kb)
- m.3243A>G
- at least 35 distinct mtDNA missense variants
- at least 12 nuclear DNA pathogenic variants
- one phenotype and multiple genotypes
T or F. Chronic progressive ophthalmoplegia is mostly seen in adults
T!
Pt cant move their eyes in the direction they are instructed to
chronic progressive ophthalmoplegia
Kearns-Sayre syndrome phenotype
progressive extenral ophthalmopelgia, onset <20 yrs
pigmentary retinopathy
one of:
- CSF protein >1g/L
- cerebellar ataxia
- <3 block
Kearns-Sayre syndrome other features
bilateral deafness
myopath
dysphagia
DM
hypoparathyroidism
dementia
T or F. Kearns-Sayre syndrome is a multisystem disorder
T!
Most common inheritance of Kearns-Sayre syndrome
de novo
Leigh syndrome
- subacute necrotizing encephalomyelopathy
- onset 3-12 mos (after viral infection)
- decompensation = elevated lactate in blood and/or CSF
- psychomotor retardation/regression
- hypotonia (low muscle tone), spasticity, movement disorders like chorea, cerebella ataxia, peripheral neuropathy
- extra-neurologic manifestations = hypertrophic cardiomyopathy
- ~50% die by 3 y/o
basal ganglia is affected in this syndrome
LEigh syndrome
MELAS
mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes
MELAS onset
2-40 y/o
MELAS phenotypes
stroke-like epis
encephalopathy with seizures/dementia
muscle weakness, exercise intolerance
recurrent headaches
recurrent vomiting
hearing impairment
peripheral neuropathy
learning disability
short stature
biochemical features of MELAS
lactic acidemia
ragged red fibres in muscle biopsies
genotypes for MELAS
m.3243A>G in MT-TL1
- 80%
MT-ND5 <10%
Leber hereditary optic neuropathy phenotype
- young adults = bilateral, painless subacute visual failure
- 90% lose vision before 50 y/o
- males = 4-5x more likely to be affected
- some individuals, usually F, develop MS-like illness
onset of LHON
Leber…
2nd to 3rd decade of life
genotypes for LHON
- m.3460G>A in MTND1
- m.11778G>A in MTND4 (more in Asians)
- m.14484T>C in MT-ND6 (founder effect; French Canadian)
MERFF phenotype
myoclonic epilepsy with ragged red fibers
- myoclonus (often first symptom)
- generalized epilepsy
- ataxia
- weakness
- exercise intolerance
- dementia
- onset childhood to adulthood
genotype MERFF
MT-TK (90%) variant:
- m.8344A>G
- m.8356T>C
- m.8363G>A
- m.8361G>A
NARP phenotype
neurogenic muscle weakness, ataxia, retinitis pigmentosa
- proximal neurogenic muscle weakness with sensory neuropathy
- ataxia
- seizures
- pigmentary retinopathy
- onset in early childhood
- episodic deterioration, often in association with viral illness
genotypes of NARP
> 50% have two variants in MT-ATP6
- m.8993T>G MOST COMMON
- m.8993T>C
list of primary mitochondrial diseases associated with nuclear genome pathogenic variants
POLG
SUCLA2
nuclear encoded Leigh syndrome spectrum
Sengers
Primary coenzyme Q10 deficiency
onset of POLG related mitochondrial diseases
infancy to late adulthood
T or F. POLG = one genotype multiple phenotypes for one genotype
T
one of most severe phenotypes for POLG
AHS = Alpers-Huttenlocher syndrome
- childhood onset
- progressive severe encephalopathy with intractable epilepsy and hepatic failure
childhood myocerebrohepatopathy spectrum (MCHS)
- POLG
- first few months to 3 y/o onset
- developmental delay or dementia
- lactic acidosis
- myopathy with failure to thrive
MEMSA
myoclonic epilepsy myopathy sensory ataxia
POLG
without opthalmoplegia
ataxia neuropathy spectrum
ANS
- 90% individuals have ataxia and neuropathy
- 2/3 seizures
- 1/2 ophthalmoplegia
- clinical myopathy rare
POLG
encodes DNA polymerase y subunit 1
- 15q26.1
- essential in replicaton of mtDNA
- biallelic pathogenic variants result in POLG-related primary mitochondrial diseases
inheritance pattern for POLG
aut rec
mitochondrial POLG enzyme structure
homotetramer
- 140 kD catalytic subunit = POLG1
- 55 kD accessory subunit = POLG2
SUCLA2-related mtDNA depletion syndrome
encephalomyopathic dorm with methylmalonic aciduria
onset in infancy
median survival up to 2nd decade
phenotypic features of SUCLA
developmental delay
hypotonia
dystonia
muscular atrophy
sensorineural hearing impairment
growth failure
feeding difficulties
multisystem
location of SUCLA2
13q14.2
SUCLA2 encodes…
beta subunit of succinul-CoA ligase
- mitochondrial tricarboxylic acid cycle enzyme (KREBS)
SUCL composed of…
- alpha subunit = SUCLG1
- beta subunit = CULG2
how is SUCL important for mtDNA synthesis ?
- forms complex with mitochondrial nucleoside diphosphate kinase
- mt nucleoside diphosphate kinase is involved in synthesis of mt nucleotides
mt nucleotides used to synthesize mtDNA
nuclear encoded Leigh syndrome spectrum
- decompensation = levated lactate levels during intercurrent illness
- developmental delay/regression
- transient or prolonged stabilization or even improvement
- eventual progressive neurologic decline
onset of nuclear encoded Leigh syndrome spectrum
3-12 mos
- 50% die by age three
T or F. nuclear encoded Leigh syndrome spectrum = one phenotype and multiple genotypes
T
a mtDNA maintenance defect (mt depletion syndrome)
Sengers syndrome
Sengers onset
neonatal period
Sengers phenotypic features
hypotonia
hypertrophic cardiomyopathy
cataracts
mutation for Sengers
biallelic pathogenic variants in AGK
- encodes acylglycerol kinase
- chr 7q34
a lipid component of mt resp chain, classified as mt resp disorder
primary coenzyme Q10 def
- reduction of CoQ10 levels in tissues or cultured cells associated
lipid soluble component of mt inner membrane
coenzyme Q10
this is critical for e- transport in the mt resp chain
coenzyme Q10
this carries electrons from complex I and II to complex III
coenzyme Q10, participates in ATP production
phenotypes of coenzyme Q10 deficiency
- neurologic = encephalopathy, progressive cerebellar atrophy, ataxia with intellectual disability, seizures, peripheral neuropathy
- renal = steroid resistant nephrotic syndrome, proteinuria, slowly progressive end-stage kidney disease
- ophthalmologic = retinopathy, optc atrophy
- muscle = weakness, exercise intolerance
- sensorineural hearing loss
onset of coenzyme Q10 deficiency
birth to seventh decade
multisystem disorders =
primary mt disorders
variant interpretation requires this
biochemical investigations
