mitochondrial genetics Flashcards
heteroplasmy
- results in variable penetrance and variable expressivity in mitochondrial disorders
- only some mitochondria that are passed on carry disease-causing mutation (WT and mutant)
complex V deficiency
not clinically testable in US
failure in mitochondrial formation
only caused by nuclear DNA mutation
complex II
only comprised of nuclear subunits
mitDNA
-comprised of 37 genes
+13 proteins
+22 tRNAs
+2 RNAs-6S and 12S
-no introns, no recombination
-continuous replication with high mutation rate relative to nuclear DNA and no histone regulation
-disease causing mutations occur in a tissue-specific fashion
LHON c.11778G>A
protective vision mutation in certain ethnic groups
homoplasmy
can be 100% WT or mutant mitochondrial line presence
threshold effect
specific heteroplasmy load that can be tolerated in a tissue for a specific mtDNA mutation before showing signs of pathology
mit dz recurrence risk
- 1-4% if mother shows no signs or symptoms
- up to 50% if mom is symptomatic
- testing recommended in parents and siblings of a proband to reveal level of heteroplasmy and manage them
high lactic acid
- common reason for mitochondrial referral
- not sensitive or specific though
- complications: using a tournicate or having a child fight the blood stick can spike the levels, handling also tricky
plasma and CSF AAs
- high alanine
- high glycine, tyrosine, proline or sarcosine
urine OAs
- TCA intermediates
- ethyl malonate
- 3-methyl-glutaconate
- dicarboxylic acids
plasma acylcarnitines
- low free levels
- elevated bound:free ratio
- elevations suggestive of FAODs
Leigh syndrome genetics
-can be caused by mitochondrial and nuclear gene mutations; HIGH mtDNA mutation load
-involvement of brainstem, basal ganglia and cerebellum
+subacute necrotization
Brain MRI
- symmetric sign abnormality in deep gray matter
- transient v progressive
- cerebellar atrophy
brain MRS
-provides chemical picture of brain
+not always specific
-shows decreased L-NAA, which is synthesized in mitochondria
-see lactate doublet in affected individuals related to increased lactic acid and a switch to anaerobic respiration
-can study different parts of brain individually
tissue based skeletal muscle analysis
- not as common anymore, though sometimes still useful
- morphology can show increased proliferation, subsarcolemmal accumulation, abnormal RRFs and Cox-deficient fibers
- electron microscopy identified increased count and structural anomalies
ETC activity via biochemical tissue analysis
- reported relative to citrate synthase
- important to obtain the standard muscle type in biopsy
- can show isolated or multiple enzyme defects
- effects may be secondary to other conditions
CoQ10 determination via biochemical tissue analysis
- primary deficiency is rare, whereas secondary deficiency common in mit dz
- may help adjust need for changes to therapy and dosage for patient or insurance-coverage
mtDNA CNV and genetic analysis via biochemical tissue analysis
assesses for potential depletion
- <20% is considered clinical cutoff for depletion
liver biopsy analysis
- ETC deficiency
- mtDNA CNV and genetic sequencing
skin biopsy analysis
- ETC deficiency
- mtDNA and gene sequencing
- fibroblast cell line establishment (indefinitely storable)
skeletal muscle analysis
- morphology and staining
- electron microscopy
- ETC deficiency
- CoQ10 determination
- mtDNA CNV analysis and gene sequencing
molecular testing
-mostly done by NGS
+often labs can report heteroplasmy as low as 1.2%
+older testing methods not as accurate
-tissue specific testing can also be important for identifying targeted heteroplasmy and mutation burden
-WES may be helpful because it includes del/dup analysis and avoids multiple panels being run, but still can’t diagnose all cases
LHON presentation
-caused by mutations in MT-ND1, MT-ND4 and 4-L, and MT-ND6
+causes degeneration of the retinal ganglia and optic nerve, disc and vascular changes
+3 common mutations, mostly homoplasmy in blood
-development of painless, bilateral, subacute vision failure
+begins with central vision loss that usually starts unilaterally and progresses
-can also see arrhythmia and neurologic involvement (tremors, peripheral neuropathy, myopathy, MS-like features)
*males more likely to have vision loss than females
LHON management
- cardiac eval
- neuro eval
- optho eval
- avoidance of mitochondrial toxins
MELAS presentation
-80% caused by MT-TL1 mutation m.3243A>G (leucine tRNA)
-normal early development with seizures, headaches, exercise intolerance with vomiting and proximal limb weakness starting between 2-10y or delayed between 10-40y
+sz and stroke-episodes usually only present before 40y and can lead to hemiparesis, cortical blindness, progressive cognitive and motor impairment
-can also see dementia, HL, myopathy, short stature
-can see elevated LA+or-RRF
Leigh syndrome phenotype
- psychomotor retardation and regression
- develop feeding problems with FTT, hypotonia, ataxia, seizures, respiratory failure, hearing and vision loss with or without nystagmus
MERRF
- tRNA lysine mutations
- seen with ataxia and myoclonus and abnormal muscle stains
- can also develop peripheral neuropathy, optic atrophy, hearing loss
- short stature and lipomas common
NARP
- ATPase 6 mutations (complex V)
- adult onset, often non-specific lactic acidosis related
- peripheral neuropathy, cerebellar ataxia, pigmentary neuropathy
KSS
-mtDNA deletion that presents before 20yo
+sometimes only detectable in CNS and muscle
+mostly de novo
-causes ptosis, progressive external ophthalmoplegia and pigmentary retinopathy
+one of the following also part of diagnosis: CSF protein >100mg/dL, conduction heart block, or cerebellar ataxia
-hormone imbalance may also occur (i.e. DM, hypoparathyroidism), as can short stature, hearing loss, dementia, limb weakness
Pearson syndrome
-mtDNA deletion that presents and is fatal in infancy
+mutation detectable in blood
-causes anemia, exocrine pancreas dysfunction
Progressive external ophthalmoplegia (PEO)
- mtDNA deletion detectable in muscle with onset in adulthood
- ptosis and inability to move eyes laterally
POLG mutations
-spectral disorder ranging from severe infantile to adult onset
-gene is the only mtDNA pol that functions in replication and repair
-150+ mutations
+exonuclease activity loss causes PEO
+linker region loss causes ataxia, vasiculations
+replication activity loss leads to severe Alpers phenotype (sz, cortical blindness, dementia, liver dz)
suspecting mitochondrial mutation
- more likely to be AR nuclear mutation if infantile onset with lactic acidosis
- more likely to be mtDNA mutations if later onset-mutations must buildup
management
-no treatment, but treat symptoms
+no fasting, no febrile periods
+vaccinate, avoid illness, avoid toxic drugs (valproate, statins, etc)
+may give vitamin cocktails and antioxidants
-guidelines with anesthesia due to recovery difficulty
-arginine or citrulline therapy for stroke
(Mitochondrial NeuroGastroIntestinal Encephalomyopathy) MNGIE
- fatal AR condition
- characterized by ptosis with ophthalmoplegia, GI dysmotility, cachexia, peripheral neuropathy, leukoencephalopathy and mitochondrial abnormalities
TK2 disease
-mt thymine kinase deficiency leads to early onset and average death within 2.5y
-causes progressive skeletal and respiratory muscle failure
+decreased movement, gait abnormalities, tremor
-see elevated CK and lactic acid levels
