mitochondrial genetics

Question 1

heteroplasmy

Answer 1

• results in variable penetrance and variable expressivity in mitochondrial disorders
• only some mitochondria that are passed on carry disease-causing mutation (WT and mutant)

Question 2

complex V deficiency

Answer 2

not clinically testable in US

Question 3

failure in mitochondrial formation

Answer 3

only caused by nuclear DNA mutation

Question 4

complex II

Answer 4

only comprised of nuclear subunits

Question 5

mitDNA

Answer 5

-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

Question 6

LHON c.11778G>A

Answer 6

protective vision mutation in certain ethnic groups

Question 7

homoplasmy

Answer 7

can be 100% WT or mutant mitochondrial line presence

Question 8

threshold effect

Answer 8

specific heteroplasmy load that can be tolerated in a tissue for a specific mtDNA mutation before showing signs of pathology

Question 9

mit dz recurrence risk

Answer 9

• 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

Question 10

high lactic acid

Answer 10

• 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

Question 11

plasma and CSF AAs

Answer 11

• high alanine

- high glycine, tyrosine, proline or sarcosine

Question 12

urine OAs

Answer 12

• TCA intermediates
• ethyl malonate
• 3-methyl-glutaconate
• dicarboxylic acids

Question 13

plasma acylcarnitines

Answer 13

• low free levels
• elevated bound:free ratio
• elevations suggestive of FAODs

Question 14

Leigh syndrome genetics

Answer 14

-can be caused by mitochondrial and nuclear gene mutations; HIGH mtDNA mutation load
-involvement of brainstem, basal ganglia and cerebellum
+subacute necrotization

Question 15

Brain MRI

Answer 15

• symmetric sign abnormality in deep gray matter
• transient v progressive
• cerebellar atrophy

Question 16

brain MRS

Answer 16

-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

Question 17

tissue based skeletal muscle analysis

Answer 17

• 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

Question 18

ETC activity via biochemical tissue analysis

Answer 18

• 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

Question 19

CoQ10 determination via biochemical tissue analysis

Answer 19

• 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

Question 20

mtDNA CNV and genetic analysis via biochemical tissue analysis

Answer 20

assesses for potential depletion

- <20% is considered clinical cutoff for depletion

Question 21

liver biopsy analysis

Answer 21

• ETC deficiency

- mtDNA CNV and genetic sequencing

Question 22

skin biopsy analysis

Answer 22

• ETC deficiency
• mtDNA and gene sequencing
• fibroblast cell line establishment (indefinitely storable)

Question 23

skeletal muscle analysis

Answer 23

• morphology and staining
• electron microscopy
• ETC deficiency
• CoQ10 determination
• mtDNA CNV analysis and gene sequencing

Question 24

molecular testing

Answer 24

-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

Question 25

LHON presentation

Answer 25

-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

Question 26

LHON management

Answer 26

- cardiac eval - neuro eval - optho eval - avoidance of mitochondrial toxins

Question 27

MELAS presentation

Answer 27

-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

Question 28

Leigh syndrome phenotype

Answer 28

- psychomotor retardation and regression - develop feeding problems with FTT, hypotonia, ataxia, seizures, respiratory failure, hearing and vision loss with or without nystagmus

Question 29

MERRF

Answer 29

- 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

Question 30

NARP

Answer 30

- ATPase 6 mutations (complex V) - adult onset, often non-specific lactic acidosis related - peripheral neuropathy, cerebellar ataxia, pigmentary neuropathy

Question 31

KSS

Answer 31

-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

Question 32

Pearson syndrome

Answer 32

-mtDNA deletion that presents and is fatal in infancy +mutation detectable in blood -causes anemia, exocrine pancreas dysfunction

Question 33

Progressive external ophthalmoplegia (PEO)

Answer 33

- mtDNA deletion detectable in muscle with onset in adulthood - ptosis and inability to move eyes laterally

Question 34

POLG mutations

Answer 34

-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)

Question 35

suspecting mitochondrial mutation

Answer 35

- 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

Question 36

management

Answer 36

-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

Question 37

(Mitochondrial NeuroGastroIntestinal Encephalomyopathy) MNGIE

Answer 37

- fatal AR condition - characterized by ptosis with ophthalmoplegia, GI dysmotility, cachexia, peripheral neuropathy, leukoencephalopathy and mitochondrial abnormalities

Question 38

TK2 disease

Answer 38

-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