Clinical-Pathological Conference

Saba Ahmad, MDAssistant Professor of Clinical Pediatrics

Division of NeurologyJuly 26, 2013

Final Diagnosis?

A Mitochondrial Disorder

Caused by a mtDNA deletion detected by Southern Blot performed on the muscle biopsy

Is it MELAS?Yes. But it doesn’t really matter.

Extremely heterogeneous group of disorders characterized by failure of mitochondria to meet the energy needs of a cell

What are Mitochondrial Disorders?

Organelles found in most cells

Known as the cellular power plants

Have their own DNA and replicate independently within the cell

Inherited only from mothers

Mitochondria

Dominant role is the production of ATP by oxidative phosphorylation

Also involved in cell signaling, Calcium homeostasis, apoptosis, its own replication

Mitochondrial function

Making Energy

Krebs's (TCA) Cycle

Electron Transport Chain

In order to do their jobs, mitochondria need about 1500 proteins

BUTmtDNA only has 37

genes.

mtDNA

mtDNA encodes for a total of 13 peptides of the ETC found in complexes I, III, IV, and V (the other 67 are nuclear)

2 mtDNA genes encode rRNA, and 22 encode tRNA

mtNA continued

The vast majority of genes involved in mitochondrial function are actually nuclear chromosomal DNA◦ Proteins are synthesized in the cell nucleus and

imported into the organelle A mitochondrial organelle can contain 2-10

copies of its mtDNA There is this thing called heteroplasmy

More about mtDNA

What is Heteroplasmy?

Mosaicism

Analogous to chromosomal mosaicism where nuclear DNA within an individual can vary amongst various tissues

The presence of different mtDNA populations within a cell: At conception, you get a bunch of mitochondria from mom, and they randomly separate as cells divide in in the embryo

Heteroplasmy

A cell can have some mitochondria that have a mutation in the mtDNA and some that do not.

Homoplasmy refers to a cell that has a uniform collection of mtDNA: either completely normal mtDNA or completely mutant mtDNA.

A unique feature of mtDNA is that, at cell division, the mitochondria sort randomly among daughter cells. Therefore, in cells where heteroplasmy is present, each daughter cell may receive different proportions of mitochondria carrying normal and mutant mtDNA.

To put it another way

This explains why mothers can carry a low mutation load (have a low level of heteroplasmy) and be asymptomatic, and have much more severely affected children-because when eggs are formed, they get random distributions of mitochondrial populations

ALSO, Different tissues have varying degrees of

sensitivity to mutation loads, related to energy requirements: CNS, myocardium, skeletal muscle, retina, kidneys very commonly involved in mitochondrial disease

Heteroplasmy

Lets go back to embryology and

Pretend we have a zygote undergoing replication….

Heteroplasmy explains why the same mtDNA mutation may cause LOTS of different phenotypes

Nuclear genes are responsible for a large portion of mitochondrial function: replication, ATP synthesis pathways

Normal mitochondria can accumulate mutations over time due to high replication rate, lack of error checking, mitochondrial toxins (mutations in mtDNA occur at a rate of 10-100x that of nuclear DNA)

Maternally inherited mitochondrial mutations which are generally heteroplasmic at birth, undergo a loss of heteroplasmy over time from cell replication

What makes mitochondria dysfunctional?

Nervous system: ataxia, dystonia, chorea, athetosis, myoclonus, leukodystrophy, seizures, infantile spasms, cerebral atrophy, myopathy, neuropathy, stroke, deafness, headache/migraine, developmental delay, regression, dementia, tremor, bradykinesia, retinopathy, optic atrophy, ophthalmoplegia

Constitutional: exercise intolerance, growth failure, microcephaly, cachexia, lactic acidosis, hyperalaninemia, SIDS

GI: nausea, vomiting, dysmotility, pseudoobstruction, constipation, diarrhea, pancreatitis, liver failure, dysphagia, abdominal pain

Endocrine: diabetes, hypothyroidism, hypoparathyroidism, ovarian failure, Addison’s, hypopituitarism

Renal: FSGS, renal cysts, nephrotic syndrome, RTA

Cardiac: cardiomyopathy, conductions defects (WPW), bundle branch block, CAD/atherosclerosis, sudden death

Psychiatric: psychosis, mood disorders, autism spectrum disorders

Neoplastic: lymphomas, renal cell carcinomas, leiomyomatosis, pheochromocytomas

And much much more…

What can happen when mitochondria are not working (a short list)

She had mtDNA testing for the mtDNA point mutations known to cause MELAS and MERRF in 2004. It was negative.

She was found to have mtDNA deletion. The extent of the deletion, genes involved, or her percentage of heteroplasmy is unknown.

Now back to our patient: Is it MELAS?

Based on her clinical presentation she had 1-encephalopathy (manifested by seizures

and regression)2-lactic acidosis3-strokes

(she also had RTA, failure to thrive, pancreatic dysfunction, myopathy)

Why is it MELAS?

Mitochondrial disorders are not single gene-single phenotype disorders.

It is useful to recognize that there is a constellation of features that would indicate a possible mitochondrial disorder, but the specific clinical syndrome is becoming increasingly less useful to identify the underlying genetic problem

Why doesn’t it matter?

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

Genetic abnormalities associated with Leigh’s syndrome

Most proteins involved in mitochondrial functioning are actually nuclear in origin

They undergo more typical inheritance patterns: AD, AR, X-linked

The presentation tends to be earlier in life, often catastrophic, due to the fact that all cells are typically equally affected (though tissues might be unequally affected due to energy needs)

Nuclear genes responsible for mitochondrial function

When you have some combination of Lactic acidosis Myopathy Developmental regression Failure to thrive Unexplained cardiomyopathy Retinopathy/eye movement problems Sensorineural hearing loss

Common reasons to start thinking about a Mitochondrial disorder

When its really weirdWhen its really badWhen you can’t find another explanation

And in some cases….

Work up the patient in a stepwise manner: amino/organic acids (elevated alanine?), lactate/pyruvate, acylcanitine profile, ammonia. Consider storage diseases, VLCFA’s

MRI/MRS for radiographic characteristics (does it look like Leigh’s? MELAS?), lactate peak

LP for cells, protein, amino acids, lactate/pyruvate. neurotransmitters (movement disorders, progressive epilepsy)

Consider muscle biopsy to measure ETC chain enzyme activity, look for ragged red fibers

Consider whole mitochondrial genome sequencing, deletion/duplication analysis

As we know, the clinical phenotype can only take you so far.

Mitochondrial diseases are typically progressive (either slowly or catastrophically)

There is no cure for these disorders. EXTREMELY limited evidence that

supplements like CoEnzyme Q10 and carnitine may slow progression of the disease (but no randomized trials)

Natural History

Studies to see if mitochondrial dysfunction is implicated in a host of other diseases: Alzheimer’s, Parkinson’s, atherosclerotic disease, autism

Hot area of research:It may not be just for kids anymore

Thank you!Questions?