4. mitochondrial diseases

Question 1

number of mitochondria in diff cells

Answer 1

highest n: oocytes (100000)

high n: heart, brain, muscles

very low n: sperms (none enter into the fertilised oocyte)

no mitochondria: RBCs

Question 2

describe mtDNA

Answer 2

-double stranded
-contains 37 genes: 13 for polypeptides involved in oxidative phosphorylation, 24 for functional RNAs (22tRNA and 2rRNA)
-contains a heavy (H) strand and a light (L) strand (depending on content of GC bases)
-does NOT contain introns
-derive from bacteria (endosymbiotic theory)

Question 3

relationship between nuclear and mitochondrial DNA

Answer 3

SYMBIOTIC: cellular homeostasis is under the control of BOTH

-13pp for ox. hosphorylation coded for by mtDNA and the other 80 units by nuclear DNA
-mutations in BOTH genomes can cause mitochondrial disease, they just have diff types of inheritence patterns
-mitochondrial DNA has higher mutation rate than nuclear bcos of free radical formation in respiration,+ the fact that it doesnt have repair mechanisms

Question 4

inheritance pattern of mitochondrial disorders

Answer 4

MATERNAL: all mitochondria in oocyte are contributions from the mother, bcos mitochondria of sperm are all in the tail, but only head of sperm goes towards fertilisation

Question 5

what are the features of mtDNA (5)

Answer 5

1. polyplasmy
2. replicative segregation
3. homoplasmy/ heteroplasmy
4. threshold effect
5. maternal transmission

Question 6

polyplasmy def

Answer 6

Each cell contains hundreds or thousands of mitochondria and each mitochondrion harbors multiple copies of mtDNA (av. 5-10)

Question 7

relative segregation def

Answer 7

-segregation in mitochondrial DNA is not tightly controlled bcos there is an absence of spindles
-Hence the sorting of mtDNA among newly synthesised mitochondria is RANDOM (hence giving rise to homoplasmy and heteroplasmy)

Question 8

homoplasmy and heteroplasmy def

Answer 8

HOMO: mitochondria have all identical copies of mtDNA (very rare bcos there is a very low prob that in replicative segragation, copies will randomly be shared 100% equally)

HETERO: a stochastic distribution of mutant and wild-type mtDNAs

Question 9

threshold effect

Answer 9

in heteroplasmic conditions, the % of mutant (vs wild type) mtDNA copies can determine whether the individual can manifest a disorder or not

If the level of mutant mitochondria exceeds a certain threshold value, the cell will express the dysfunction

Question 10

mitochondrial bottleneck scenario meaning

Answer 10

explains the fact that an unaffected mother can transmit a mitochondrial disease and produce an affected child + explain reduced penetrance and variable expression:

1. mother in heteroplasmic state doesnt reach threshold value of mutated mitochondria hence doesnt present phenotype
2. during the formation of primary oocyte only a SELECT number of mtDNA are transferred into each oocyte
3. Rapid replication of mitochondria during oocyte maturation to amplify copies significantly for embryo development
4. In the case that there were more MUTATED mitochondria transferred to the primary oocyte, the amplification can increase their % past threshold value, and offspring has disease

Question 11

what tissues are usually involved in mutations in mtDNA

Answer 11

those of high aerobic energy demand (nervous sytem, eyes, heart) bcos of free radical production + damage

Question 12

age of onset for mitochondrial disease

Answer 12

MITOCHONDRIAL MUTATIONS: high incidence at pubertal development (6-25) bcos of increase in energy consumption, but can occur at any age

NUCLEAR GENOME MUTATIONS (in 80 pp involved in mitochondrial metabolism): usually manifestation starts at a young age + congenital disorders in newborns

Question 13

how are mit disorders diagnosed

Answer 13

-some individuals are able to undergo DNA testing (when the clinical presentation is characteristic of mitochondrial dysfunction and hence physician can predict it is a mt disorder)

OTHERWISE:
-fmaily history/ tree analysis
-lactic acid conc measurement (degree of anaerobic resp)
-muscle biopsy for histo analysis
-mtDNA molecular analysis to locate a specific mutation

Question 14

types of mit mutations

Answer 14

1. class 1: nuclear gene disorders
2. class 2: point mutations (deletion /insertion) of mtDNA

Question 15

class 1 mutation disorders

Answer 15

NUCLEAR MUTATIONS:
1. oxidative phosphorylation disorders manifesting early (leigh syndrome)

2. Disorders with secondary effects on oxidative phosphorylation (eg Friedreich ataxia)
3. defects in nuclear proteins for mit integrity (progressive opthalmoplegia)

Question 16

class 2 mutation disorders

Answer 16

1. point mutations in tRNA/rRNA genes which affect proteins produced (translation)
2. large deletions causing rearrangements

eg -> myoclonic epilepsy with ragged-red fibers (MERRF) affecting muscles and NS, where red ragged fibers are found histologically as a cause of lactic acidemia

Question 17

treatment of mt disorders

Answer 17

Baby created by the couple + a female donor so that mt disorder isnt passed on

method 1 = pronuclear transfer (mit replacement therapy): donor egg mitochondria is used instead of mother’s mutant ones.

method 2 = maternal spindle transfer: nucleus removed from mother’s oocyte and is placed inside donor egg (that has alr had nucleus extracted) + egg fertilised by father