Mitochondrial DNA and Non-Mendelian Inheritance Carolyn K. Suzuki, Ph.D. Dept of Biochemistry

1. Mitochondrial DNA and Non-Mendelian Inheritance Carolyn K. Suzuki, Ph.D. Dept of Biochemistry

2. Essential functions of mitochondria Biosynthesis of amino acids nucleotides steroid hormones heme Oxidative phosphorylation Oxidation of fatty acids Apoptotic cell death

4. Proteins localized in mitochondria are: • encoded by two genomes- • nuclear DNA (nDNA) • mitochondrial DNA (mtDNA) • produced by two different protein synthesis machineries • cytosolic • mitochondrial • majority of mitochondrial proteins are encoded by nuclear genes, which are synthesized in cytosol and post-translationally imported into mitochondria. • 13 mitochondrial proteins are encoded by mtDNA and synthesized in the mitochondrial matrix.

5. D-loop non-coding region control region Organization of the mitochondrial genome tRNAs D-loop: displacement loop HSP and LSP: heavy- and light- strand promoters for transcription OH: origin of replication

6. Characteristics of animal mtDNAs: Circular Small in size ~16 kb in man 5-10 copies of mtDNA / mitochondrion ~1,000 mitochondria / cell ~1% of cellular DNA Encode: 13 proteins large and small rRNA tRNAs NO INTRONS- polycistronic mRNAs Mitochondrial genetic code has different genetic code as compared to that in nucleus UGA = tryptophan not STOP AGA = STOP not arginine AUA = methionine not isoleucine

7. Mitochondrial genome encodes proteins of the oxidative phosphorylation pathway * subunits encoded by mtDNA

8. Uncoupling proteins (UCPs) H+ transport into the matrix respiration (electron transport) is uncoupled from ATP synthesis chemical energy released as heat

10. siblings

11. Random segregation of mitochondria and mtDNA contributed to fertilized egg

13. Threshold effect Different tissues have different energy needs and thus, a different tolerance for mtDNA mutations For example, if 70% of mtDNA is mutated in various tissues is mitochondrial and cellular dysfunction observed in all cases? Not necessarily. TissueEvidence of disease Liver asymptomatic Heart dysfunction Brain dysfunction Muscle dysfunction Threshold sensitivity is also affected by nuclear genetics, environment, age.

14. Side reaction electron transport is electron transfer to oxygen generating oxygen radicals O2- O2- O2-

15. Mitochondrial electron transport chain- generates reactive oxygen species (ROS) OUT IN e- e- e- e- e- e- e- e- superoxide hydroxyl radical .OH MnSOD H2O2 H2O2 H2O2 + Cyt C Cyt C Cyt C Fe2+ + hydrogen peroxide e- e- O2 O2 O2 O2 . . O- O- ADP ATP Q H2O H2O H+ H+ H+H+H+ H+ H+H+ H+H+ OH- H+H+ H+H+ 2OH- NAD+ H+`H+ H+` NADH Krebs Cycle Animated by Peter Rabinovitch Background after Mandavilli et al, Mutation Research 509:121 (2002)

16. OUT IN Krebs Cycle mtDNA ROS can damage DNA, proteins and lipids hydroxyl radicals are highly reactive leading to damage of protein, lipids and DNA Mandavilli et al, Mutation Research 509:127 (2002)

17. reactive oxygen species reactive oxygen species

18. Mitochondrial DNA and aging

19. Mitochondrial haplotypes associated with longevity Masashi Tanaka and colleagues • Accumulation of somatic mtDNA mutations is proposed to be a major contributor to aging and degenerative diseases. MtDNA deletions and point mutations are detected during an individuals lifetime. • Different mtDNA haplotypes are linked to differences in longevity. • Study of Japanese centagenarians identified Mt5187C A resulting in a Leu to Met substitution the Complex I subunit protein ND2 subunit.

20. The mechanism underlying increased longevity of individuals with the Mt5187C A is not known. Tanaka speculates that the introduced methionine may function as an antioxidant by efficiently scavenging oxygen radicals as proposed by Levine et al. PNAS 93:15036 (1996).

21. Mitochondrial DNA mutations directly linked to human disease

22. Mitochondrial DNA mutations and aging mtDNA replicative errors versus accumulated damage

23. mtDNA REPLICATION Synthesis of RNA primer Mitochondrial RNA polymerase- homology to bacteriophage RNA polymerases single subunit TFAM- transcription factor activator of mitochondria TFB1M and TFB2M- mitochondrial transcription factor Newly synthesized RNA remains hybridized to mtDNA RNA primer is cleaved to provide 3’OH RNase MRP (RNase mitochondrial RNA processing) mtDNA replication requires: POLG- mtDNA polymerase g- consists of a and b subunits a- catalytic subunit b- accessory subunit, primer recognition and processivity Polymerizing activity and 3’ to 5’ exonuclease activity. High fidelity (1 error for every 500,000 bases), proofreading capability. Reverse transcriptase activity mtSSB- mtDNA single stranded-DNA binding protein Twinkle- mtDNA helicase, homology to bacteriophage T7 helicase

24. Twinkle OH HSP and LSP- heavy and light strand promoters mtRNA pol- mitochondrial RNA polymerase TFAM and TFBM- transcription factors mtSSB- mitochondrial single-strand DNA binding protein Twinkle- DNA helicase OH- origin of heavy strand replication

25. RNase MRP- RNase mitochondrial RNA processing mtRNA pol- mitochondrial RNA polymerase TFAM and TFBM- transcription factors

26. OH

27. One theory of aging accumulation of mtDNA mutations and mitochondrial dysfunction

28. Transgenic mice expressing a mutant mtDNA polymerase (POLG- encoded by a nuclear gene) accumulate mtDNA mutations and exhibit premature aging and reduced lifespan

29. Production of homozygous knock-in mice- mtDNA mutator mice expressing a variant of mtDNA polymerase a subunit chromosome-encoded catalytic subunit of mtDNA polymerase lacking 3'-5' exonuclease activity lacking proof-reading activity purified recombinant polymerase has: reduced exonuclease activity no decrease in DNA polymerase activity

30. Mice express a proof-reading mutant of the mtDNA polymerase (POLG) catalytic subunit A • Transgenic mice exhibit • - 3-5-fold increase in somatic mtDNA point mutations • weight loss • reduced subcutaneous fat • alopecia- hair loss • kyphosis- curvature of the spine • osteoporosis • anemia, (20% lower than wild-type) • reduced fertility • heart enlargement • reduced lifespan • Demonstrates a causal link between increased somatic • mtDNA mutations and aging.