MITOCHONDRIAL DNAMUZAFFAR KHAN ALAM KHAN MD.,

GROUP-3 STUDENT OF TSMU

OBJECTIVES:

WHAT IS MITOCHONDRIAL GENOME?

CHARACTERISTICS.

NUCLEAR DNA VS MITOCHONDRIAL DNA.

MATERNAL INHERITANCE

INTERACTIONS BETWEEN MITOCHONDRIAL AND NUCLEAR GENOMES

MUTATIONS IN MTDNA AND DISEASE

MITOCHONDRIAL GENETIC BOTTLENECK

MITOCHONDRIAL GENOME:

This genome consists of a circular chromosome,16.5kb in size that is located inside the mitochondrial organelle,not in the nucleus.

Most cells contain at least 1000 mtDNA molecules distributed among hundreds of individual mitochondria.

Not all the RNA and protein synthesized in a cell are encoded in the DNA of the nucleus.

It contains 37 genes, and encodes 2 types of rRNA and 22 tRNAs

Genes encode 13 proteins that are subunits of enzymes of oxidative phosphorylation

The remaining 74 polypeptides of the oxidative phosphorylation complex are encoded by the nuclear genome.

NUCLEAR DNA VS MITOCHONDRIAL DNA

0%

20%

40%

60%

80%

100%

1%

99%

DNA

Nuclear DNA

found in nucleus of the cell

2 sets of 23 chromosomes

maternal and paternal

can "discriminate between individuals of the same maternal lineage“

double helix

bounded by a nuclear envelope

DNA packed into chromatin

Mitochondrial DNA

found in mitochondria of the cell

each mitochondria may have several copies of the single mtDNA molecule

maternal only

cannot "discriminate between individuals of the same maternal lineage“

Circular

free of a nuclear envelope

DNA is not packed into chromatin

Nuclear DNA vs. Mitochondrial DNA

CHARACTERISTICS

Is inherited exclusively from the mother!

mtDNA is a circular shape single chromosome

It is only 16 kb in length - contains 16,600 bp

Codes for 37 genes

Contains 22 tRNA and 2 rRNA coding genes

Encodes 13 proteins that are subunits of oxidative phosphorilation

Contains only exons, no introns

Has no reparation system – high mutation rate especially in D-loop!

No crossing over

Replicative segregation, homoplasmy & heteroplasmy

Myoclonic epilepsy with ragged-red fibers

More than 100 different rearrangements and 100 different point mutations have been identified in mtDNA that can cause human disease,often involving the CNS and musculoskeletal system

The diseases that result from these mutations show distinctive pattern of inheritance because of 3 unusual features of mitochondria.

1.Replicative Segregation

2.Homoplasmy and Heteroplasmy

3.Maternal Inheritance

Replicative segregation

At cell division, multiple copies of mtDNA in each of the mitochondria in a cell replicate and sort randomly among newly synthesized mitochondria.

The mitochondria in turn are distributed randomly between the 2 daughter cells. This is called as replicative segregation.

The first unique feature of mitochondria is the absence of tightly controlled segregation seen during mitosis and meiosis of the 46 nuclear chromosomes.

Homoplasmy and Heterolasmy

One daughter cell may by chance receive mitochondria that contain only a pure population of normal mtDNA or a pure population of mutant mtDNA(Homoplasmy)

The daughter cell may receive a mixture of mitochondria some with and some without mutation(Heteroplasmy)

Maternal Inheritance of mtDNA

Sperm mitochondria are generally eliminated from the embryo so that mtDNA is inherited from the mother.

All children of a female who is homoplasmic for a mtDNA mutation will inherit the mutation

None of the offspring of a male carrying the same mutation will inherit the defective DNA

Maternal inheritance of a homoplasmic mtDNAmutation causing Leber Hereditary optic neuropathy is known.

Features of Maternal Inheritance

1.Number of mtDNA molecules within the developing oocyte is reduced before being amplified to the huge total seen in mature oocytes.This restriction and subsequent amplification of mtDNA during oogenesis is termed Mitochondrial genetic Bottleneck.

2.Variablity in the percentage of mutant mtDNAmolecules seen in the offspring of a mother with heteroplasmy for a mtDNA mutation arises from the sampling of only a subset of mtDNAs during oogenesis.

Both the nuclear and mitochondrial genomes produce polypeptides of oxidative phosphorylation, thus the phenotypes associated with mutations in the nuclear genes are often indistinguishable from those due to mtDNA mutations

The nuclear genome encodes approximately 200 factors required for the maintenance and expression of mtDNA or for the assembly of proteins, involved in oxidative phosphorylation

Interactions between mitochondrial and nuclear genomes

This is why often mtDNA is referred to as the “slave” of the nuclear DNA, because mtDNA depends on many nuclear genome-encoded proteins for its replication and the maintenance of its integrity.

Thus, diseases of oxidative phosphorylation arise not only from mutations in the mitochondrial genome but also from mutations in nuclear genes that encode oxidative phosphorylation components.

Mutations in many of these nuclear genes can lead to disorders with phenotype similar to that of the mtDNA diseases.

The mitochondrial genome has a very high mutation rate, 10- to 17-fold higher than that observed in nuclear DNA. Although mtDNA repair systems do exist and , they are not sufficient to counteract the oxidative damage sustained by the mitochondrial genome.

Protective histones are also lacking.

Oxygenation process is high percentage.

The mtDNA mutation rate can be increased by environmental agents or by mutation of nuclear genes involved in mtDNA maintenance

Why is mitochondrial mutation so high?

About 1 in 4,000 children in the US develop mitochondrial disease by the age of 10 years

mtDNA genome mutates 10 times more frequently than does nuclear DNA

More than 100 different rearrangements and about 100 different point mutations that are disease-causing have been identified in mtDNA.

The clinical phenotype resulting from mtDNA mutations is diverse, however the diseases of central-nervous or muscular-skeletal systems are most common.

Pleiotropy and variable expressivity is common in different affected family members (due to heteroplasmy).

Pleiotropy – multiple phenotipic effects of a single allele or pair of alleles.

Mutations in mtDNA and disease

It has been identified in mtDNA:

(1) missense mutations in the coding regions of genes that alter the activity of an oxidative phosphorylation protein;

(2) point mutations in tRNA or rRNA genes that impair mitochondrial protein synthesis;

(3) Deletions or duplications of the mtDNA molecule. They are generally somatic in origin, although a small proportion is inherited, in some diseases.

THREE TYPES OF MUTATIONS

All the children of a female who is homoplasmic for a mtDNA mutation will inherit the mutation, whereas none of the offspring of a male carrying the same mutation will inherit the defective DNA.

Mitochondrial Genetic Bottleneck

The number of mtDNA molecules within developing oocytes is reduced before being subsequently amplified to the huge total seen in mature oocytes.

This restriction and subsequent amplification of mtDNA during oogenesis is termed the mitochondrial genetic bottleneck.

Thus, mothers with a high proportion of mutant mtDNA molecules are more likely to produce eggs with a higher proportion of mutant mtDNA and therefore are more likely to have clinically affected offspring than are mothers with a lower proportion.

Also, for reasons unknown, deleted mtDNA molecules are generally not transmitted from clinically affected mothers to their children.

MERRF (Myoclonic Epilepsy with Ragged Red Fibres)

MELAS (Myopathy, Epilepsy, Lactic acidosis, Stroke-like episodes)

LHON (Leber’s Hereditary Optic atrophy)

Kearn-Sayre (eye problems, heart block, ataxia and loss of coordination)

Leigh syndrome (rare severe brain disease in infancy, also heart problems)

Some diseases associated with mtDNA

References

Ibrahim Okumus and Y. Çiftci / Turk. Turkish Journal of Fisheries and Aquatic Sciences 3: 51-79 (2003)

ARIAGNA LARA,* JOSE´ LUIS PONCE DE, Molecular Ecology Resources (2010) 10,421–430

Vallone, P.M., Just, R.S., Coble, M.D., Butler, J.M., Parsons, T.J. (2004) A multiplex allele-specific primer extension assay for forensically informative SNPs distributed throughout themitochondrial genome. Int. J. Legal Med., 118: 147-157. [Protocol for 11plex SNP assaydeveloped at NIST] [Genotyper macro for mtSNP 11plex]

Coble, M.D., Just, R.S., O'Callaghan, J.E., Letmanyi, I.H., Peterson, C.T., Irwin, J.A.,Parsons, T.J. (2004) Single nucleotide polymorphisms over the entire mtDNA genome thatincrease the power of forensic testing in Caucasians. Int. J. Legal Med., 118: 137-146.

Coble, M.D. (2004) The identification of single nucleotide polymorphisms in the entiremitochondrial genome to increase the forensic discrimination of common HV1/HV2 types inthe Caucasian population. PhD dissertation, George Washington University, 206 pp.

www.google.co.in/mtdna/wikipedia/in