Title of Thesis

Prevalence of Various Types Of Mitochondrial DNA Mutation In Type 2 Diabetes Mellitus In Pakistan

Author(s)

Maryam Sharif

Abstract
Two basic abnormalities have been observed in the development of T2DM which include impaired synthesis and release of insulin secretion by β cells of pancreas and decreased insulin sensitivity. For the last 10 – 20 years, with more advancement in the studies and techniques, basic concepts about exact mechanism involved in the pathogenesis of T2DM have been modified, but the progression in this domain has been facing many difficulties. In spite of a lot of hard work the basic fundamental molecular events are still to be explored completely. Studies have shown that the individuals carrying diabetogenic mitochondrial DNA (mtDNA) mutations have decreased insulin response and impaired glucose tolerance. It was proposed that ATP generating mitochondrial oxidative phosphorylation system, in the pancreatic β cells, plays a pivotal role in the synthesis and release of insulin in reaction to elevated blood glucose level. Mitochondria contain their own DNA which is extrachromosomal and distinguishable from the nuclear genomic DNA. Total mitochondrial DNA content accounts for up to 0.5% of the total genomic DNA in a nucleated somatic cell. Mitochondrial DNA (mtDNA) has been described to have only "Maternal Inheritance". Therefore, if there is any mutation in the maternal mtDNA, it will be transmitted to all of her siblings. But if father has mutant mtDNA, it is neither transmitted nor influences his children. Human mitochondrial DNA comprises of 37 genes, 13 of which code for polypeptides forming part of the OXPHOS system. Whereas, the other 24 genes code for 22 transfer ribonucleic acids (tRNAs) and 2 ribosomal ribonucleic acids (rRNAs), 12S rRNA and 16S rRNA. Thirteen messenger ribonucleic acids (mRNAs) are also produced.
Mitochondrial DNA has 10 times higher chance to develop a mutation as compared to nuclear genome. MtDNA mutations accumulate sequentially through maternal lineage and can be detected in almost every gene of the mitochondrial DNA. MtDNA mutations are linked with a variety of diseases, ranging from rare muscular syndromes to common disorders like diabetes mellitus and Alzheimer’s disease. These pathogenic mtDNA mutations disrupt the OXPHOS system affecting the energy supply to the cells. The energy supply deficit leads to the development of a disease state. Beta cells of pancreas require more energy and hence are liable to be affected more due to any disruption in OXPHOS system. MtDNA mutations are linked with diabetes mellitus as these mutations lead to defective release of insulin from the beta cells. However, insulin sensitivity is normal. Diabetes mellitus is a common and the predominant hallmark associated with various mitochondrial diseases. The most common diabetogenic heteroplasmic point mutation in mtDNA tRNA gene is A3243G. It is considered to affect transcription and translation of mtDNA encoded tRNALeu(UUR). It was found to be a major cause of maternally inherited diabetes accompanied with sensorineural hearing defect – a new subtype of diabetes mellitus which was given the name of “Maternally Inherited Diabetes and Deafness (MIDD)”. A3243G point mutation in the tRNALeu(UUR) gene is considered to be strongly linked with the pathogenesis of MIDD. Many other mtDNA mutations are capable to make the human beings more susceptible to develop diabetes. Approximately twenty (20) mtDNA mutations have been detected and found to be associated with maternally inherited diabetes mellitus like homoplasmic mutations i.e. G1888A, T4216G, A4917G and T14709C. Out of these twenty mutations, A3243G mutation is constantly identified in 0.1–1.5 % of the diabetics Moreover, for some reason, the cells in the cochlear portion of the ear are also found to be more susceptible to energy deficiency. Hence, any disruption in ATP production can also affect the normal hearing power of the patients with mitochondrial diabetes. This leads to the development of “Sensory-neural deafness” along with diabetes. Hence, due to the combination of these two defects, this disease got its present name of MIDD, “Maternally Inherited Diabetes and Deafness”. MtDNA A3243G mutation leads to an overall decrease in the tRNALeu(UUR), defective aminoacylation and absence of proper post translational modification of tRNA and proteins encoded by mtDNA.
In Pakistan, no information and study was available to evaluate the types of mitochondrial DNA mutation seen more frequently in population affected with type 2 diabetes mellitus. This study was conducted to ascertain the prevalence of the A3243G substitution in a mitochondrial tRNALeu(UUR) gene in type 2 diabetes mellitus. We could not identify any A-to- G mutation at position 3243 of mitochondrial leucine tRNA gene in the patients with maternally inherited mitochondrial diabetes phenotype as well as first degree relatives of these diabetics. In conclusion, the A3243G mutation in mitochondrial tRNALeu(UUR) gene was not found to be a frequent cause of T2DM in Pakistani population.
Unfortunately, the heteroplasmy of the mutation is the lowest in the peripheral blood leukocytes and the highest in the affected tissues. In our study, peripheral blood leukocytes were used to isolate total and then mitochondrial genome. So, the chance to detect this mutation was lower in leukocytes and it might have hampered the detection of this mutation. Moreover, ~0.7% decline in the heteroplasmy levels in leukocytes is seen per year. Anyhow we believe that the results of our study in collaboration with the earlier studies can cater a guideline for further research. More precise techniques can be developed to identify novel mutations as well as to analyze post mitotic tissues.

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