Mitochondrial DNA and Type II Diabetes Mellitus
Introduction
1.1 Evolution of Mitochondrial DNA
In 1966, Lynn Margulis wrote a paper entitled “On the Origin of Mitosing Cells”. In her paper, Margulis came up with an influential theory that has been shaping modern science ever since. Although it was hard for her to publish it in magazines, she was determined that the similarity between mitochondria and bacteria was far more meaningful than sheer coincidence.That theory is now famously known as the Endosymbiosis theory. Endosymbiosis explains the origin of eukaryotes. It is suggested that the first step in the origin of eukaryotes from prokaryotes was related to survival in the new oxygen-containing atmosphere: an aerobic prokaryotic microbe (i.e. the protomitochondrion) was ingested into the cytoplasm of a heterotrophic anaerobe. This endosymbiosis became obligate and resulted in the evolution of the first aerobic amitotic amoeboid organisms. By hypothesis, some of these amoeboids ingested certain motile prokaryotes. Eventually these, too, became symbiotic in their hosts. The association of the motile prokaryote with the amoeboid formed primitive amoeboflagellates. In these heterotrophic amoeboflagellates classical mitosis evolved. The evolution of mitosis, insuring an even distribution of large amounts of nucleic acid (i.e. host chromosomes containing host genes) at each cell division, must have taken millions of years. It most likely occurred after the transition to the oxidizing atmosphere, since all eukaryotic organisms contain mitochondria and are fundamentally aerobic. [1]
When the amoeboids ingested the prokaryotes, they also took in the genetic material of the prokaryotes. This genetic material is termed as mitochondrial DNA. A few years after the publication of the paper, scientists were able to study and sequence genomes of different species. In fact, the year 2014 saw more than a thousand new mitochondrial genome sequences deposited in GenBank—an almost 15% increase from the previous year. Hundreds of peer-reviewed articles accompanied these genomes, making mitochondrial DNAs (mtDNAs) the most sequenced and reported type of eukaryotic chromosome. [2]
1.2 The Human Mitochondrial DNA
The human mitochondrial DNA was sequenced and the different genes and their products-RNAs and tRNAs- are under constant study. The human mitochondrial DNA (mtDNA) is a double-stranded, circular molecule of 16 569 bp and contains 37 genes coding for two rRNAs, 22 tRNAs and 13 polypeptides. The mtDNA-encoded polypeptides are all subunits of enzyme complexes of the oxidative phosphorylation system. [3]
The sequence shows extreme economy in that the genes have none or only a few noncoding bases between them, and in many cases the termination codons are not coded in the DNA but are created post-transcriptionally by polyadenylation of the mRNAs.[4]
1.3 Type II Diabetes Mellitus
Diabetes, type 2: One of the two major types of diabetes, the type in which the beta cells of the pancreas produce insulin but the body is unable to use it effectively because the cells of the body are resistant to the action of insulin. Although this type of diabetes may not carry the same risk of death from ketoacidosis, it otherwise involves many of the same risks of complications as does type 1 diabetes (in which there is a lack of insulin).[5] Type 2 diabetes used to be known as adult-onset diabetes, but today more children are being diagnosed with the disorder, probably due to the rise in childhood obesity. There's no cure for type 2 diabetes, but losing weight, eating well and exercising can help manage the disease. If diet and exercise aren't enough to manage your blood sugar well, you may also need diabetes medications or insulin therapy.[6]
The Human Mitochondrial DNA and Type II Diabetes Mellitus
Multiple studies show that mutations on the different sections of the mitochondrial DNA could be reasons for the development of type 2 diabetes. The following are a few of those many studies.
1. A gradual development of pancreatic β-cell dysfunction upon aging, rather than insulin resistance, is the main mechanism in developing glucose intolerance. Carriers of the A3243G mutation show during a hyperglycemic clamp at 10 mmol/l glucose a marked reduction in first- and second-phase insulin secretion compared with noncarriers. The molecular mechanism by which the A3243G mutation affects insulin secretion may involve an attenuation of cytosolic ADP/ATP levels leading to a resetting of the glucose sensor in the pancreatic β-cell, such as in maturity-onset diabetes of the young (MODY)-2 patients with mutations in glucokinase. Hepatic glucose production may be another factor that becomes deregulated by the A3243G mutation. A mitochondrial dysfunction in muscle is expected to lead to a higher lactate flux to the liver, fueling gluconeogenesis. At this time, no data are available on hepatic glucose production and its suppression by insulin in carriers of the A3243G mutation.[7]
2. Variants in mitochondrial DNA (mtDNA) could be associated with type 2 diabetes because ATP plays a critical role in the production and release of insulin. Diabetes can be precipitated both by mtDNA mutations and by exposure to mitochondrial poisons. The risk of inheriting diabetes from an affected mother is greater than that from an affected father, but this is not explained by maternally inherited diabetes and/or deafness (MIDD) caused by the 3243G : C mtDNA point mutation, which accounts for less than 0.5% of cases of diabetes. A common mtDNA variant (the 16189 variant) is positively correlated with blood fasting insulin, but there are no definitive studies demonstrating that it is associated with diabetes. [8]
Section 2 Microbiome
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Conclusion
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References
- ↑ [Lynn Sagan, On the origin of mitosing cells, Journal of Theoretical Biology, Volume 14, Issue 3,1967,Pages 225-IN6,ISSN 0022-5193,https://doi.org/10.1016/0022-5193(67)90079-3.(http://www.sciencedirect.com/science/article/pii/0022519367900793)
- ↑ Smith DR. The past, present and future of mitochondrial genomics: have we sequenced enough mtDNAs?. Brief Funct Genomics. 2016;15(1):47–54. doi:10.1093/bfgp/elv027
- ↑ Jan-Willem Taanman, The mitochondrial genome: structure, transcription, translation and replication, Biochimica et Biophysica Acta (BBA) - Bioenergetics, Volume 1410, Issue 2,1999, Pages 103-123, ISSN 0005-2728, https://doi.org/10.1016/S0005-2728(98)00161-3.
- ↑ Anderson, S., Bankier, A., Barrell, B. et al. Sequence and organization of the human mitochondrial genome. Nature 290, 457–465 (1981) doi:10.1038/290457a0
- ↑ Jr. WCS. Definition of Diabetes, type 2. MedicineNet. https://www.medicinenet.com/script/main/art.asp?articlekey=2976. Published January 25, 2017.
- ↑ Type 2 diabetes. Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/type-2-diabetes/symptoms-causes/syc-20351193. Published January 9, 2019.
- ↑ Maassen, J. Antonie, et al. “Mitochondrial Diabetes.” Diabetes, American Diabetes Association, 1 Feb. 2004, https://diabetes.diabetesjournals.org/content/53/suppl_1/S103.short.
- ↑ Poulton, Joanna, Luan, et al. Type 2 diabetes is associated with a common mitochondrial variant: evidence from a population-based case–control study. OUP Academic. https://academic.oup.com/hmg/article/11/13/1581/744945. Published June 15, 2002.
Edited by [Beimnet Beyene Kassaye], student of Joan Slonczewski for BIOL 116 Information in Living Systems, 2019, Kenyon College.
