|Keywords (Extracted from title, table of contents and
abstract of thesis)|
Prevalence, Mitochondrial, DNA,
Mutation, Diabetes, Mellitus, mechanism, pathogenesis, Pakistan,
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.