Abstract

Diabetes mellitus is a chronic debilitating non-communicable disease prevalent throughout the world. There are two different types of diabetes; the type 1 diabetes usually presents in children and young adults, and the type 2 diabetes, a most frequent age-related condition usually noted among the adults aged over 40 years. The type 1 diabetes results due to an immunological reaction against insulin and the insulin secreting cells. The type 2 diabetes can occur due to various factors that include genetic predisposition, lifestyle disorders, insulin resistance, and lack of adequate insulin production. Since lifestyle management is an adjustable risk factor for diabetes, may people with genetic predisposition could delay the onset of clinical diabetes. Further there is an increasing need to understand the genetics behind the signaling pathways involved in the development of type 2 diabetes, which could pave the way for formulating, and implementing therapeutic, and preventive strategies.

1. Introduction

Type 2 diabetes mellitus (T2DM) is a major public health problem worldwide. The incidence of this disease increases with age, physical inactivity, unhealthy diet, obesity, genetic predisposition, impaired glucose tolerance; and the prevalence rates of diabetes is increasing in children ^{1, 2}. There are many genes contributing to this disease, out of which 36 genes account for 10% of overall genetic component of the disease ³. Previous studies indicate that there could be a 3.5-fold risk of acquiring the disease if only one of the parent is diabetic, and a 6-fold increased risk if both parents are diabetic. Thus, genetic factors appear to contribute up to 30-70% cases of T2DM ^{4, 5}. The inheritance pattern of T2DM is polygenic and heterogenous, hence multiple genes are involved, and different combination of genes play a role in development of T2DM.

2. The Signaling Pathways Involved in the Development of Diabetes

There are two approaches available for genetic factor analysis which include the candidate gene analysis and the linkage analysis. The candidate gene analysis examines a specific gene with a possible role of it in the disease progression. The major disadvantage of this approach is that it assumes a gene involvement in disease progression before testing. This approach helped in identification of numerous genes, but only fraction of them have been further confirmed in follow-up studies ^{7, 8, 9, 10, 11}. The linkage analysis identifies various genes through their genomic position, and hence this type of gene analysis helps in monogenic disorders (Maturity Onset Diabetes of Young (MODY), neonatal diabetes and rare forms of childhood obesity) ^{12, 13, 14}. The new approach based on single nucleotide polymorphism for genetic studies was found successful to detect genetic variations with modest phenotypic effects ¹⁵.

3. Wnt Signaling Pathways (Wingless-related Integration Site)

These are a group of signal transduction pathways made of proteins that pass signals from outside of a cell through cell surface receptors into the cell. These proteins attach to the receptors on plasma membrane and activate different Wnt pathways via paracrine (cell–cell communication) and autocrine (same–cell communication) mode of communications ^{16, 17}. This pathway plays an important role in cell proliferation, migration, cell fate specifications, and body axis patterning ¹⁸.

Wnt proteins are lipid modified glycoproteins which perform lipid modifications predominantly by palmitolyation (addition of palmitic acid) of cysteine residues ¹⁹. There are three Wnt signaling pathways which include the Canonical Wnt pathway, Non-canonical planar cell polarity pathway, and the Non-canonical Wnt/Calcium pathway.

All the three Wnt pathways are activated by binding of Wnt protein ligand to a Frizzled family receptor, which passes the signal to Disheveled inside the cell. The clinical importance of this pathway lies in the fact that mutations may be responsible for a variety of diseases like breast and prostate cancer, glioblastoma (a type of brain cancer), and T2DM. The Wnt Canonical pathway acts via beta Catenin and regulates transcription of gene ^{20, 21}. Non-canonical planar cell polarity pathway doesn’t involve beta catenin, and LRP (Lipoprotein Receptor related protein) receptors. The Non-canonical Wnt/Calcium pathway also does not involve beta Catenin and helps in regulation of intra cellular calcium levels ²². The flow charts depicting the canonical Wnt pathway, Non-canonical planar cell polarity pathway, and the Non-canonical Wnt/Calcium pathway are shown in Figure 1, Figure 2, and Figure 3 respectively.

Canonical Wnt pathway increases the sensitivity of the cell to insulin, especially Wnt 10b protein increases the insulin sensitivity in skeletal muscle cells. This pathway increases the cells insulin sensitivity by increasing glucose uptake by the cells mediated by enhanced glucose transporters ²³. Over expression of Wnt 5b increases the susceptibility to type 2 diabetes because of its role in adipogenesis. Wnt signaling pathway also activates mitochondrial biogenesis, which increases the production of ROS (Reactive Oxygen Species), damages DNA, and leads to acute hepatic insulin resistance or injury-induced insulin resistance ^{24, 25}. Mutation in LRP 5/6 leads to development of diabetes and obesity. Beta catenin / TCF, mutation in one of transcription factor regulated by Wnt signaling. TCF7L2 also increases the susceptibility to T2DM ²⁶. The canonical Wnt pathway is attenuated due to competition between Fork head box transcription factor subgroup O (FOXO) protein and T cell transcription factor (TCF) protein because of ageing together with T2DM ²⁷.

This signaling is important in embryonic development including body axis patterning, cell fate specification, cell proliferation, and cell migration. The mutation of this pathway may lead to breast, prostate cancer, glioblastoma, and T2DM ²⁸.

TCF7L2 is a gene which codes for a protein acting as a transcriptional factor ²⁹. It is a member of Wnt signaling pathway, and the stimulation of this pathway leads to association of beta catenin with B cell Lymphoma-9 (BCL9) gene. It was noted that translocation of this gene to nucleus could activate target gene and represses proglucagon synthesis in enteroendocrine cells (cells of GIT and pancreas with endocrine function) ^{30, 31}.

The single nucleotide polymorphism (SNP) within the TCF7L2 gene is most significant marker associated with T2DM risk. SNPs in this gene are linked to higher risk of T2DM and gestational diabetes as noted by previous studies ^{32, 33}. Frameshift mutation in this gene may lead to colorectal cancer ^{34, 35}.

It is a nuclear receptor of type II class encoded by PPARG gene. There are mainly two isoforms including the gama1(found in all tissues except muscle) and gamma 2 (present in adipose tissue and intestine) ³⁶. It regulates fatty acid storage and glucose metabolism ³⁷

PPARG is activated on binding with various natural agents like arachidonic acids and arachidonic acid metabolites like 5-hydroxyeicasotetraenoic acid (HETE) and 15-HETE, 5-Oxoeicosatetraenoic acid etc. ^{38, 39}. The activation of PPARG inturn inhibits the growth of cultured human breast, gastric, lung, prostate and other cancer cell lines ⁴⁰. PPARG has a role in pathogenesis of obesity, diabetes, atherosclerosis, and cancer. Thiazolidinediones used in the treatment of diabetes targets PPARG to lower serum glucose without increasing pancreatic insulin secretion.

Fat mass and obesity associated protein (FTO)/alpha-ketoglutarate-dependent dioxygenase (AlkB)

FTO is an enzyme coded by FTO gene and it oxidatively demethylates DNA (deoxy ribonucleic acid). FTO gene is expressed in both fetal and adult tissues ⁴¹. It demethylates methyl adenosine of mRNA, tRNA, rRNA, small nuclear RNA(snRNA) in eukaryotes as well as prokaryotes ^{42, 43, 44}. Thus, FTO helps in processing of mRNA and other nuclear RNAs ⁴⁵.

FTO is indirectly associated with T2DM as it increases the body fat distribution; may lead to metabolic syndrome, and increased fasting insulin ^{46, 47}. FTO gene in a carrier state increases the risk of Alzheimer’s disease ⁴⁸.

Potassium channel, inwardly rectifying subfamily Jmember11(KCNJ11)/BIR/HHF2/PHHI/IKATP/TNDM3/KIR6.2

Potassium channels are present in all mammalian tissues and they carry out various physiological responses. The protein encoded by this gene is an integral membrane protein, which is inward-rectifier type and the function of this protein is to allow potassium to flow into the cell ⁴⁹. Its function is controlled by G-protein associated with sulphonylurea receptor(SUR) ⁵⁰.

Mutation or alteration in this gene may lead to an autosomal recessive disorder with unregulated insulin secretion, persistent hyperinsulinemic hypoglycemia of infancy (PHHI), non-insulin dependent type II diabetes (NIDDM), transient neonatal diabetes, and permanent neonatal diabetes (PNDM) ^{51, 52}.

Neurogenic locus notch homolog protein2 (NOTCH2)

It belongs to type-1 transmembrane protein family ⁵³. The domains of this protein are extracellular and can help in binding to epidermal growth factors (EGF) ⁵⁴. It plays an important role in developmental processes by controlling cell fate decisions and conserves intercellular signaling pathways by regulating interactions between physically adjacent cells ⁵⁵. It functions as a receptor for membrane bound ligands and play a significant role in vascular, renal and hepatic development ⁵⁶.

Mutation in this gene may lead to removal of PEST domain, escapes non-sense mRNA decay and leads to Hajdu-cheney syndrome, a progressive and severe bone disorder ^{57, 58, 59} and genetic variation in NOTCH2 among the people exposed to arsenic increases the susceptibility to T2DM ⁶⁰.

Wolframin (WFS)

It is a transmembrane protein that act as a channel selective for cations ⁶¹, and it plays an important role during stress conditions and beta cell apoptosis ⁶². Mutation in exon8, of this gene leads to Wolframan Syndrome, characterized by insulin dependent diabetes mellitus (IDDM) and bilateral progressive optic dystrophy, psychiatric illness, autosomal dominant deafness (DFNA6), and congenital cataract ^{63, 64}.

Insulin-like growth factor2 mRNA binding protein2 (IGF2BP2)

It is a growth factor binding protein ⁶⁵ which binds to 5’ UTR (Untranslated region) of insulin-like growth factor 2 (IGF2) mRNA and regulates its translation ^{66, 67}. Thus, involved in pancreatic development and stimulates insulin production. Mutation/alteration in this gene could cause diabetes mellitus.

Solute Carrier family 30 (zinc transporter) member 8 (SLC30A8)

It is a zinc transporter involved in insulin storage and secretion ⁶⁸. Certain alleles of this gene (P.Trp325 Arg) increases the risk of T2DM and also elevates glucose and proinsulin levels ^{69, 70, 71}. Thus, the inhibitor of this zinc transporter serves as therapeutic strategy in preventing T2DM ⁷².

Juxtaposed with another zinc finger protein (JAZF1)/TAK1-interacting protein 27(TIP27)/Zinc finger protein 802 (ZNF 802)

This is a Zinc finger protein, which acts as a transcriptional repressor ⁷³. There are different isoforms for this protein. However, variation in any form of this protein increases the risk for prostate cancer and abnormal insulin response ⁷⁴.

Haematopoietically expressed homeobox (HHEX)

This gene codes for a member of transcription factor involved in hematopoietic differentiation ⁷⁵. HHEX acts as transcription promoter or inhibitor ^{76, 77}. It promotes developmental processes of organs like liver, thyroid, and forebrain ⁷⁸. It interacts with SCL (Stem Cell Leukemia) factor, a significant transcription factor for blood and endothelial cell differentiation ⁷⁹. It interacts with promyelocytic leukemia protein, represses VEGFA, inhibits endothelial cell differentiation ⁸⁰. Because of its role in pancreatic development it might influence the insulin release and insulin sensitivity. Mutation in this gene in experimental mice showed defects in forebrain development, heart, vasculature, liver, monocyte and thyroid abnormalities.

Prospero homeobox 1 protein (PROX1)

It is a transcriptional factor that plays a key role in cell fate determination, neurogenesis and embryonic development of liver, pancreas, heart and lymphatic system ⁸¹. It specifically interacts with hepatocyte nuclear factor (HNG alpha), which activates CYP7A1 gene (involved in the bile acid synthesis (cholesterol homeostasis) and phosphoenol pyruvate carboxy kinase (PEPCK) gene (involved in gluconeogenesis) ^{82, 83}. It represses the activity of acid related orphan receptors like ROR and regulates circardian rhythms ⁸⁴. Mutation in this gene or functional deletion of it leads to abnormal cellular proliferation via down regulation of cell cycle inhibitors like p27(kip1) and p57 (kip2) ^{85, 86}. Such changes may lead to hematological carcinomas, biliary carcinomas, obesity and indirectly to T2DM.

Diacyl glycerol kinase beta (DGKB)

This gene encodes diacyl glycerol kinase, a beta enzyme found in neurons which regulates intracellular availability of second messenger, diacylglycerol (DAG) for various cellular activities ⁸⁷. The DAG stimulates protein kinase C (PKC) involved in insulin secretion. High levels of calcium in beta cells of pancreas activates release of DAG and insulin secretion via PKC ⁸⁸. Thus, mutation of this gene has a role to play in development of T2DM.

Potassium voltage gated channel subfamily Q member 1(KCNQ1)

This gene is involved in the regulation of potassium channels. They are involved in the transport of potassium ions and produce membrane potential. These channel proteins are active in the inner ear, cardiac muscle, kidney, intestine and pancreas. Proper ion balance is maintained by these channels for hearing and to maintain regular cardiac rhythms. Mutation in this gene leads to atrial fibrillation, long Q wave syndrome, hearing loss, and may leads to improper insulin secretion from islet cells of pancreas ^{89, 90}.

Cyclin dependent kinase inhibitor 2A / 2B (CDKN2A / 2B)

These are adjacent genes located on chromosome 9, which encodes two protein p16^INK4a and p15^INK4b. P16 inhibits cyclin dependent kinase 4 (cdc4), a regulator of pancreatic beta cell replication and its overexpression leads to pancreatic islet hypoplasia and diabetes ^{91, 92}.

Hepatocyte Nuclear Factor 1 homeobox A (HNF1A/ IDDM20/LFB1/MODY3/TCF1)

This gene encodes for a protein required for the transcription factor expressed in liver. It regulates expression of liver specific genes ⁹³. It helps in glucose homeostasis, renal glucose absorption, insulin secretion and transcription ⁹⁴. A mutation in this gene may lead to maturity onset diabetes of young type 3 (monogenic diabetes), hepatocellular adenoma and carcinoma of ovary ^{95, 96, 98, 99}.

Hepatocyte Nuclear Factor 1B (HNF1B / TCF2/ VHNF1 / MODY5)

This gene encodes for a protein which binds to DNA, and forms a homo or hetero dimer with HNF1A. It has role in nephron development as well as in regulating the development of embryonic pancreas, parathyroid gland, liver, and brain. Thus, mutation in this gene leads to maturity onset of Diabetes (MODY5) ⁹⁹. Cystic renal disease is the prominent features of HNF1B gene mutation. Mutation in this gene is initially associated with hypomagnesemia, early onset diabetes, gout, hyperparathyroidism, elevated liver enzymes, and congenital anomalies of urinary tract ¹⁰⁰.

Gastric Inhibitory Polypeptide Receptor (GIPR/PGQTL2 / Insulinotropic polypeptide receptor)

This gene codes for a protein which is coupled with G protein and acts as a receptor for gastric inhibitory polypeptide (GIP). It was initially identified for its activity in gut for inhibiting gastric acid secretion and gastrin. Later it was known for its activity in insulin secretion in the presence of elevated glucose levels. The oral glucose causes intestinal cells to secrete insulin secretagogues, mainly glucagon-like peptide 1 (GLP1) and gastric inhibitory peptide leading to higher insulin response than intravenous glucose load. Thus, it could be concluded that GIPR gene mediates insulin secretion after oral glucose stimulation or after eating ¹⁰¹. A defect in enteroinsular axis (the relationship between insulin secretion and the entrance of a glucose load into the intestine) may contribute to pathogenesis and progression of diabetes ¹⁰².

Hepatocyte nuclear factor 4 alpha (HNF4A / TCF14 / FRTS4/ MODY1)

It is a nuclear transcription factor which binds to DNA and forms a homodimer and regulates expression of several hepatic genes. HNF4A is also expressed on pancreatic beta cells. It regulates expression of genes required for glucose transport and metabolism, sex hormone binding globulin (has a role in decreasing insulin resistance) and interacts with TCF1 ¹⁰³. This gene also has a role in development of liver, kidney and intestine. Mutation in this gene leads to maturity onset diabetes (MODY1) and abnormal expression of islet cells of pancreas.

Melatonin receptor 1B (MTNR1B / FGQTL2 / MT2)

This gene codes for a membrane protein, and when it is coupled with G protein, forms a receptor for melatonin and is found primarily in the retina and the brain. It participates in light dependent functions of retina and is involved in neurobiological effects of melatonin ¹⁰⁴. It is expressed in human beta islets cells of pancreas ^{105, 106}. Mutation in this gene is associated with increased average blood sugar levels and thus increases the risk for developing type 2 diabetes ¹⁰⁷.

Zinc finger BED Domain containing protein3 (ZBED3/Axin interacting protein)

It is a member of zinc finger domain protein superfamily and this gene interacts with Axin, a core protein of Wnt/beta catenin signaling pathway ¹⁰⁸. It is a novel secretory protein found abundantly in muscle and adipose tissue. Its activity increases in parallel to glucose levels. The serum concentrations of this protein have been found to increase from pre-diabetic to diabetic stage indicating its role in insulin signaling pathway ¹⁰⁹. The mutation of this gene thus may lead to obesity and type 2 diabetes.

Cyclin dependent kinase 5 regulatory subunit associated protein -1like (CDKAL1)

It is a member of methyl thio transferase family and is found in human pancreatic cells, skeletal muscles, and brain. It is involved in methyl thio modification in t-RNA lysine residues of CDK5 protein ¹¹⁰. CDK5 is a serine/threonine protein kinase that regulates insulin secretion from pancreatic cells at high glucose load, protect pancreatic cells from gluco toxicity ¹¹¹. This gene is involved in controlling first phase of insulin exocytosis (in response to meals the rapid secretion of insulin) of beta cells of pancreas by mechanisms other than CDK5 mediated regulation ¹¹². CDKAL1 gene mutation leads to loss of inhibitory effect of CDK5 and increases the risk for type 2 diabetes.

GLIS Family Zinc Finger Protein 3 (GLIS/ZNF515/NDH)

This gene belongs to zinc finger protein family and encodes for a nuclear protein. It functions both as repressor and activator of transcription and is involved in the development of pancreatic beta cells, thyroid, eye, liver, and kidneys ¹¹³. Mutation in this gene showed dilated pancreatic tubules and could be responsible for neonatal diabetes due to insufficient pancreatic beta cells ¹¹⁴.

Glucokinase (hexokinase4) Regulator (GCKR/GKRP/FGQTL5)

It codes for a protein produced by hepatocytes, binds with Glucokinase enzyme (GK), thereby controls both activity and intracellular location of glucokinase enzyme involved in glucose metabolism ¹¹⁵. This gene modulates GK activity according to the levels of glucose and then moves into nucleus in inactive form ^{116, 117}. Thus, low glucose levels inactivate this gene. Mutation of this gene leads to diabetes (MODY2), hyper insulinemic hypoglycemia ¹¹⁸.

Growth factor receptor bound protein 14 (GRB14 / GRB 14 adapter protein)

This gene encodes for a growth factor receptor binding protein, which is expressed in testis, ovary, heart, liver, skeletal muscles, kidney, and pancreas. It interacts with insulin receptors and insulin-like growth factor receptors and has an inhibitory effect on tyrosine kinase signaling and on insulin receptor signaling ^{119, 120}. It is a potent inhibitor of insulin stimulated MAPK3 phosphorylation and regulates PDPK1(Phosphoinositol phosphate dependent phosphorylation of PDK1) membrane translocation in response to insulin stimulation. Recruits PDPK1 to activated insulin receptor and promotes PKB/AKT1 phosphorylation and transduction of insulin signal ^{121, 122}. Overexpression of this gene results in decreased insulin stimulation, glycogen synthesis, and dysregulation of insulin mediated pathways. Mutation of this gene is associated with reduced insulin sensitivity.

B Cell CLL / Lymphoma 11A (BCL11A/ EV19 / HBFQTL5 / ZNF856)

This gene encodes a zinc finger protein and its action is like proto-oncogen Bcl-6. The hemopoietic cell differentiation down regulates the activity of this gene, and therefore has a role in suppression of fetal hemoglobin production and sickle cell anemia ^{124, 125}. Recent studies indicate that this gene is a novel breast cancer gene and is expressed in luminal progenitor cells of mammary gland and its involvement in breast cancer progression could be via p21 transcription ¹²⁶. It has a role in both B and T lymphocyte development and beta cell function. This gene is associated with non-metabolic syndrome, and is not associated with abdominal obesity, but is a risk factor for type 2 diabetes ¹²⁷.

RNA Binding Motif Single Stranded Interacting protein 1(RBMS1/MSSP/SCR2/ C2orf12)

This gene codes for a protein which binds with either single stranded DNA/RNA and are involved in DNA replication, gene transcription, cell cycle progression and apoptosis. It binds to single stranded DNA binding protein that interacts with upstream region of c-myc gene ¹²⁸. This gene together with ITGB6 gene (Integrin coding transmembrane protein) is expressed in heart, muscle, liver and adipose tissue. RBMS1 gene is involved in the development of type 2 diabetes through inflammatory response as well as intake high fat diet.

Insulin receptor substrate 1 (IRS1/HIRS-1)

It is an adapter protein involved in the transmission of signals from the insulin and insulin-like growth factor-1(IGF-1) receptors to intra cellular pathways like PI3K/Akt and Erk MAP kinase pathways. Tyrosine phosphorylation of the insulin receptors/ binding of ligand to IGF-1 stimulates IRS1, which in turn activates PI3K pathway and MAP kinase pathway ^{129, 130}. Presence of fatty acids, TNF alpha, AMPK phosphorylates serine proteins which degrades IRS1 leads to insulin resistance and related changes leading to diabetes ¹³¹. Abnormality of IRS1 is also associated with various types of cancers including the colorectal, prostrate, lung and breast cancers.

Thyroid Adenoma associated gene (THADA/KIAA1767)

It helps in maintaining thyroid epithelium/follicular epithelium of thyroid ^{132, 133} and is involved in death receptor pathway of apoptosis. Any variations of this gene are associated with insulin sensitivity, reduced GLP1 induced insulin release, and decreased beta cell mass due to increased apoptosis ¹³⁴. Mutation of this gene may lead to thyroid cancers and type 2 diabetes.

Beta galactoside alpha -2,6-sialyltransferase 1(ST6GAL1 / SIAT1 / ST6N)

This gene encodes a member of glycosyltransferase, which transfers sialic acid from CAMP – sialic acid to galactose – containing substrates found in Golgi apparatus. It has a role in pancreatic beta cell function and in glycosylation of complex gangliosides ¹³⁵. The defective action of this gene was found to have impaired intra cellular lipid and lipoprotein transport ¹³⁶. Overexpression of sialyated proteins can be seen in various cancers including the pancreatic, gastric, colon, breast, bladder and brain cancers. The raised sialyated proteins further activates PI3K/AKT pathway and thus could lead to T2DM.

ADAMTS9

ADAMTS9 codes for matrix metalloproteins which help in the breakdown of structural matrix proteins like aggrecan, versican, and breavican ¹³⁷, and transport of proteins from endoplasmic reticulum (ER) to golgi ¹³⁸. These proteins are essential for blood coagulation, ovulation, extracellular matrix turnover, and have an important role in angiogenesis and cancer. These proteins also activate proinflammatory cytokines like interleukin (IL-1β) and tumor necrosis factor (TNF-∞). The activated cytokines destroy beta cells, causes beta cell death and phosphorylates serine residues of insulin receptor substrate (IRS-1), and inhibit glucose induced insulin secretion ¹³⁹. Experimental evidence shows that IL-1β induces SOCS-3, an inhibitor of insulin signaling pathway ¹⁴⁰. Thus, ADAMTS9 gene potentially impairs islet cell functioning via induction of proinflammatory cytokines (IL-1β & TNF-∞) resulting in T2DM.

ADCY5 (Adenyl Cyclase 5)

It is a member of Adenyl cyclase enzymes, which are membrane bound enzymes. These enzymes are coded on ADCY5 gene and help in the formation of cyclic Adenosine monophosphate(cAMP) from Adenosine triphosphate (ATP) ¹⁴¹. The activity of ADCY5 is stimulated by G-protein and inhibited by Protein Kinase A (PKA) and calcium. It also helps in regulating cytosolic calcium levels in response to raised blood sugar levels. Thus, it has a role in calcium dependent insulin secretion ¹⁴². It also helps in protecting cardiac muscle against apoptosis and restoring function of beta cells of pancreas. Mutation of this gene effects the functions of neighboring genes like SEC22, and PDIA5 which are involved in vesicle trafficking and protein folding ¹⁴³. The misfolded proteins impair the conversion of proinsulin to Insulin, resulting in increased release of proinsulin from beta cells of pancreas, and beta cell dysfunction, leading to raised blood glucose levels ¹⁴⁴.

TLE4 (Transducin – like – Enhancer of Split 4)/ ESG ( Enhancer of split groucho )4/ Groucho related gene (GRG)4

This gene codes for TLE or Groucho protein, which plays an important role in sex determination, segmentation and neurogenesis. In humans groucho proteins interacts with histone deacetylase to form a multiprotein – DNA complex which repress chromatin structure ¹⁴⁵. TLE4 gene suppresses the activation of a protein Pax2, which are DNA binding transcriptional factors that regulates kidney growth, and the development of special regions of nervous system like eye, ear and hind brain. The phosphorylation process of Pax2 is done by c-Jun N- terminal Kinase (JNK), which is inhibited by TLE4 gene ¹⁴⁶. The JNK activates insulin signaling pathway, and its activity positively influences insulin secretion ¹⁴⁷. Thus, TLE4 gene interaction with Pax2 not only stops JNK transcription, but also inhibits insulin secretion, and indirectly contributes to the development of T2DM.

PTPRD (Protein Tyrosine Phosphate Receptor type D)

This gene codes for a protein which belongs to the class of tyrosine phosphatases and is involved in the receptor signaling. It is expressed in skeletal muscles, pancreas and brain. PTPRD proteins are involved in signaling pathways that help in regulating various cellular processes like cell growth, differentiation, and oncogenic transformation. The deficiency of this gene in experimental studies reveal an impaired learning, memory, early growth retardation, neonatal mortality and posture as well as motor defects ¹⁴⁸. PTPRD, and its role in T2DM can be explained by the fact that it activates PPAR-G2, a gene which is a high-risk factor for the development of T2DM. PPAR-G2(Peroxisome Proliferator – Activated receptor Gamma) is a nuclear receptor protein coding gene that plays an important role in fatty acid storage and glucose metabolism ¹⁴⁹. PPAR-G2 activation increases the expression of PTPRD gene, which inhibits STAT3 (Signal Transducers and Activators of Transcription) oncoproteins. STAT proteins are involved in cell transformation and inhibit apoptosis. This helps us to know the role of this gene as tumor suppressor. Exact role of PTPRD gene can be understood by its epigenetic silencing which is found in the progression of T2DM from middle to late stages of the disease, but never in the early stages ¹⁵⁰. Thus, PTPRD role in T2DM is due to sequence of activation of other risk alleles of the gene which is indicated in the late stages of the disease.

Arf (ADP-ribosylation factor)-GAP (GTP associated protein) with Rho-GAP domain, ANK repeat and PH domain-containing protein 1(ARAP1)

ARAP1 activates ADP (Adenosine diphosphate) activity as well as Rho (a transcriptional Protein), and GTPase (Guanosine triphosphate) which regulates membrane trafficking and actin cytoskeletal reorganization ¹⁵¹. It is located near Golgi and has a role in regulating the members of Arf, Rho, and GTPase family ¹⁵². Experimental evidence shows that there is a decreased release of proinsulin from islet cells of pancreas in response to glucose stimulation after 1 hour due to overexpression of ARAP1 gene. This shows that normally proinsulin is secreted from islet cells of pancreas along with mature insulin in small amounts and when this gene is overexpressed it decreases or stops the release of proinsulin and thus, regulates proinsulin release via its ARF – GAP domain. It was also observed that ARAP1 has a role in insulin packing, trafficking, or exocytosis and an overexpression may stop release of all forms of insulin (preproinsilin, proinsulin, insulin) and contributes to defective functioning of beta cells of pancreas resulting in T2DM ¹⁵³.

SPRY2 (Sprouty homologue 2)

This gene codes for sprouty protein 2, which are developmental proteins involved in cell signaling. It suppresses the insulin-receptor and epidermal growth factor (EGF) receptor transduced MAPK (Mitogen Activated Protein Kinase) pathway ¹⁵⁴. since insulin receptor and EGF receptor are both involved in tyrosine kinase signaling, it also inhibits tyrosine kinase signaling. SPRY2 interacts with WFS1 gene and indirectly has a role in the development of T2DM. The role of WFS 1 gene is to keep endoplasmic reticulum (ER) stress free and further helps in proper folding of proteins. Because WFS1 gene is expressed only in beta cells of pancreas and is required for proinsulin folding proteins and processing in the ER of beta cells of pancreas, the loss of function of WFS1 gene causes abnormal calcium homeostasis in ER, which leads to ER stress and loss of insulin secretion ¹⁵⁵. SPRY2 inhibits phospho ionositol kinase (PIK) activity (role in calcium signaling), decreases calcium efflux from ER, creates a stress to ER, and activates unfolded protein response (UPR). UPR tries to remove misfolded proteins, and if activated for a long time, it could cause insulin resistance as it decreases the phosphorylation of insulin receptors and subsequently delays insulin signaling pathway. If UPR is not able to clear accumulated misfolded protein it activates apoptotic enzymes leading to apoptosis (cell death) ¹⁵⁶.

DUSP9 (Dual Specific Phosphatase 9)

This gene codes for dual specific phosphatase 9, an enzyme which belongs to the family of phosphatases and can remove phosphate (dephosphorylate) both from tyrosine as well as serine/threonine residues at their catalytic sites ¹⁵⁷. This dephosphorylates members of mitogen activated protein kinase superfamily MAPK, ERK (Extracellular signal Regulated Kinase), JNK (c-Jun N-terminal Kinase), SAP (Stress Activated Protein), P³⁸ (a class of MAPK) which are involved in cell proliferation and differentiation. As it acts on MAPK it is also called as MAP Kinase Phosphatase -4(MKP-4). The structure of MKP-4 reveals a cAMP dependent protein kinase phosphorylation site near to the c-terminal end of this gene which helps in mediating PKA (Protein Kinase A) pathway, mitogen and stress–activated pathways ¹⁵⁸. It was observed that inhibitory role of MKP-4 on MAP Kinase family initially inhibits cell proliferation and glucose uptake of certain cells but do not completely block the action of insulin. Thus, overexpression of DUSP9 gene improves insulin resistance especially in a stressed environment and can be used as a site of therapeutic intervention in treating T2DM.

4. Conclusion

The genetic reasons for the development of T2DM involves a complex interlinked signaling cascade. There are certain genes which influence post translational modifications (ARAP1, ADCY5, SPRY2, FTO), some genes related to metabolism (RBMS, HNF4A, HNF1A, PROX1, PPARG, GCKR), some involved in the development of pancreas (GLIS, HNF1B, HHEX, IGF2BP2), and a few are related to insulin secretion (GRB14, CDKAL1, ZBED3, GIPR). There are also some genes (SLC30A8) involved in insulin storage, glycosylation (ST6GAL1), apoptosis (THADA), cancer (BCL11A), metal related gene (NOTCH2), and drug related genes (KCNJ11). Understanding the pathways, and the genetics behind the pathways could help us in designing therapeutic interventions to reverse as well as prevent T2DM.

References