Pathogenicity of TCF7L2 gene for diabetes mellitus type 2

Diabetes Mellitus is a group of diseases caused by higher glucose levels in the blood. These diseases include problems in insulin secretion, improper insulin function, or maybe both, an imbalance in the metabolism of carbohydrates, fat, and protein [Kharroubi 2015].

Three main types of diabetes mellitus are:

Type-1-Diabetes Mellitus (T1DM)

This type of diabetes mostly occurs in children and individuals before adult age. It is also known as juvenile diabetes [Skyler 2017]. The autoimmune death of endocrine pancreatic cells causes T1DM, which results in an inadequate supply of insulin. When one or more environmental factors contribute to this autoimmune destruction process, genetically vulnerable individuals who are euglycemic, asymptomatic, and have positive, relevant autoantibodies experience it. This process often develops over several months or years. Long latency periods between symptoms of hyperglycemia and diabetes are due to the high proportion of cells that must first be killed or become dysfunctional for diabetes to become overtly apparent [Paschou 2018].

Gestational Diabetes Mellitus

During pregnancy, women who have never been diagnosed with diabetes acquire chronic hyperglycemia, known as gestational diabetes mellitus (GDM), a significant pregnancy complication. This hyperglycemia is usually brought on by regular insulin resistance, poor glucose tolerance, and dysfunctional pancreatic beta-cells. A family history of diabetes in any form or being overweight or obese are risk factors for GDM, as well as higher age. Increased risk of T2DM, macrosomia, and deli-
very difficulties for the infant are among the effects of GDM. Additionally, the child has a higher long-term risk of obesity, T2DM, and cardiovascular disease. Globally, GDM affects about 16.5 % of pregnancies, expected to rise as the obesity pandemic spreads. Several management techniques exist, including insulin and lifestyle changes, but no effective treatment or preventative method exists now. The poorly understood molecular pathways underpinning GDM are contributing to this [Plows 2018]. It is the most commonly observed form of diabetes occurring during pregnancy. Secretion of insulin increases and insulin function remains the same at the start of pregnancy or may slightly change. At the end of the second trimester or the start of the third trimester, insulin function increasingly becomes defective, gestational diabetes mellitus appears and remains till delivery [Alfadhli 2015].

Type-2-Diabetes Mellitus (T2DM)

T2DM, one of the most widespread metabolic illnesses, is brought on by a confluence of two main factors: improper insulin secretion by pancreatic beta-cells and nasty insulin response in insulin-sensitive tissues. The molecular mechanisms involved in insulin production, release, and recognition are closely regulated because these actions are crucial for maintaining glucose homeostasis. The development of the disease is attributed to a metabolic imbalance caused by defects in any of the mechanisms involved in these processes [Galicia-Garcia 2020].

Persistent higher glucose levels arise slowly, so T2DM remains undetected and asymptomatic for so long. However, there is a rising risk of cardiovascular diseases. It includes 90-95 % of the total diabetic people who face insulin resistance and somewhat insufficient insulin secretion. Obese individuals are more likely to develop this disease because obesity is a major cause of insulin resistance. Ketoacidosis rarely develops in these subjects due to other diseases or infections [Kharroubi 2015].

About the nature of the pathogenesis of the disease, unusually high blood glucose levels are caused by a breakdown of the feedback loops between insulin action and insulin secretion. As a result of decreased insulin secretion caused by beta-cell malfunction, the ability of the body to maintain physiological glucose levels is constrained. Conversely, insulin resistance helps reduce glucose uptake in adipose tissue, muscle, and the liver while increasing glucose synthesis in the liver. All of these processes occur early in the pathophysiology and help to cause the disease to manifest, but beta-cell dysfunction is typically more severe than insulin resistance. However, hyperglycemia increases, and T2DM progresses when insulin resistance and beta-cell dysfunction are present [Zheng 2018].

Jan et al. evaluated the perspectives of various researchers regarding the etiology and treatment of diabetes mellitus. According to their research, the main causes of diabetes include inheritance, radiation, dysregulation, and pancreatic ailments. Similarly, the researcher concluded that engaging in regular physical activity, eating a healthy diet, and taking various curative medications can assist in managing and lessening diabetes mellitus symptoms in the general population [Jan 2017].

Prevalence of T2DM

Almost 462 million people have diabetes, which represents 6.28% population in the world. In 2017 T2DM was ranked 9th after 1 million deaths due to it. In contrast, it was ranked 18th in 1990. Recent research shows that developed countries are more under the effect of diabetes than underdeveloped, economically poor countries. Males show a higher degree of this disease than females [Khan 2020].

Genetic Cause of T2DM

It has been thought for many years that T2DM transfers from one generation to another. Recent research on familial history showed that close relatives are three times more at risk of getting this disease than other individuals (Florez 2003). Different candidate genes have been recognized for this disease because of their role in beta cell function, the action of insulin, i.e., metabolism of glucose, and other risk factors for diabetes [Barroso 2003]. According to genome-wide analyses of familial linkage, only a few T2DM putatively related areas were discovered using this technique over ten years: CAPN10, ADIPOQ, HNF4A, ENPP1, and TCF7L2. However, only the connections of the HNF4A and TCF7L2 loci with T2DM risk were later verified by GWAS analysis, raising questions regarding the significance of the other loci. The discovery of TCF7L2 as a gene associated with T2DM susceptibility was the single notable achievement of the pre-GWAS era. The largest influence on T2DM risk to date is conferred by a common intronic SNP within TCF7L2 [Bonnefond 2015].

Transcription factor 7-like 2 gene

TCF7L2 (previously TCF4-transcription factor 4) is a transcription factor that belongs to the T-cell factor/lymphoid enhancer binding factor family (TCF/LEF), a group of transcription factors that bind to DNA via a high-mobility group (HMG) domain. TCF7L2 gene is localized on chromosome 10q25 and encodes a 215.9 kb nucleotide sequence. The TCF7L2 gene has received much attention because of its strong genetic link to T2DM [Del Bosque-Plata 2021]. The TCF7L2 gene is a key component of the Wnt (wingless-related integration site) signaling pathway, which comprises a complex network of interacting proteins with cellular intercommunications regulated at multiple levels, resulting in various effects, and is originally linked to developmental biology. Proteins that are part of the Wnt network, such as TCF7L2, have been linked to various common diseases and cancer models, demonstrating the importance of this developmental pathway in the pathogenesis of human diseases [Van Amerongen 2009]. The human TCF7L2 gene was first sequenced in colorectal cancer (CRC) cell lines (Duval 2000). TCF7L2 is made up of 18 exons that splice in a variety of ways in various tissues. The sequence portions of this gene that correspond to functional domains are substantially conserved. Exon 1 of human TCF7L2 belongs to the β-catenin binding domain, while exons 10 and 11 relate to the HMG box binding domain (hTCF4). Between species, both domains are largely conserved. TCF7L2 also has highly conserved sequences in exon 18’s 30 region and splicing patterns similar to those of the transcription factor T-cell factor 1 (TCF1) gene. Two reading frames are conserved in Drosophila melanogaster and Caenorhabditis elegans, and two more reading frames are found in these spliced transcripts [Del Bosque-Plata 2021]. TCF7L2 is a member of the high-mobility group box family of transcription factors that regulate the Wnt signaling pathway. The Wnt pathway is involved in islet cell proliferation and differentiation in the pancreas. T2DM in humans could be caused by a mutation in the TCF7L2 gene, which is involved in the Wnt signaling pathway [Maschio 2016]. The major effector of Wnt signaling is β-cat/TCF, which is generated by free β-catenin and a member of the TCF family, specifically TCF7L2. According to a study, TCF7L2, a transcriptional co-activator of β-catenin, has a critical role in modulating insulin secretion, and TCF7L2 overexpression reduces insulin synthesis [Huang 2018].

Wnt signaling has been revealed to control myogenesis and adipogenesis and is essential for cell growth, motility, and proper embryogenesis. It is also necessary for the pancreas and islets to develop during the embryonic stages [Weedon 2007]. The Wnt signaling pathway comprises seven transmembrane domains that function as ligands for receptors that control human developmental and adult processes and 19 secreted glycoproteins with 22 or 24 highly conserved cysteine residues [MacDonald 2009]. The Wnt/β-catenin pathway affects several nuclear-effector proteins with various functional properties and triggers various tissue-specific responses, including regulating gene transcription and transduction (e.g., TCF proteins). Expression of four members of the TCF/LEF family (TCF7, TCF7L1, TCF7L2 and LEF1) and their numerous isoform variants is regulated by Wnt signals through a transcriptional interrupter that partially modulates binding with β-catenin and the binding of co-activator compounds [Mao 2011].

Pathogenicity of TCF7L2

TCF7L2 gene has been observed to cause T2DM in various ways, such as a mutation in a particular variant, i.e., rs7903146, as an important part of Wnt signaling by binding to β-catenin to perform significant roles, reducing incretin (GIP and GLP-1) function, causing β-cell apoptosis, proinsulin conversion and changing β-cell responsivity, increasing adipogenesis by making beta cells weak, and regulation of glucose metabolism. All of these factors have been briefly discussed in this review.

SNPs of TCF7L2 Gene

T2DM has been associated with the TCF7L2 gene SNP (Single nucleotide polymorphism) rs7903146, which can be used as a genetic marker for T2DM [Syamsurizal 2019]. A previous study that evaluated TCF7L2 mRNA in pancreatic islets from T2DM and non-diabetic human deceased organ donors found that higher gene expression was associated with the rs7903146 and rs12255372 T alleles [Pang 2013].

The TCF7L2 gene contains the most highly related T2DM locus. TCF7L2 SNPs elevate the prevalence of T2DM among variant bearers. The TCF7L2 T2DM risk factor was honed to the ancestral T allele of an SNP, rs7903146, through replication in West African and Danish T2DM case-control studies as well as expanded Icelandic research [Helgason 2007]. A study that examined 43 SNPs in African Americans, including the previously discovered DG10S478 microsatellite, discovered that the trait-defining polymorphism associated with T2DM risk was rs7903146. In these studies, the SNP rs7903146 located in intron 3 of the TCF7L2 gene is considered the causal variant. However, the physiological mechanism underlying the connection between changes and T2DM is yet unknown. The most likely possible risk variants were SNPs rs7903146 and rs12255372, which were shown to have strong LD with the microsatellite marker DG10S478. The most statistically significant associations with T2DM were rs7903146-T and rs12255372-T alleles [Palmer 2011].

A regulatory process determines T2DM risk because the mutations in risk-related variants are identified in an intronic region rather than an exon. Studies show that the locus self-regulates T2DM risk by binding a transcriptional protein complex across rs7903146 inside TCF7L2 [Xia 2015]. A study discovered that intronic TCF7L2 variants regulate various transcript isoforms, which may have diverse physiological roles in the production of T2DM. Acyl-CoA synthetases 5 (ACSL5), which are involved in lipid synthesis and fatty acid oxidation, may be related to insulin resistance [Mondal 2010]. Knocking down ACSL5, a crucial regulator of the overall energy metabolism of the body, may help avoid obesity and insulin resistance [Bowman 2016]. TCF7L2 may control the expression of ACSL5 because it has a causal variation in a region that affects ACSL5 expression. Because of the connection between ACSL5 and insulin sensitivity, treating T2DM by reducing ACSL5 enzyme activity may be advantageous [Xia 2016].

Wnt Signaling

The Wnt signaling pathway has been related to cellular metabolism and is a critical regulator of cell growth and differentiation [MacDonald 2009]. Canonical Wnt signaling is initiated when extracellular Wnt molecules bind to cell surface receptors on target cells. In response, β-catenin is stabilized and moves into the nucleus, which stimulates TCF/LEF transcription factors (Miravet 2002). To silence Wnt-mediated gene expression when no Wnt ligand is present, TCF transcription factors associate with TLEs, transcriptional repressors [Cavallo 1998]. TCF/LEF family member TCF7L2 is one of the different components of the Wnt signaling pathway known to regulate metabolic disease in humans [Geoghegan 2019].

Wnt signaling plays a variety of roles in cell differentiation and maturation, as well as in the secretion and action of insulin. It has been demonstrated that intestinal endocrine L-cell secretion of the hormone glucagon-like peptide-1 (GLP-1) depends on Wnt signaling via the nuclear receptor TCF7L2. Therefore, a change in this route could result in less GLP-1 being secreted, impacting insulin release after a meal and developing new beta cells from ductal precursor cells. Such a hypothesis is supported by the consensus result that the TCF7L2 risk alleles are related to decreased insulin secretion. With rigorous phenotyping and GLP-1 measurements of well-matched non-diabetic persons with or without the risk genes, this is undoubtedly something that can and should be studied. In research, mice with the TCF7L2 gene fully deleted pass away soon after birth [Smith 2007].

Reduced Incretin Function

The role of altered incretin function in the etiology of T2DM has recently come under discussion, despite studies suggesting that the diminished incretin effect in T2DM may be an outcome rather than a cause of the diabetic condition. It has been proposed that it acts as a genetic mediator for a high risk of T2DM [Knop 2007].

A transcription factor that binds to β-catenin and activates the transcription of several genes, including intestine proglucagon, is encoded by the transcription factor-7-like 2 (TCF7L2) gene. Numerous populations have repeatedly demonstrated that TCF7L2 variants are closely linked to T2DM and decreased insulin production [Lyssenko 2007]. Numerous studies have looked into the potential contribution of altered incretin effects to reduced insulin production in carriers of TCF7L2 polymorphisms, given that the incretin GLP-1 is derived from proglucagon.

Incretin impact was dramatically reduced by 20 % in carriers of the risk (T) allele who had T2DM or impaired glucose tolerance, according to the first study, compared to non-carriers. Another study looked into the function of GLP-1. GLP-1 infusion revealed a substantial reduction in GLP-1-induced insulin secretion in carriers of the risk allele, despite the GLP-1 concentration per se being identical in carriers and non-carriers of the risk allele. It is interesting to note that in isolated human pancreatic islets, TCF7L2 has been proven to control β-cell survival and operation. TCF7L2 polymorphisms may consequently have a diabetogenic effect through impairment of incretin action. This appears to be accomplished through decreased GLP-1 action without a change in GLP-1 secretion. However, additional research is necessary to comprehend the precise mechanisms regulating this phenomenon [Schäfer 2007].

Moreover, when TCF7L2 is silenced, both the GLP-1 receptor and the glucose-dependent insulinotropic polypeptide receptor (GIPR) show lower expression and also show lower expression in pancreatic islets from diabetic people [Hansson 2010]. Individuals who carry the TCF7L2 risk allele have a marked decrease in GLP-1-induced insulin production, possibly due to an underlying abnormality in the islets. Alterations in TCF7L2 expression may also impact GLP-1/GIP activity. Previously, it was found that TCF7L2-depleted isolated human islets had a severe reduction in the potentiation of GLP-1-induced glucose-stimulated insulin secretion [Schäfer 2007].

Apoptosis of β-cells

GLP-1 prevents cell death by activating apoptosis-inhibiting protein kinase (AKT). Furthermore, poor AKT activation has been linked to decreased GSK-3 inactivation [Katoh 2005]. The degradation of β-catenin by GSK-3 limits TCF7L2 activation and, in turn, enhances β-cell apoptosis via the Wnt signaling pathway. TCF7L2-depleted islets had nearly undetectable levels of p-AKT. In normal, but not in TCF7L2-depleted, islets, GLP-1, and GIP caused AKT phosphorylation. If AKT phosphorylation were to be lost, nuclear accumulation and Foxo1-mediated transcription would be suppressed [Kim 2005].

A decrease in the quantity and effectiveness of pancreatic islet β-cells mainly causes the development of T2DM. TCF7L2 affects pancreatic islet-cell survival, which has an impact on β-cell functioning. Since active TCF7L2 may be necessary for pancreatic-cell proliferation, carriers of an at-risk allele for TCF7L2 are thought to be more likely to develop T2DM [Yao 2015]. It was proposed that the risk-associated allele in TCF7L2 increased the expression of an inhibitory TCF7L2 isoform with decreased transcriptional activity through the p53-induced-nuclear protein 1 (p53INP1) pathway.

Additionally, it has been observed that Wnt signaling is triggered during the differentiation of adipose-derived stem cells (ADSCs) into islet β-cells, suggesting that TCF7L2 regulates β-cell survi-
val via the Wnt signaling pathway. When TCF7L2 was less active, β-cell apoptosis increased, ultimately affecting insulin secretion. Diabetes mellitus type 2 might be treated by finding a mechanism to stop β-cells from death, similar to how geniposide works [Zhou 2012].

Regulation of Glucose Metabolism

Investigating a possible causal link between TCF7L2 and T2DM was done in mice expressing multiple copies of TCF7L2 using a BAC transgenic system. Since human TCF7L2 was overexpressed in several tissues, glucose regulation was compromised [Savic 2011]. Inconsistent results have been found from studies aiming to determine how TCF7L2 affects systemic glycemia in different organs (fig. 2). Patients with TCF7L2 risk alleles have higher levels of TCF7L2 expression in the pancreas than the general population, and TCF7L2 deficiency in pancreatic β-cells in mice resulted in normal β-cell activity and insulin secretion [Boj 2012]. TCF7L2 removal in the liver improves glucose tolerance and reduces gluconeogenesis in mice, while TCF7L2 up-regulation in the liver leads to glucose intolerance in mice [Boj 2012]. More study is needed to determine the involvement of TCF7L2 in other metabolically active tissues to understand how TCF7L2 contributes to the complex etiology of T2DM [Shu 2008].

Increase in Adipogenesis

According to an analysis of the genome-wide distribution of TCF7L2 binding and gene expression in adipocytes, TCF7L2 directly regulates genes associated with cellular metabolism and cell cycle control. A high-fat diet causes weight gain, impaired glucose intole-rance, reduced insulin sensitivity, and increased subcutaneous adipose tissue mass in an animal model with conditional deletion of TCF7L2 in adipocytes. Additionally, fasting-induced free fatty acids are released, and triglyceride-hydrolase expression is decreased. After three days and months, high-fat feedings cause impaired insulin sensitivity and glucose intolerance. According to the research, mice exposed to a high-fat diet are predisposed to developing diabetes if TCF7L2 is lost early [Geoghegan 2021].

TCF7L2 plays critical metabolic and developmental processes in adipose tissue, as shown in Fig 1. TCF7L2 and Wnt signaling plays a complex role in regulating adipogenesis, and adipocyte TCF7L2 inactivation stimulates adipocyte hypertrophy and peripheral and hepatic insulin resistance. Adipogenesis in the 3T3-L1 cell line and the methods by which the Wnt signaling pathway and miRNAs regulate it have also been investigated. Studies have shown that overexpressing TCF7L2 without its β-catenin binding domain stimulates spontaneous adipogenesis in both preadipocyte 3T3-L1 cells and skeletal muscle precursor cells. Based on these results, we may say that TCF7L2, without its binding ligand, inhibits adipogenesis. Some researchers believed this was an oversimplified representation of TCF7L2 and Wnt signaling activity in adipogenesis. First, fully differentiated adipocytes express various Wnt components, such as TCF7L2 and multiple Wnt ligands and receptors. Secondly, the transcriptional activity of TCF7L2 can be altered by its interactions with other transcription factors, most notably β-catenin. The interaction of TCF7L2 with β-catenin and other transcriptional partners demonstrates its resilience. TCFs suppress Wnt target genes by recruiting nuclear cofactors when β-catenin is absent, effectively silencing the genes. Third, adipogenesis can be stimulated by numerous secreted Wnt ligands such as Wnt4, Wnt5a, and Wnt5b. The TCF7L2 protein is upregulated in 3T3-L1 and primary ASCs during adipogenesis [Chen 2018]. Overexpression of β-catenin influences insulin secretion through an unknown mechanism. Its discovery could pave the way for a brand-new T2DM treatment method [Sorrenson 2016].

Proinsulin conversion and β-cell responsivity

Proinsulin, a prohormone precursor to insulin, is produced in the islets, specialized pancreas areas. Proinsulin concentrations higher than mature insulin levels can signal the onset of T2DM and approaching insulin resistance. The disruption of the proinsulin processing pathway is another theory about how TCF7L2 triggers apoptosis in β-cells [Mykkänen 1997]. TCF7L2 silencing has been proven to affect insulin vesicle trafficking. In addition to having a higher chance of developing T2DM, people who carry the risk alleles rs1225537 and rs7901346 in TCF7L2 also have higher proinsulin levels and a higher proinsulin-to-insulin ratio. According to a meta-analysis, the T-allele of the TCF7L2 rs7903146 is a major risk factor for poor proinsulin conversion [Silbernagel 2011]. One of the mechanisms through which TCF7L2 induces T2DM may be impaired proinsulin conversion. When glucose from the breakdown of dietary carbs begins to enter the bloodstream, the beta cells begin to create insulin. However, in people with T2DM, β-cells would probably not react promptly to the blood glucose levels. There were 120 people investigated, of whom half had the TT and half the CC homozygotes at rs7903146. They found that β-cell responsiveness varied considerably between genotypes and was somewhat decreased in the TT genotype group, suggesting that a genetic variation of TCF7L2 reduces glucose tolerance by affecting glucagon and insulin production [Shen 2015].

Conclusions

Regarding the risk for T2DM, TCF7L2 is the most significant T2DM gene currently known. Under its complicated expression with many mRNA isoforms, the TCF7L2 gene can regulate a wide range of functions as a downstream effector of the canonical Wnt/β-catenin signaling pathway. Various diseases and conditions are linked to this gene, such as a mutation in gene variants, adipogenesis, downregulation of insulin production by reducing incretin function, β-cells apoptosis, and regulation of glucose metabolism, including several types of cancer. More research is necessary to understand better the precise molecular pathways behind TCF7L2-mediated pathogenicity and its potential as a therapeutic target in treating T2DM. Researchers are still delving into the functional implications of the genetic variant linked to T2DM.

Manuskript eingegangen: 11. August 2022
Manuskript angenommen: 29. June 2023

Conflicts of interest
The authors declare no conflicts of interest.

Erschienen in: Diabetes, Stoffwechsel und Herz, 2023; 32 (5) Seite 233-239