Modeling Genetic Risk of β‐Cell Dysfunction in Human Induced Pluripotent Stem Cells From Patients Carrying the <i>MTNR1B</i> Risk Variant

Disruptions in circadian rhythm, partly controlled by the hormone melatonin, increase the risk of type 2 diabetes (T2D). Accordingly, a variant of the gene encoding the melatonin receptor 1B (MTNR1B) is robustly associated with increased risk of T2D. This single-nucleotide polymorphism (SNP; rs10830963; G-allele) is an expression quantitative trait locus (eQTL) in human pancreatic islets, conferring increased expression of MTNR1B, which is thought to perturb pancreatic β-cell function. To understand this pathogenic mechanism in detail, we utilized human induced pluripotent stem cells (hiPSC), derived from individuals with T2D carrying the MTNR1B G-allele. Patient-derived fibroblasts were reprogrammed to hiPSC and single-base genome editing by CRISPR/Cas9 was employed to create isogenic lines of either the C/C or G/G genotypes (nonrisk and risk, respectively). In addition, the human embryonic stem cell (hESC) line (HUES4) was subjected to genome editing to create isogenic lines of either the C/C or G/G genotypes. hiPSC and hESC were differentiated into β-like cells, using a 50-day 2D protocol. Single-base genome editing generated cells with the desired genotype at a success rate of > 90%. Expression of stage-specific markers confirmed differentiation of both hiPSC and hESC into β-cells. MTNR1B mRNA levels were consistently low in differentiated β-cells, precluding quantitative analysis of gene expression. Western blot analyses indicated slightly higher levels of the MTNR1B protein in differentiated β-cells carrying the risk allele, which is in accord with the notion that rs10830963 (G-allele) functions as an eQTL in β-cells. Insulin secretion in response to the combination of high glucose and IBMX was comparable between genotypes, whereas the addition of melatonin appeared to reduce insulin secretion more efficiently in cells carrying the G-allele. While our data suggest elevated MTNR1B protein levels in stem cell-derived β-like cells carrying the risk allele, these cells do not appear to be sufficiently mature to establish rs10830963 as an eQTL at the mRNA level. The observed nominal increase in melatonin sensitivity in G-allele-carrying cells is suggestive of a functional contribution of rs10830963 to β-cell dysfunction; however, this interpretation remains tentative and will require further validation in more mature β-cell models.