# A new form of diabetes caused by INS mutations defined by zygosity, stem cell and population data

## TL;DR

This study shows that a specific insulin gene mutation causes diabetes only when inherited from both parents, using patient data and stem cell models.

## Contribution

The study establishes INS R6C as a recessive loss-of-function mutation causing monogenic diabetes, using multi-modal analyses.

## Key findings

- Homozygous R6C individuals develop early-onset diabetes, while heterozygotes do not show significant diabetes risk.
- Homozygous R6C β cells show preproinsulin accumulation and reduced insulin secretion.
- Transcriptomic analysis reveals downregulation of ER-related pathways in homozygous R6C β cells.

## Abstract

The INS c.16 C > T (insulin p.Arg6Cys, R6C) variant was reported to cause autosomal dominant monogenic diabetes, yet its pathogenicity has been questioned. R6C preproinsulin exhibits impaired translocation into the endoplasmic reticulum (ER). We explored R6C pathogenicity using integrative clinical, genetic, and functional approaches.Homozygous INS R6C individuals presented early-onset insulin-treated diabetes, whereas heterozygous carriers showed variable or absent glycemic phenotypes. Population-level analysis revealed no significant enrichment of diabetes among heterozygotes. Heterozygous R6C patient’s induced pluripotent stem cell (iPSC)-derived pancreatic β cells exhibited minimal defects, while homozygous R6C β cells displayed preproinsulin accumulation and reduced insulin content and secretion. In vivo, homozygous R6C β cell transplants recapitulated insulin deficiency and responded poorly to GLP-1 receptor agonist. Homozygous R6C β cells had a gene signature of attenuated translation, translocation and ER related pathways.Our findings establish R6C as a recessive loss-of-function mutation, prompting a clinical reassessment of heterozygous R6C carriers. This study highlights the power of population genetic databases, patients’ iPSC-based modeling and multi-modal variant classification frameworks for dissecting the consequences of genetic variants in monogenic diabetes.

This study redefines the INS c.16 C > T (insulin p.Arg6Cys, R6C) variant as a recessive cause of monogenic diabetes. Through integrative clinical, genetic, and induced pluripotent stem cell (iPSC)-based functional analyses, it distinguishes homozygous pathogenicity from benign heterozygosity and elucidates the underlying endoplasmic reticulum (ER)-translocation defect in pancreatic β cells.

The INS R6C variant acts recessively, causing diabetes in homozygous (HOM) individuals.Heterozygous (HET) carriers lack population-level enrichment for diabetes.iPSC-β cells from homozygous (HOM) patients showed preproinsulin accumulation and impaired insulin content and secretion.Transcriptomic profiling revealed downregulation of translation, translocation, and ER-related pathways, supporting a loss-of-function mechanism.

The INS R6C variant acts recessively, causing diabetes in homozygous (HOM) individuals.

Heterozygous (HET) carriers lack population-level enrichment for diabetes.

iPSC-β cells from homozygous (HOM) patients showed preproinsulin accumulation and impaired insulin content and secretion.

Transcriptomic profiling revealed downregulation of translation, translocation, and ER-related pathways, supporting a loss-of-function mechanism.

This study redefines the INS c.16 C > T (insulin p.Arg6Cys, R6C) variant as a recessive cause of monogenic diabetes. Through integrative clinical, genetic, and induced pluripotent stem cell (iPSC)-based functional analyses, it distinguishes homozygous pathogenicity from benign heterozygosity and elucidates the underlying endoplasmic reticulum (ER)-translocation defect in pancreatic β cells.