Characterization of Insulin Ilpr Sequences for Their Ability to Adopt a G-Quadruplex Structure

Abstract

A major genetic factor linked to the progression of type 1 diabetes occurs in the insulin-linked polymorphic repeat region (ILPR) located 363 bp upstream of the human insulin gene. Genetic studies have shown that individuals with class I repeats (30–60) are predisposed to the development of type 1 diabetes while individuals with longer repeats are protected. Previous research has suggested that some sequences found within the ILPR can adopt a G-quadruplex structure, and this finding has lead to speculation that G-quadruplexes may control insulin expression in certain circumstances. Unfortunately, relatively little study has been done on whether sequences found in the ILPR can adopt a quadruplex fold. In this study, we have utilized circular dichroism, thermal difference spectroscopy and ultraviolet (UV) melting studies to examine the first seven common repeat sequences (A-G) found in the ILPR. We find that sequences A-E adopt a quadruplex fold while sequences F and G likely do not. Examination of sequence B and a single nucleotide variant, B2, revealed that both folded into a G-quadruplex. This result casts doubt on previous studies suggesting that the formation of a quadruplex was related to the ability of ILPR sequences to regulate transcription.

Keywords:

• insulin-linked polymorphic repeat region (ILPR)
• G-quadruplex
• type 1 daibetes
• human insulin gene
• class I allele repeats

Acknowledgments

We thank Wayne State University and the Juvenile Diabetes Research Foundation (SMF) for funding. We are grateful to Dr. Andrew Feig (Department of Chemistry, WSU) for the use of his circular dichroism instrument.

Notes

^a Melting temperature determined from the first derivative plot of the melting curves shown in Figure 3.