Characterization of insulin ILPR sequences for their ability to adopt a G-quadruplex structure

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.