Analysis for co-occurring sequence features identifies link between common synonymous variant and an early-terminated NPC1 isoform

Abstract

Direct assessment of allelic phase for DNA and RNA features of diploid genomes has been challenging for Sanger sequencing, due to its allele-conflating base-calling signal. Massively parallel sequencing technologies are based on the generation of a continuous copy of a single strand sequence segments, thus preserving the allelic relation between the features of the original molecules. We have performed a transcriptome-wide search for co-occurrence of variant nucleotides and exon-intron boundaries positioned within the length of a single sequencing read. Analysis of 75 human transcriptomes from retinal pigment epithelia (RPE), glioblastoma, low-grade brain tumor, breast cancer and colon cancer, have identified an association between the synonymous variant rs1140458 and an early-terminated NPC1 isoform lacking exons 19-25. Higher proportion of molecules bearing the variant nucleotide (versus the reference) incorporates the intron (P < 0.0001), which turns the last codon of exon 18 into a stop codon. The significance is highest in RPE cells (P = 3.88 × 10-12). NPC1 protein is involved in the control of the cholesterol trafficking. NPC1 mutations lead, in an autosomal recessive manner, to the neurological disorder Niemann-Pick syndrome type C (NP-C), and, ablation of NPC1 causes age-progressive retinal degeneration in mice and drosophila. The vast majority of the NP-C causative variants consist of missense/nonsense substitutions, small indels, and, intronic splice variants. Rs1140458 is a common exonic synonymous substitution that has never been linked to alternative splicing or pathogenicity. Our analysis suggests that rs1140458 may affect the levels of the functional NPC1 protein, and to contribute to some of the cholesterol-implicated cellular phenotype.