DISC1 splice variants are upregulated in schizophrenia and associated with risk polymorphisms

Abstract

Disrupted-In-Schizophrenia-1 (DISC1) is a promising susceptibility gene for major mental illness, but the mechanism of the clinical association is unknown. We searched for DISC1 transcripts in adult and fetal human brain and tested whether their expression is altered in patients with schizophrenia and is associated with genetic variation in DISC1. Many alternatively spliced transcripts were identified, including groups lacking exon 3 (Delta3), exons 7 and 8 (Delta7Delta8), an exon 3 insertion variant (extra short variant-1, Esv1), and intergenic splicing between TSNAX and DISC1. Isoforms Delta7Delta8, Esv1, and Delta3, which encode truncated DISC1 proteins, were expressed more abundantly during fetal development than during postnatal ages, and their expression was higher in the hippocampus of patients with schizophrenia. Schizophrenia risk-associated polymorphisms [non-synonymous SNPs rs821616 (Cys704Ser) and rs6675281 (Leu607Phe), and rs821597] were associated with the expression of Delta3 and Delta7Delta8. Moreover, the same allele at rs6675281, which predicted higher expression of these transcripts in the hippocampus, was associated with higher expression of DISC1Delta7Delta8 in lymphoblasts in an independent sample. Our results implicate a molecular mechanism of genetic risk associated with DISC1 involving specific alterations in gene processing.

Fig. 1.

Schematic representation of the genomic organization of alternative splice variants of the human DISC1 gene. The continuous line represents the genomic sequence, on which known exons of DISC1 and TSNAX are shown as rectangles and are numbered. Exons C' located between the TSNAX gene and the DISC1 gene, E68 in intron 2, E77, E126, E133, and E96 in intron 3, E179 in intron 6, E152 in intron 8, 9b in intron 9, and E62 in intron 12 and the exon previously identified as TSNAX/DISC1 exon B (6) are shown as smaller rectangles. For each transcript (numbered on the right from 1–54), the number of clones in which the transcript has been identified is shown per a total number of clones examined in each cloning experiment of end-to-end PCR products. Ovals indicate the locations of stop codons for the ORF starting with either DISC1 exon 1 or TSNAX exon 1. Open ovals indicate premature termination codons (PTC); closed ovals indicate normal stop codons. The predicted molecular weight of each protein isoform was calculated using the Compute pI/Mw tool (Swiss Institute of Biotechnology). Variants used in transfection experiments are marked with asterisks.

Fig. 2.

(A) Expression of DISC1 and TSNAX transcripts in the hippocampus of patients with schizophrenia and normal controls (means ± SE). Abbreviations: Δ3, deletion of exon 3; Δ7Δ8, deletion of exons 7 and 8; DISC 3/4, transcripts with exons 3 and 4 present; DISC_All, transcripts with exon 2 present (a majority of all transcripts); Esv1, extra short variant; L, full length variant; Lv, long variant; TSNAX 1/2, all TSNAX and TSNAX/DISC1 fusion transcripts; TSNAX 3/4, all TSNAX transcripts and a part of TSNAX/DISC1 fusion transcripts; TSNAX 5/6, only TSNAX transcripts. **, Significantly different from controls, P < 0.05; *, different from controls, P < 0.10. (B) Association of rs821597 (SNP9) and Δ3 mRNA expression in the hippocampus. (C) Associations between rs6675281 (SNP5) and rs821616 (SNP10) and Δ7Δ8 mRNA expression in the hippocampus. *, Significantly different from other groups, P < 0.05. Error bars in A–C are SD of the mean.