Role of chimeric transcript formation in the pathogenesis of birth defects.

Chimeric transcripts are formed by chromosomal aberrations. Little is known about the role of chimeric transcripts in the pathogenesis of birth defects. We reanalyzed RNA-seq data in alignment map files from the peripheral blood of 56 patients in whom the diagnoses could not be confirmed by standard exome analysis and transcriptome analysis to screen for chimeric transcripts using a dedicated software, ChimPipe. Chimeric analysis led to a diagnosis in two of the 56 patients: (a) the first patient had a chimeric transcript spanning the causative gene ZEB2 and the GTDC1 gene in its neighboring locus. RNA-seq revealed reads spanning exon 5 of ZEB2 and exon 7 of GTDC1. Whole genome sequencing revealed a 436-kb deletion spanning intron 4 of ZEB2 and intron 7 of GTDC1 and the diagnosis of Mowat-Wilson syndrome was made. (b) The second patient had a chimeric transcript spanning the causative gene KCNK9 and the TRAPPC9 gene in its neighboring locus. RNA-seq revealed reads spanning exon 21 of TRAPPC9 and exon 1 of KCNK9. Whole genome sequencing revealed a 186-kb deletion spanning intron 20 of TRAPPC9 and intron 1 of KCNK9 in this patient. KCNK9 gene is a maternally expressed imprinted gene. The diagnosis of Birk-Barel syndrome was made. Thus, both patients had chimeric transcripts that were directly involved in the pathogenesis of the birth defects. The approach reported herein, of detecting chimeric transcripts from RNA-seq data, is unique in that the approach does not rely on any prior information on the presence of genomic deletion.