Long-read DNA and RNA sequencing reveal an intronic retrotransposon insertion in TCOF1 causing Treacher Collins syndrome.

Treacher Collins syndrome (TCS) is a craniofacial genetic disorder caused by loss-of-function variants in TCOF1, POLR1B, POLR1C, or POLR1D. Here, we describe two previously undiagnosed paternal half-siblings affected with clinical TCS, and their apparently unaffected father. Diagnostic short-read RNA sequencing) identified aberrant expression of TCOF1 and optical genome mapping detected a large genomic insertion therein. Long-read genome sequencing (lrGS) resolved a deep intronic 3.5 kb SINE-VNTR-Alu (SVA) retrotransposon insertion in intron 17 of TCOF1. Long-read RNA sequencing (lrRNA-seq) demonstrated that the insertion was partially exonized inducing isoform switch to the shorter non-canonical TCOF1 isoform c. SVA insertion was confirmed in both half-siblings, and we detected mosaicism in the father. This work demonstrates the potential of lrRNA-seq and lrGS, to identify pathogenic variants in unexplained genetic disorders.