Exome sequencing in 51 early onset non-familial CRC cases

Colorectal cancer (CRC) cases with an age of onset <40 years suggests a germline genetic cause. In total, 51 simplex cases were included to test the hypothesis of CRC as a mendelian trait caused by either heterozygous autosomal dominant or bi-allelic autosomal recessive pathogenic variants.

The cohort was whole exome sequenced (WES) at 100× coverage. Both a dominant- and recessive model were used for searching predisposing genetic factors. In addition, we assayed recessive variants of potential moderate risk that were enriched in our young-onset CRC cohort. Variants were filtered using a candidate cancer gene list or by selecting variants more likely to be pathogenic based on variant type (e.g., loss-of-function) or allele frequency.

We identified one pathogenic variant in PTEN in a patient subsequently confirmed to have a hereditary hamartoma tumor syndrome (Cowden syndrome) and one patient with a pathogenic heterozygous variant in PMS2 that was originally not identified by WES due to low quality reads resulting from pseudogenes. In addition, we identified three heterozygous candidate missense variants in known cancer susceptibility genes (BMPR1A, BRIP1, and SRC), three truncating variants in possibly novel cancer genes (CLSPN, SEC24B, SSH2) and four candidate missense variants in ACACA, NR2C2, INPP4A, and DIDO1. We also identify five possible autosomal recessive candidate genes: ATP10B, PKHD1, UGGT2, MYH13, TFF3.

Two clear pathogenic variants were identified in patients that had not been identified clinically. Thus, the chance of detecting a hereditary cancer syndrome in patients with CRC at young age but without family history is 2/51 (4%) and therefore the clinical benefit of genetic testing in this patient group is low. Of note, using stringent filtering, we have identified a total of ten candidate heterozygous variants and five possibly biallelic autosomal recessive candidate genes that warrant further study.