Chromosomal translocation disrupting the SMAD4 gene resulting in the combined phenotype of Juvenile polyposis syndrome and Hereditary Hemorrhagic Telangiectasia

Background: Patients with germline variants in SMAD4 can present symptoms of both juvenile polyposis syndrome (JPS) and Hereditary Hemorrhagic Telangiectasia (HHT): JP-HHT syndrome. Next-Generation Sequencing (NGS) techniques disclose causative sequence variants in around 90% of HHT patients fulfilling the Curaçao criteria. Here we report a translocation event involving SMAD4 resulting in JP-HHT. Methods: A patient fulfilling the Curaçao criteria was analyzed for variants in ENG, ACVRL1, and SMAD4 using standard techniques. Whole-genome sequencing (WGS) using both short-read NGS technology and long-read Oxford Nanopore technology was performed to define the structural variant and exact breakpoints. Results: No pathogenic variant was detected in ENG, ACVRL1, or SMAD4 in DNA extracted from blood. Due to abortus habitualis, the proband´s daughter was submitted for chromosomal analysis, and a cytogenetically balanced chromosomal reciprocal translocation t(1;18)(p36.1;q21.1) was detected in the daughter and the patient. The balanced translocation segregated with both gastrointestinal cancer and HHT in the family. WGS provided the exact breakpoints of the reciprocal translocation proving disruption of the SMAD4 gene. Discussion: A disease-causing reciprocal translocation between chromosome 1 and 18 with a breakpoint in the SMAD4 locus co-segregated with JP-HHT in an extended family. This observation warrants further analysis for chromosomal rearrangements in individuals with clinical HHT or JP-HHT of unknown cause.