PO-059 Cancer-predisposing variants in alternatively spliced TP53 exons

Introduction Germline pathogenic variants in the canonical TP53 gene cause Li-Fraumeni, a severe cancer predisposition syndrome characterised by early onset of multiple tumours, particularly breast cancer, brain tumours, soft-tissue sarcomas, osteosarcomas and leukaemia. Genetic variation in the alternatively spliced TP53 exons 9β and 9γ, each encoding a different carboxy-termini, have been poorly studied. Therefore, the clinical significance of germline variation in these alternative exons remains unknown. In the search for novel colorectal cancer susceptibility genes, we identified a germline variant in an alternative TP53 exon 9 and we show that this variant is associated with a predisposition to multiple cancer types. Material and methods Exome sequencing was performed on 118 unexplained familial and/or early-onset colorectal cancer patients. Somatic TP53 and cancer hotspot mutations were assessed in tumour tissues of carriers. TP53 isoform expression and non-sense mediated decay were studied in cell lines from colorectal cancer patients using (allele specific) qPCR and Western blotting. Results and discussions We identified a germline heterozygous variant in an alternative TP53 exon 9 in twelve individuals from three families severely affected by colorectal cancer and multiple other cancer types. The carriers developed one or more primary tumours after 40 years of age, including colorectal cancer, breast cancer, thyroid cancer and Non-Hodgkin lymphoma. Only two out of eleven tumours harboured a somatic TP53 mutation and a single tumour showed loss of heterozygosity of the alternative exon 9 variant. The cell line carrying the TP53 variant showed (i) an allelic imbalance towards the expression of variant containing transcripts and (ii) a higher expression of the alternative TP53 exon 9 isoforms compared to wild-type cell lines. Increased protein expression of the alternative exon 9 isoforms could be confirmed in the cell line using Western blotting. The increased expression was not caused by disturbance of non-sense mediated decay mechanisms. Conclusion We have identified a variant in an alternative TP53 isoform that predisposes carriers to multiple cancers. Carriers presented at a slightly older age and with a different spectrum of tumours compared to classical Li-Fraumeni syndrome patients. Therefore we recommend the screening of the alternative exons 9 of the TP53 gene in all unexplained cancer-prone families.