Screening And Validation Of Susceptibility Genes In Familial Non-medullary Thyroid Cancer

The incidence of thyroid cancer has increased annually over the past 40 years.Approximately 5%of all cases of thyroid cancer are inherited.Familial non-medullary thyroid cancer（FNMTC）is categorized into two groups:syndromic and nonsyndromic.Syndromic FNMTC is associated with predominantly nonthyroid tumors,such as familial adenomatous polyposis,PTEN hamartoma（classic Cowden syndrome）,Carney complex type 1,Werner’s syndrome,and DICER1 syndrome,for which the susceptibility genes have been identified.The other group is nonsyndromic FNMTC,in which the main feature is thyroid cancer;this form accounts for 95%of all the familial cases.Nonsyndromic FNMTC is considered when two or more first-degree relatives are diagnosed with thyroid cancer,but the patient has no risk factors for thyroid cancer.The susceptibility genes for nonsyndromic FNMTC are still unclear,although some low-penetrance susceptibility variants in NKX2-1,FOXE1,HABP2,and SRGAP1 and several candidate chromosomal loci have recently been described but most have not been validated in follow-up studies.We performed whole genome sequencing of peripheral blood DNA samples from two affected family members with FNMTC.CHEK2 transcript expression and the protein levels of CHK2 and p53 were evaluated in the thyroid tissues from two affected members of the kindred and sporadic PTC cases.The entire CHEK2 coding sequence was examined by Sanger sequencing in blood DNA samples from 242sporadic PTC patients.We identified a novel heterozygous germline mutation in CHEK2 ^c.417C→A that was detected in all available affected members of a kindred with FNMTC.This variant was found in only 1 out of 264,200 persons in the Genome Aggregation Database and the NHLBI Trans-Omics for Precision Medicine program.The CHEK2^c.417C→A variant introduces a premature termination codon（Y139X）.We found reduced CHK2 protein expression in tumor samples from the two patients who carried the variant as compared with sporadic cases without the variant.The Y139X loss-of-function variant led to reduced p53 phosphorylation and decreased p53 protein level.In addition,two rare missense variants（R180C and H371Y）in CHEK2 were identified in 5（2%）of 242 patients with sporadic thyroid cancer.Our findings suggest that the CHEK2 ^c.417C→Avariant may be associated with FNMTC.In this study,the Y139X mutation resulted in a strong decrease in mutant m RNA in the affected family members,suggesting that the NMD pathway may be triggered.NMD-triggered loss-of-function causes disease owing to haploinsufficiency,as exemplified by the generation of premature stop codons in the SOX10 gene,causing Waardenburg syndrome.Thus,our results and others’data already mentioned suggest that some CHEK2 mutants may contribute to tumorigenesis through the haploinsufficiency mechanism due to low CHK2 protein levels.Our study highlights DNA repair deficiency as a potential mechanism driving FNMTC susceptibility.In addition,whether the Y139X variant results in cases of FNMTC in other families and confers a predisposition to other hereditary cancers needs to be investigated.Together,our study provides the first evidence identifying that CHEK2 Y139X variant may be associated with FNMTC.Our findings will contribute to the understanding of FNMTC.