1. Epigenomic and Cancer SDHC Promoter Methylation, A

ID: 00770 Type: COMUNICACIÓN ORAL Topic: 1. Epigenomic and cancer

SDHC promoter methylation, a novel pathogenic mechanism in parasympathetic paragangliomas

CRISTOBAL BERNARDO-CASTIÑEIRA1 , NURIA VALDES2 , MARIO F. FRAGA3 , MARIA DOLORES CHIARA1

1) Hospital Universitario Central de Asturias (HUCA), Instituto de Investigación Sanitaria del Principado de Asturias (ISPA), CIBERONC, Instituto Universitario de Oncología del Principado de Asturias (IUOPA), Universidad de Oviedo 2) Hospital Universitario Central de Asturias (HUCA), Instituto de Investigación Sanitaria del Principado de Asturias (ISPA) 3) Hospital Universitario Central de Asturias (HUCA), Instituto de Investigación Sanitaria del Principado de Asturias (ISPA), Nanomaterials and Nanotechnology Research Center (CINN-CSIC) , Universidad de Oviedo

Introduction:

Paragangliomas (PGLs) are rare neuroendocrine tumors that arise from paraganglia tissue. Approximately, 40% of PGLs are familiar due to germline mutations in about 10 susceptibility genes. Germline mutations in the succinate dehydrogenase A, B, C, and D genes (collectively, SDHx) typically predispose to the development of PGLs arising at the paraganglias of parasympathetic or sympathetic neuroendocrine systems. These gene alterations cause loss of function of the mithocondrial complex II which is the base for the tumor development via not well defined mechanisms. However, little is known about pathogenic events that promote tumor development, in the non- inherited PGLs.Although SDHB immnostaining of tumors with SDHx mutations is typically negative absence of SDHB-immunostaining has also been found in a rare subset of sporadic PGLs that lack SDHx mutations and other mutations in known susceptibility genes. This provides an opportunity to unravel mechanisms, others than genetic alterations, involved in SDH silencing which could be involved in the pathogenesis of sporadic PGLs.

Objectives:

To investigate the molecular pathogenic events in a sporadic case of PGL without any mutations in known susceptibility genes and negative SDHB immunostaining (hereafter SDHBneg/SDHxwt). To this end, we performed a comprehensive molecular characterization of the tumor and compared genetic/epigenetic events with those of healthy paraganglia and other PGLs with or without SDHx mutations.

Methods:

We performed integrated approaches of multi-platform data that include exome sequencing, genome-wide methylation array, DNA bisulfite modification and pyrosequencing, CGH array, MLPA, immunohistochemistry and RT-qPCR using tumor and blood samples of a SDHBneg/SDHxwt . We compared those data with those of other PGLs with or without SDHx mutations.

Conclusions:

The genomic data revealed somatic SDHC promoter hypermethylation and loss of the 1q23.3 region containing the SDHC gene. This correlated with decreased SDHC mRNA and protein levels. Another genetic event affected the VHL gene which showed decreased DNA copy number (CN), associated with low VHL mRNA levels, and absence of VHL protein detected by immunohistochemistry. Additionally, the tumor displayed a pseudohypoxic phenotype consisting in over-expression of the hypoxia- inducible factor (HIF-1?) and miR-210, and down- regulation of the iron-sulfur cluster assembly enzyme (ISCU) involved in SDHB maturation. This profile resembles that of SDHx- or VHL-mutated PGLs but not of PGLs with decreased VHL-CN, pointing to SDHC rather than VHL as the pathogenic driver. Collectively, these findings demonstrate the potential importance of both, the SDHC epigenomic event and the activation of the HIF-1?/miR-210/ISCU axis in the pathogenesis of SDHBneg/SDHxwt PGLs. Moreover, this is the first case of a sporadic parasympathetic PGL that carries silencing of SDHC, fulfilling the two-hit Knudson’s model for tumorigenesis.