Background: The enzyme succinate dehydrogenase (SDH) functions in the citric acid cycle and loss of function of this enzyme can lead to the development of phaeochromocytoma/paraganglioma (PPGL), gastrointestinal stromal tumour (GIST) and renal cell carcinoma. A germline mutation in one of the four genes (SDH-A/B/C/D) encoding the SDH complex is the most common mechanism of SDH inactivation causing SDH deficiency and is routinely screened for in clinical practice. SDH deficiency can also arise due to epigenetic silencing of the SDHC gene, but clinical testing for an SDHC epimutation is not widely available in the UK or Europe.
Objectives: I) To develop a pyrosequencing based assay for the detection of SDHC epimutations ii) to identify diagnostic pathways for the detection of an SDHC epimutation in clinical practice.
Design: SDHC promoter methylation analysis of 32 paraffin embedded tumours (including 17 GIST and 15 PPGL), matched normal tissue and germline DNA was performed using a pyrosequencing technique and correlated with SDHC gene expression.
Results: SDHC promoter methylation was identified in 18.7% (6 /32). All 6 cases had a presenting diagnosis of SDH deficient wtGIST and 3/6 cases had a multiple tumour syndrome including GIST, PPGL and pulmonary chondromas. SDHC hypermethylation correlated with reduced expression of the SDHC gene. No case of constitutional SDHC promoter hypermethylation was detected on analysis of germline DNA samples. Whole genome sequencing of germline DNA from three cases with a SDHC epimutation, did not identify any causative sequence anomalies to account for the focal hypermethylation in the SDHC promoter region in the tumours of these three cases.
Conclusion: This analysis has enabled us to recommend a diagnostic workflow for the detection of an SDHC epimutation in a service setting with the aim of improving the diagnosis, long term management and possible treatment options for patients with SDH deficient tumours in the future.