Alterations of the SDHD gene locus in midgut carcinoids

Mutation screening in a Norwegian cohort with pheochromocytoma

Pheochromocytomas (PHEOs) are neuroendocrine tumours, originating from chromaffin cells in the adrenal medulla. They are either sporadic or hereditary. It is important to identify the hereditary cases, so that patients and relatives with germline mutations can be offered regular surveillance. The objective of this study was the detection of pathogenic germline mutations in a cohort of Norwegian PHEO patients. Blood samples and/or formalin-fixed, paraffin-embedded tissue specimens, were collected from 60 patients who were operated upon between 1986 and 2004 at two university hospitals in Norway. DNA mutation analyses were performed successfully in the 42 blood samples and in one of the paraffin-embedded tissue specimen in VHL, RET, SDHB, SDHC, SDHD and NF1. In all, 32 different DNA variants were observed, of which 8 were classified as pathogenic (19 %), or possibly pathogenic; three in NF1, two in RET and VHL and one in SDHB. Two variants were observed in one patient, one in SDHB and one in NF1. Three of these variants are, to the best of our knowledge, new ones; two in NF1 [c.950_51insGCTGA, (p.Glu318LeufsX59) and c.1588G > A, (p.Val530Ile)] and one in VHL (c.308C > T, p.Pro103Leu). In conclusion the overall incidence of germline mutations in genes associated with familial PHEO was found to be of the same order of magnitude in the present Norwegian series as in those from other countries. Two new NF1 variants and one new VHL gene variant were detected.

No evidence for involvement of SDHD in neuroblastoma pathogenesis

BACKGROUND

Deletions in the long arm of chromosome 11 are observed in a subgroup of advanced stage neuroblastomas with poor outcome. The deleted region harbours the tumour suppressor gene SDHD that is frequently mutated in paraganglioma and pheochromocytoma, which are, like neuroblastoma, tumours originating from the neural crest. In this study, we sought for evidence for involvement of SDHD in neuroblastoma.

METHODS

SDHD was investigated on the genome, transcriptome and proteome level using mutation screening, methylation specific PCR, real-time quantitative PCR based homozygous deletion screening and mRNA expression profiling, immunoblotting, functional protein analysis and ultrastructural imaging of the mitochondria.

RESULTS

Analysis at the genomic level of 67 tumour samples and 37 cell lines revealed at least 2 bona-fide mutations in cell lines without allelic loss at 11q23: a 4bp-deletion causing skip of exon 3 resulting in a premature stop codon in cell line N206, and a Y93C mutation in cell line NMB located in a region affected by germline SDHD mutations causing hereditary paraganglioma. No evidence for hypermethylation of the SDHD promotor region was observed, nor could we detect homozygous deletions. Interestingly, SDHD mRNA expression was significantly reduced in SDHD mutated cell lines and cell lines with 11q allelic loss as compared to both cell lines without 11q allelic loss and normal foetal neuroblast cells. However, protein analyses and assessment of mitochondrial morphology presently do not provide clues as to the possible effect of reduced SDHD expression on the neuroblastoma tumour phenotype.

CONCLUSIONS

Our study provides no indications for 2-hit involvement of SDHD in the pathogenesis of neuroblastoma. Also, although a haplo-insufficient mechanism for SDHD involvement in advanced stage neuroblastoma could be considered, the present data do not provide consistent evidence for this hypothesis.