Adrenal GIPR expression and chromosome 19q13 microduplications in GIP-dependent Cushing's syndrome

Abstract

Background GIP-dependent Cushing's syndrome is caused by ectopic expression of glucose-dependent insulinotropic polypeptide receptor (GIPR) in cortisol-producing adrenal adenomas or in bilateral macronodular adrenal hyperplasias. Molecular mechanisms leading to ectopic GIPR expression in adrenal tissue are not known. Methods We performed molecular analyses on 14 adrenal samples obtained from 12 patients with overt GIP-dependent Cushing's syndrome and from one patient with GIP-dependent aldosteronism. Results Nascent RNA FISH showed that GIPR expression in all adenoma and hyperplasia samples occurred through transcriptional activation of a single allele of the GIPR gene. While no abnormality was detected in proximal GIPR promoter methylation, we identified by CGH-array somatic duplications in chromosome region 19q13.32 containing the GIPR locus in the adrenocortical lesions derived from 3 patients. In 2 adenoma samples, the duplicated 19q13.32 region was rearranged with other chromosome regions, whereas a single tissue sample with hyperplasia had 19q duplication only. These duplications were further confirmed by DNA FISH. Among the 5 genes encompassed in the smallest region of overlap, only GIPR was over expressed. We demonstrated in vitro that juxtaposition with cis-acting regulatory sequences such as glucocorticoid response elements in the newly identified genomic environment drives abnormal expression of the translocated GIPR allele in HEK293 T and H295R cells. Conclusion Altogether, our results provide insight into the molecular pathogenesis of GIP-dependent Cushing's syndrome, occurring through monoallelic transcriptional activation of GIPR driven in some adrenal lesions by structural variations.