Alterations of RET oncogene in human adrenal tumors

Review: Should patients with apparently sporadic pheochromocytomas or paragangliomas be screened for hereditary syndromes?

CONTEXT

The recent identification of germline mutations of the mitochondrial complex II genes in variants of paraganglioma/pheochromocytoma syndrome has enlarged the number of known causative genes for hereditary pheochromocytoma. A question confronting clinicians is whether they should screen patients with apparently sporadic pheochromocytomas for unsuspected germline mutations of some or all of the seven genes known to cause hereditary paraganglioma or pheochromocytoma (NF1, VHL, RET, MEN1, SDHD, SDHC, and SDHB). A positive answer was suggested by a report that placed the estimate of hereditary disease in apparently sporadic pheochromocytoma as high as 24%.

EVIDENCE ACQUISITION

We applied clinically useful criteria to a review of the literature, defining cases of apparently sporadic pheochromocytoma as those without a suspicious personal or family history, with a focal, unilateral pheochromocytoma, and presenting at age less than 50 yr.

EVIDENCE SYNTHESIS

We reduced the overall estimate of unsuspected hereditary pheochromocytoma patients with apparently sporadic pheochromocytoma to approximately 17%. Mutations in only three genes (VHL, SDHB, and SDHD) accounted for almost this entire minority, and unsuspected RET mutation was rare. Costs, coverage by insurance, the potential effect on insurability, and deficient information for populations outside of referral centers should be considered before recommending genetic testing in patients with apparently sporadic presentations of pheochromocytomas.

CONCLUSION

We recommend genetic testing for patients with an apparently sporadic pheochromocytoma under the age of 20 yr with family history or features suggestive of hereditary pheochromocytoma or for patients with sympathetic paragangliomas. For individuals who do not meet these criteria, genetic testing is optional.

The molecular pathogenesis of hereditary and sporadic adrenocortical and adrenomedullary tumors

Modern imaging modalities lead to frequent detection of adrenal masses, most of them incidental findings. Although the majority of adrenocortical and adrenomedullary tumors are benign, there are no reliable clinical and laboratory markers to distinguish most of them from malignant neoplasms. The molecular mechanisms underlying the pathogenesis of these tumors have recently begun to be unraveled. A fruitful avenue for the elucidation of tumorigenesis has been the study of adrenal tumors that are manifestations of hereditary or postzygotic genetic syndromes, because one knows the "first hit", i.e. the primary gene defect. In contrast, in sporadic adrenal tumors the first hit, possibly a somatic mutation of a tumor-related gene, is unknown, and therefore the sequence of genetic alterations is difficult to establish. In this article we review in addition to our own work the literature on molecular aspects of adrenocortical and adrenomedullary tumorigenesis.

Mutant K-ras oncogene regulates steroidogenesis of normal human adrenocortical cells by the RAF-MEK-MAPK pathway

The result of our previous study has shown that the K-ras mutant (pK568MRSV) transfected human adrenocortical cells can significantly increase cortisol production and independently cause cell transformation. The aim of this study is to investigate the effect of the active K-ras oncogene on the cortisol production in normal human adrenocortical cells. First we used isopropyl thiogalactoside to induce the inducible mutant K-ras expression plasmid, pK568MRSV, in the stable transfected human adrenocortical cells. The result showed that the increase of RasGTP levels in transfected cells was time-dependent after isopropyl thiogalactoside induction. Additionally, results from Western blot analysis revealed significant elevation in phosphorylation of c-Raf-1 and Mitogen-activated protein kinase. We also detected the levels of mRNA encoding Cholesterol side-chain cleavage enzyme (P450(SCC)), 17alpha-Hydroxylase/17,20-lyase (P450(c17)) and 3beta-Hydroxysteroid dehydrogenase (3betaHSD) were increased in human adrenocortical cells transfected with mutant K-ras after IPTG treatment. The increase of mRNA amount in P450(scc) P450(c17) and 3betaHSD and the elevation of cortisol level were inhibited with a pretreatment of PD098059, a specific extracellular signal-regulated kinase inhibitor. In our previous report, we proved that lovastatin, a pharmacological inhibitor of p21(ras) function, also reversed the increase of cortisol level in mutant K-ras stably transfected human adrenocortical cells. Taken together, these findings proved that the active mutant Ras enhanced not only cell proliferation but also steroidogenesis in steroidogenic phenotype cells by activating Raf-MEK-MAPK related signal transduction pathway. Therefore, we believe that K-ras mutants influence regulation of steroidogenesis in adrenocortical cells through RAF-MEK-MAPK pathway.

Morphologic change and elevation of cortisol secretion in cultured human normal adrenocortical cells caused by mutant p21K-ras protein

In our previous study on the tumorigenesis of human functional adrenal tumors, we observed a high frequency of K-ras point mutations in clinical specimens. Furthermore, we cloned the mutated K-ras gene from the tumors and inserted it into vectors to transfect normal bovine adrenocortical cells to express the mutated K-ras gene. The mRNA level of steroidogenic enzymes such as cholesterol sidechain cleavage enzyme (P450SCC), 17alpha-hydroxylase/17,20-lyase (P450c17), and 3beta-hydroxysteroid dehydrogenase (3betaHSD) in the mutant K-ras stably transfected cells were elevated. Cultured normal adrenocortical cells from donors and patients with adrenocortical tumors were then transfected with mutant K-ras expression plasmids constructed from human adrenal tumors. Stable transfectants grew faster than normal cells. Additionally, morphologic change was observed in the transfected cells. Moreover, when the synthesis of hormones was analyzed, the mRNA of P450SCC, P450C17, and 3betaHSD was found to have increased, and the level of cortisol was 18 to 25 times that in control cells. The increased steroid hormone production in mutant K-ras-transfected cells was reversed by lovastatin, a pharmacologic inhibitor of p21ras function. These results, combined with previous reports of steroidogenic K-ras in bovine adrenocortical cells, suggest that the K-ras oncogene is involved in steroidogenesis in human adrenocortical cells.

Genetic background of adrenocortical tumor development

The increasing occurrence of incidentally discovered benign adrenocortical tumors has become a clinical dilemma because of the difficulties in differentiating them from their malignant counterpart. Adrenocortical tumors are associated with familial cancer syndromes such as the Beckwith-Wiedemann syndrome, the Li-Fraumeni syndrome, the Carney complex, multiple endocrine neoplasia type 1, congenital adrenal hyperplasia, and the McCune-Albright syndrome. Genetic events are known to take place on the chromosomal and gene level in sporadic adrenocortical tumors.