Multiple endocrine neoplasia type 5: emerging evidence and clinical perspectives.

Multiple endocrine neoplasia type 5 (MEN5) is a newly proposed hereditary syndrome caused by heterozygous germline loss-of-function mutations in the MAX gene, which encodes MYC-associated factor X (MAX). Initially only linked to pheochromocytoma/paraganglioma (PCC/PGL) syndrome, MAX mutations have also been associated with other tumors. This review synthesizes current knowledge on the discovery, pathogenesis, clinical features, diagnosis, and management of MEN5. The pathogenesis of MEN5 involves disruption of tumor suppressor MAX-MXD heterodimers in the context of MAX loss, leading to increased cell proliferation. Tumor development in MAX-associated disease is consistent with the two-hit mechanism, in which a heterozygous germline mutation is followed by somatic inactivation of the remaining allele. Clinically, MEN5 is characterized by a diverse tumor spectrum, with PCC/PGL and pituitary adenoma being the most common manifestation. Other reported conditions include primary hyperparathyroidism, renal oncocytoma, and neuroblastoma, but the connection between the mutation and development of some of these tumors has not been definitively proven. Evidence from published reports suggests a parent-of-origin effect, with higher penetrance more commonly observed when the pathogenic variant is paternally inherited. Despite recent progress, the epidemiology, penetrance, and genotype-phenotype associations of MEN5 require further investigation. Taken together, MEN5 represents a novel hereditary endocrine neoplasia syndrome with a broader tumor spectrum than initially suspected. The MAX gene is included in several diagnostic gene panels, and endocrinologists should be aware of the associated disorders. The limitations of current knowledge highlight the need for further research to better characterize its clinical features, pathogenesis, and management.