Pure mucosal neuroma syndrome, not MEN2B

The characteristic phenotypic features of multiple endocrine neoplasia type 2B (MEN2B) include marfanoid body habitus, thickened corneal nerves and mucosal neuromas. Rarely, patients can present with pure phenotypic features of MEN2B but without the RET gene mutation or associated endocrine conditions such as medullary thyroid cancer and phaeochromocytoma. We describe a case of a mid-30s male who was referred by the local optician for assessment of conjunctival lesions. Ophthalmology assessment revealed prominent corneal nerves and conjunctival neuromas. Due to the known association with MEN2B, an urgent referral was made to our endocrinology department. He exhibited a marfanoid body habitus and tongue neuromas. Genetic testing did not reveal a pathogenic variant in the RET proto-oncogene. Exome sequencing revealed a heterozygous pathogenic son of sevenless-1 frameshift mutation, suggestive of pure mucosal neuroma syndrome (MNS). Pure MNS is distinct from MEN2B with a lack of association with other endocrinopathies; therefore, unnecessary prophylactic treatments such as thyroidectomy can be avoided.