Extra-endocrine phenotypes at infancy in multiple endocrine neoplasia type 2B: A case series of six Japanese patients.

Multiple endocrine neoplasia type 2B (MEN2B) is an extremely rare disease, most often caused by a de novo p.Met918Thr RET mutation. Medullary thyroid carcinoma of MEN2B has a good prognosis if diagnosed by one year of age. However, diagnosis of MEN2B within the first year of life is markedly challenging owing to its high de novo occurrence and lack of clarity in terms of extra-endocrine symptoms that could aid early diagnosis. Herein, we present six cases of Japanese children with MEN2B harboring the p.Met918Thr RET variant. Exploratory data extraction was conducted using a questionnaire. The patients underwent thyroidectomy at a median age of 11 yr (range, 6-19 yr). Four of the six patients underwent neonatal hospitalization at birth without complications, and three tested positive for neuroblastoma screening at infancy. The patients presented at least one MEN2B-associated symptom before one year of age, including ganglioneuromas, pseudo-Hirschsprung disease, alacrima, bumpy lips, sucking disability, or decreased muscle tone, along with other suspected comorbidities, such as Williams or Prader-Willi syndrome. This case series demonstrates that MEN2B manifests through several extra-endocrine symptoms by the age of one year.