Germline CDKN1B loss-of-function variants cause pediatric cushing's disease with or without an MEN4 phenotype

Context: Germline loss-of-function CDKN1B gene variants cause the autosomal dominantsyndrome of multiple endocrine neoplasia type 4 (MEN4). Even though pituitaryneuroendocrine tumors are a well-known component of the syndrome, only 2 cases of Cushing'sdisease (CD) have so far been described in this setting.Aim: To screen a large cohort of CD patients for CDKN1B gene defects and to determine theirfunctional effects.Patients: We screened 211 CD patients (94.3% pediatric) by germline whole-exome sequencing(WES) only (n = 157), germline and tumor WES (n = 27), Sanger sequencing (n = 6), and/orgermline copy number variant (CNV) analysis (n = 194). Sixty cases were previously unpublished.Variant segregation was investigated in the patients' families, and putative pathogenic variantswere functionally characterized.Results: Five variants of interest were found in 1 patient each: 1 truncating (p.Q107Rfs*12) and4 nontruncating variants, including 3 missense changes affecting the CDKN1B protein scatter domain (p.I119T, p.E126Q, and p.D136G) and one 5' untranslated region (UTR) deletion (c.-29-26delAGAG). No CNVs were found. All cases presented early (10.5 ± 1.3 years) and apparentlysporadically. Aside from colon adenocarcinoma in 1 carrier, no additional neoplasms weredetected in the probands or their families. In vitro assays demonstrated protein instability anddisruption of the scatter domain of CDKN1B for all variants tested.Conclusions: Five patients with CD and germline CDKN1B variants of uncertain significance(n = 2) or pathogenic/likely pathogenic (n = 3) were identified, accounting for 2.6% ofthe patients screened. Our finding that germline CDKN1B loss-of-function may present asapparently sporadic, isolated pediatric CD has important implications for clinical screening andgenetic counselling.