Contributions of Somatic Mismatch Repair Defects to a Differential Microsatellite Profile of MEN 2A C-Cell and Adrenal Medullary Hyperplasias

Background: C-cell hyperplasias (CCH) and adrenal medullary hyperplasias (AMH) have been reported genetically heterogeneous in multiple endocrine neoplasia 2A. The contribution of DNA mismatch repair to this profile remains unknown.

Design: Microdissected samples from 22 CCH foci and 34 AMH nodules were selected for loss of heterozygosity and single nucleotide polymorphism analyses. Five polymorphic DNA regions from TP53, RB1, WT1, and NF1 were systematically studied by polymerase chain reaction-denaturing gradient gel electrophoresis. Ki-67 (proliferation) and in situ end labeling (ISEL, apoptosis) and kinetic (MIB-1/ISEL) indices were calculated in each sample. Mismatch repair was assessed by MLH1 and MSH2 sequencing and immunostaining in lesions with ≥2 abnormal microsatellite loci (microsatellite instability-high) and in lesions from a sex and age matched control group (11 foci with ≤1 abnormal microsatellite). Only informative cases were included in the final analysis.

Results: The microsatellite analysis showed microsatellite abnormalities for TP53 (12/20, 60%), RB1 (8/14, 57%) in CCH. In contrast, AMH revealed heterogeneous and lower incidence of microsatellite abnormalities for TP53 (9/31, 29%), RB1 (3/25, 12%), WT1 (9/28, 32%), and NF1 (9/19, 47%). Coexistent microsatellite abnormalities in at least two loci (microsatellite instability-high) were observed in 11 samples (20%, 5 CCH and 6 AMH), always involving TP53 in CCH and NF1 in AMH. A significant decrease of MLH1 or MSH2 protein expression with no gene mutations was identified in lesions with high microsatellite instability. The kinetic index of lesions was significantly higher in lesions with microsatellite instability-high (261.25) than in lesions with no or one abnormal microsatellite (169.38), due to a significantly lower ISEL index.

Conclusions: 1. Somatic down-regulation of mismatch repair proteins contributes to both the accumulation of microsatellite lesions and genetic heterogeneity in MEN 2A lesions, resulting in a differential microsatellite profile (predominating TP53 in CCH and NF1 in AMH). 2. MEN 2A genetically instable lesions reveal the most advantageous kinetic profile, essentially due to down-regulated apoptosis.