523 Deep Phenotyping Reveals Germline Determinants of Disease Severity in Neurofibromatosis Type 2

Abstract

INTRODUCTION: Although germline nonsense and frameshift mutations are hypothesized to confer more severe disease in neurofibromatosis type 2 (NF2), the mechanisms of genotype-phenotype association are unknown. METHODS: Patients (n = 169) with NF2 were enrolled in a natural history study (NCT NCT00598351). Deep phenotypic profiles were created (serially for 5 years) including clinical evaluations, self-reported functional measures, lifetime interventions (surgical, radiation and drug treatments), and imaging (tumor number, type, and volume). CLIA certified germline mutation analyses (n= 69) were supplemented with a custom PCR panel (NF2, LZTR1, SMARCB1, SMARCE1, and SUFU). Data were analyzed (QIAGEN CLC Genomics Workbench) for mutation type, location, and predicted protein effect (truncating/non-truncating). Low frequency variant detection was used to detect mosaicism. RESULTS: Germline mutations (nonsense-36%, splice-site-16%, and large deletions-16%) were grouped by predicted protein effect (truncating-41%, non-truncating-27%, and mosaic-33%) and mapped to the chromosomal locus. As predicted, disease severity increased in patients during the study (mean increase of 1.9 tumors/5-yrs and 3,793mm3/5-yrs). However, the disease severity was not associated with age at study enrollment (tumor number/volume) (p = 0.15/0.58). We found that truncating mutations in exons 8, 11-12 were associated with a higher tumor burden (p = 0.03). Truncating mutations at exon 8 were also associated with schwannomas (p = 0.04). Change in tumor volume during the study was predicted by younger age (p < 0.01) and truncating mutation (p = 0.05). Unlike prior studies, mortality was not predicted by truncating mutation type. CONCLUSIONS: We deeply phenotyped a diverse cohort of patients with NF2 and found that the location of truncating mutation determined the phenotype. We suspect that truncating mutations at exons 8 & 11-12 disrupt the transition from highly conserved FERM domain to the alpha-helix region leading to a severe NF2 phenotype.