Genetic and molecular mechanism for distinct clinical phenotypes conveyed by allelic truncating mutations implicated in FBN1
The molecular and genetic mechanisms by which different single nucleotide variant (SNV) alleles in specific genes, or at the same genetic locus, bring about distinct disease phenotypes often remain unclear. Allelic truncating mutations of fibrillin-1(FBN1) cause either classical Marfan syndrome (MFS) or a more severe phenotype associated with Marfanoid-progeroid-lipodystrophy syndrome (MPLS). A total of three Marfan syndrome/Marfanoid patients (2 singletons and 1 parent-offspring trio) were recruited. Targeted next-generation sequencing was performed on all the participants. We analyzed the molecular diagnosis, patient clinical features, and the potential molecular mechanism involved in the MPLS subject in our cohort. We investigated a small cohort, consisting of two classical MFS and one MPLS patient from China, whose clinical presentation included scoliosis potentially requiring surgical intervention. We provide evidence that most nonsense and frameshift mutations lead to FBN1 null alleles due to mutant mRNA transcript degradation. In contrast, the more severe disease phenotype, MPLS, is caused by mutant mRNAs that are predicted to escape the nonsense mediated decay (NMD) surveillance pathway, making a mutant protein that exerts a dominant negative interference effect to FBN1 thus generating a gain-of-function (GoF) rather than a loss-of-function (LoF) allele as in MFS. Overall, we provide direct evidence that a dominant negative interaction of FBN1 potentially explains the distinct clinical phenotype in MPLS patients through genetic and functional analysis of the first Chinese patient with MPLS. Moreover, our study expands the mutation spectrum of FBN1 and highlights the potential molecular mechanism for MPLS patients.