Novel FBN1 intron variant causes isolated ectopia lentis via in-frame exon skipping

Mutations in fibrillin-1 (FBN1) cause various clinical conditions, such as Marfan syndrome (MFS). However, the genotype-phenotype relationships underlying MFS and other conditions relevant to FBN1 mutations have not been fully elucidated. We performed whole-exome sequencing on three participants, including an affected mother-daughter pair, in a three-generation Japanese family with isolated ectopia lentis (IEL). The sequencing identified a novel single-nucleotide variant (c.1327+3A>C) in intron 11 of FBN1 that was shared between the two patients. We confirmed the co-segregation of the variant with IEL in two additional affected relatives in the family. The Combined Annotation-Dependent Depletion score of the variant was 26.1, which was indicated by SpliceAI to influence splicing, with a score of 0.93. Reverse transcription-polymerase chain reaction (RT-PCR) of mRNAs isolated from peripheral blood mononuclear cells revealed aberrant bands in all four affected individuals. Subsequent sequencing revealed that these bands originated from FBN1 transcripts lacking exon 11. The causality of the variant in the skipping of exon 11, which results in an in-frame deletion of 60 amino acids corresponding to the "hinge" region of FBN1 protein, was confirmed in a minigene experiment. Interestingly, the same result was observed for a minigene for c.1327+1G>A, a variant previously identified in two unrelated EL families without MFS manifestations. These results suggest that the c.1327+3A>C mutation in FBN1 likely leads to IEL. The findings expand our knowledge of FBN1 and provide insights into FBN1-related diseases.