Marfan syndrome caused by a mutation in FBN1 that gives rise to cryptic splicing and a 33 nucleotide insertion in the coding sequence

We have studied a patient with Marfan syndrome whose mutation was not detec ted by heteroduplex analysis. Primary cultured patient fibroblasts were met abolically labelled and found to secrete fibrillin-1 defectively when compa red with an age-matched control. Sequencing of patient cDNA, isolated by re verse transcription-polymerase chain reaction of patient fibroblast RNA, de tected a 33-bp insertion. The reading frame of the mutant allele was mainta ined and predicted the insertion of 11 amino acids at the beginning of calc ium-binding epidermal growth factor-like domain 29. Direct sequencing of ge nomic DNA detected a heterozygous G(+1)--> A transversion in intron 46 of F BN1. The 11 amino acid insertion was the consequence of the usage of a cryp tic splice site 33-bp downstream of the mutation. This is the first reporte d case of a splicing defect in FBN1 leading to the production of a full-len gth fibrillin-1 transcript containing a large amino acid insertion.