De Novo Balanced Translocations Disrupting the FBN1 Gene Diagnosed by Genome Sequencing: An Uncommon Cause of Marfan Syndrome Modifying Genetic Counseling.

Marfan syndrome (MFS) is a well-characterized rare genetic connective tissue disorder. The features of MFS are primarily skeletal, ocular, and cardiovascular and are mainly caused by single-nucleotide variants (SNVs) in the FBN1 gene (MIM#134797) located on chromosome 15q21.1. We describe two patients, a 26-year-old male and a 10-year-old female from unrelated distinct families, with clinically diagnosed sporadic MFS. After years of unsuccessful molecular diagnosis for genetic counseling purposes, genome sequencing was performed and revealed a balanced translocation in both patients: de novo t(9;15)(p13.3;q21.1) translocation in the male patient, and de novo t(15;16)(q21.1;p13.13) translocation in the female patient, respectively, disrupting intron 40 and 45 of FBN1. The other breakpoints were not clinically relevant. These translocations were confirmed by specific fluorescence in situ hybridization probes and conventional karyotyping. In the literature, only one family has been described, leading to four cases of MFS caused by balanced translocations. Genetic counseling for balanced translocations differs from SNVs and even interstitial deletions since it is not restricted to MFS recurrence, but also involves the risk of unbalanced gametes, leading to miscarriage or unbalanced progeny. In case of clinical certainty, MFS patients should be screened for balanced translocations to ensure appropriate genetic counseling.