Further evidence for FGF16 truncating mutations as the cause of X-linked recessive fusion of metacarpals 4 / 5

Background: Metacarpal 4-5 fusion (MF4; MIM#309630) is a rare congenital malformation of the hand characterized by the partial or complete fusion of the fourth and fifth metacarpals. The anomaly occurs as an isolated trait or part of a genetic syndrome. Recently, we have identified FGF16 nonsense mutations as the underlying cause of isolated X-linked recessive MF4.

Methods: In this report, we provide a detailed clinical description of a sporadic male patient showing MF4 in whom we performed Sanger sequencing of the entire coding sequence of FGF16.

Results: In addition to MF4 symptoms, the patient presented with generalized joint laxity and hypermobility. FGF16 sequencing detected a novel truncating mutation (c.474_477del; p.E158DfsX25) in exon 3 of the gene. A heterozygous mutation was found in a clinically and radiologically unaffected mother of the proband.

Conclusion: Our finding confirms that truncating mutations of FGF16 are causative for X-linked recessive metacarpal 4-5 fusion. Importantly, the mutation detected in this study was located in last exon of the gene (exon 3), like the only two FGF16 disease-causing variants identified to date. Thus, all FGF16 mutations known to give rise to this rare skeletal hand malformation are C-terminal and most probably do not result in a nonsense mediated decay. Additionally, our proband showed mild symptoms of a connective tissue disorder, as some other patients previously reported to have X-linked MF4. Therefore, we suggest that impaired FGF16 function may also be responsible for connective tissue symptoms in MF4 patients.