Mutations responsible for Larsen syndrome cluster in the FLNB protein

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Abstract
A gene for Larsen syndrome was recently described, and mutations were reported in five cases. To test whether mutations in this gene, FLNB, could explain the disease in our independent collection of sporadic and dominant Larsen syndrome cases; and to test whether mutations occurred in a non-random pattern. Missense mutations were found in each of five cases. Four of the five were new; one was reported in a sporadic case in the original Larsen syndrome study of five cases. All mutations from the two studies were compiled. Clustered mutations were observed within three filamin B protein domains: the calponin homology 2 domain, repeat 14, and repeat 15. This suggested that as few as five (of the total of 46) coding exons of FLNB could be screened to detect Larsen syndrome mutations. Four of these exons were screened in a sixth (sporadic) case and a previously reported G1691S substitution mutation detected. Mutations in FLNB may be responsible for all cases of Larsen syndrome. They appear to occur in specific functional domains of the filamin B protein. This should simplify diagnostic screening of the FLNB gene. Analyses in larger patient series are warranted to quantify this. The study confirmed the extreme variability in clinical presentation and the presence of unaffected carriers. A molecular screen would be valuable for diagnosis and genetic counselling.