Bardet-Biedl syndrome and Usher syndrome.

Bardet-Biedl syndrome (BBS) and Usher syndrome (USH) are the most prevalent syndromic forms of retinitis pigmentosa (RP), together they make up almost a quarter of the patients with RP. BBS is defined by the association of retinopathy, obesity, hypogonadism, renal dysfunction, postaxial polydactyly and mental retardation. This clinically complex syndrome is genetically heterogeneous with linkage to more than 6 loci, and 4 genes have been cloned so far. Recent molecular data present evidence that, in some instances, the clinical manifestation of BBS requires recessive mutations in 1 of the 6 BBS loci plus one or two additional mutations in a second BBS locus (tri- or tetra-allelic inheritance). USH is characterized by the combination of congenital or early-onset sensorineural deafness, RP, and variable degrees of vestibular dysfunction. Each of the three clinical types is genetically heterogeneous: 7 loci have been mapped for type 1, three loci for type 2, and two loci for type 3. Currently, 6 USH genes (MYO7A, USH1C, CDH23, PCDH15, USH2A, USH3) have been identified. Pathogenetically, mutations of the USH1 genes seem to result in defects of auditory and retinal sensory cells, the USH 2 phenotype is caused by defects of extracellular matrix or cell surface receptor proteins, and USH3 may be due to synaptic disturbances. The considerable contribution of syndromic forms of RP requires interdisciplinary approaches to the clinical and diagnostic management of RP patients.