Another mental retardation-associated syndrome is Williams Syndrome. Williams Syndrome (WS) patients exhibit an interesting idiosyncratic social behavioral syndrome—they are overly friendly. They have been described as "never meeting a stranger", that is, they treat even people that they have just met with great familiarity. Behavioral studies of WS patients show that they manifest an abnormal positive bias toward unknown individuals (24). They also exhibit spatio-visual processing deficits and mild to severe mental retardation. Williams Syndrome arises from deletions in chromosome 7 that typically include multiple genes.
Given the complex behavioral manifestations in WS patients, Williams Syndrome provides an interesting example of genetic contributions to complex social behaviors.
One identified gene in the WS locus on chromosome 7 is LIM Kinase 1 (LIMK-1; 25, 26), thus WS is associated with a heterozygous deficiency in LIMK-1. LIMK-1 is a target of the rac, rho, and PKC cascades that controls cytoskeletal organization (see figure). LIMK-1 exerts its effects through phosphorylating and inhibiting Actin Depolymerization Factor (ADF)/cofilin. ADF/cofilin binds directly to actin and promotes actin depolymerization. Thus,
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