Complete abolition of the retinal-specific guanylyl cyclase (retGC-1) catalytic ability consistently leads to leber congenital amaurosis (LCA)

PURPOSE. Leber congenital amaurosis (LCA) is the earliest and the most seve re form of all inherited retinal dystrophies. In 1996, the current investig ators ascribed the disease in families linked to the LCA1 locus on chromoso me 17p13.1 to mutations in the photoreceptor-specific guanylyl cyclase (ret GC-1) gene. So far, 22 different mutations, of which II are missense mutati ons, have been identified in 25 unrelated families. This is a report of the functional analyses of nine of the missense mutations. METHODS. cDNA constructs were generated that contained the retGC-1 missense mutations identified in patients related to the LCA1 locus. Mutants were e xpressed in COS7 cells and assayed for their ability to hydrolyze guanosine triphosphate (GTP) into cyclic guanosine monophosphate (cGMP). RESULTS. All mutations lying in the catalytic domain showed a complete abol ition of cyclase activity. In contrast, only one mutation lying in the extr acellular domain also resulted in a severely reduced catalytic activity, wh ereas the others showed completely normal activity. CONCLUSIONS. More than half the mutations identified in patients related to the LCA1 locus are truncating mutations expected to result in a total abol ition of retGC-1 activity. Concerning missense mutations, half of them lyin g in the catalytic domain of the protein also result in the complete inabil ity of the mutant cyclases to hydrolyze GTP into cGMP in vitro. In contrast , missense mutations lying in the extracellular domain, except one affectin g the initiation codon, showed normal catalytic activity of retGC-1. Nevert heless, considering that all patients related to the LCA1 locus displayed t he same phenotype, it can be assumed that all missense mutations would have the same dramatic consequences on protein activity in vivo as truncation m utations.