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Complete Abolition of the Retinal-Specific Guanylyl Cyclase (retGC-1) Catalytic Ability Consistently Leads to Leber Congenital Amaurosis (LCA)

From the Unité de Recherche sur les Handicaps Génétiques de l’Enfant, Insitut National de la Santé et de la Recherche Médicale U393, Hôpital des Enfants Malades, Paris, France.

PURPOSE. Leber congenital amaurosis (LCA) is the earliest and the most severe form of all inherited retinal dystrophies. In 1996, the current investigators ascribed the disease in families linked to the LCA1 locus on chromosome 17p13.1 to mutations in the photoreceptor-specific guanylyl cyclase (retGC-1) gene. So far, 22 different mutations, of which 11 are missense mutations, 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 expressed 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 abolition of cyclase activity. In contrast, only one mutation lying in the extracellular domain also resulted in a severely reduced catalytic activity, whereas 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 abolition of retGC-1 activity. Concerning missense mutations, half of them lying in the catalytic domain of the protein also result in the complete inability of the mutant cyclases to hydrolyze GTP into cGMP in vitro. In contrast, missense mutations lying in the extracellular domain, except one affecting the initiation codon, showed normal catalytic activity of retGC-1. Nevertheless, considering that all patients related to the LCA1 locus displayed the same phenotype, it can be assumed that all missense mutations would have the same dramatic consequences on protein activity in vivo as truncation mutations.

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