Functional characterization of missense mutations at codon 838 in retinal guanylate cyclase correlates with disease severity in patients with autosomal dominant cone-rod dystrophy
Se. Wilkie et al., Functional characterization of missense mutations at codon 838 in retinal guanylate cyclase correlates with disease severity in patients with autosomal dominant cone-rod dystrophy, HUM MOL GEN, 9(20), 2000, pp. 3065-3073
Three different mutations in codon 838 of GUCY2D, the gene for retinal guan
ylate cyclase 1, have been linked to autosomal dominant cone-rod dystrophy
at the CORD6 locus. To examine the relationship between enzyme activity and
disease severity, the three disease-causing substitutions (R838C, R838H an
d R838S) and four artificial mutations (R838A, R838E, R838L and R838K) were
generated. Assay of GCAP1-stimulated cyclase activity in vitro shows that,
compared with wild-type, R838E, R838L and R838K possess only low activity,
whereas R838A, R838C, R838H and R838S have activity equal or superior to w
ild-type at low Ca2+ concentrations. These four latter mutants showed a hig
her apparent affinity for GCAP1 than did wild-type. The Ca2+ sensitivity of
the GCAP1 activation was also altered with marked residual activity at hig
h Ca2+, the effect increasing: wildtype < R838C < R838H << R838A < R838S. W
ithin the photoreceptor, this would result in a failure to inactivate cycla
se activity at high physiological Ca2+ concentrations. Amongst the three di
sease-associated mutations, the effect correlates directly with disease sev
erity. The wild-type and R838H mutant displayed a difference in pH sensitiv
ity, with the mutant showing a higher specific activity with pH > 6.0. Site
838 is in the dimerization domain that forms a coiled-coil in the active p
rotein, A computer-aided structure prediction of this region indicates that
R838 in the wild-type breaks the structure at four helical turns, and ther
e is an increasing tendency for the structure to continue for further turns
in the order R838C < R838H,S,K << R838E < R838A < R838L.