Null missense ABCR (ABCA4) mutations in a family with Stargardt disease and retinitis pigmentosa

PURPOSE. To determine the type of ABCR mutations that seg, regate in a fami ly that manifests both Stargardt disease (STGD) and retinitis pigmentosa (R P), and the functional consequences of the underlying mutations. METHODS. Direct sequencing of all 50 exons and flanking intronic regions of ABCR was performed for the STGD- and RP-affected relatives. RNA hybridizat ion, Western blot analysis, and azido-adenosine triphosphate (ATP) labeling was used to determine the effect of disease-associated ABCR mutations in a n in vitro assay system. RESULTS. Compound heterozygous missense mutations were identified in patien ts with STGD and RP. STGD-affected individual AR682-03 was compound heteroz ygous for the mutation 2588G-->C and a complex allele, [W1408R; R1640W]. RP -affected individuals AR682-04 and-05 were compound heterozygous for the co mplex allele [W1408R; R1640W] and the missense mutation V76713, Functional analysis of the mutation V767D by Western blot and ATP binding revealed a s evere reduction in protein expression. In vitro analysis of ABCR protein wi th the mutations W1408R and R1640W showed a moderate effect of these indivi dual mutations on expression and ATP-binding; the complex allele [W1408R; R 1640W] caused a severe reduction in protein expression. CONCLUSIONS. These data reveal that missense ABCR mutations may be associat ed with RP. Functional analysis reveals that the RP-associated missense ABC R mutations are likely to be functionally null. These studies of the comple x allele W1408R; R1640W suggest a synergistic effect of the individual muta tions. These data are congruent with a model in which RP is associated with homozygous null mutations and with the notion that severity of retinal dis ease is inversely related to residual ABCR activity.