Purpose: To characterize the clinical and electroretinogram (ERG) features of our cohort of patients with Stargardt disease (STGD) exhibiting coding sequence variations in the ABCA4 gene.
Methods: Review of 76 patients with the clinical diagnosis of Stargardt disease/fundus flavimaculatus (STGD/FF) from the University of Iowa Department of Ophthalmology and Visual Sciences (41 patients) and the Casey Eye Institute (35 patients). Clinical examination, Goldmann perimetry, and electroretinography were performed on all 76 patients. Patients were divided into three groups on the basis of their funduscopic and electroretinographic features: (1) a normal ERG by the standards of the laboratory; (2) minimal rod or cone abnormalities; (3) severe ERG dysfunction. The latter category was further subdivided on the basis of a cone-dominated loss of function (C > R or "cone-rod dystrophy") or diffuse depression of rods and cones (C = R). Mutational analysis of the coding sequence of the ABCA4 gene was performed by single strand conformation polymorphism analysis followed by automated DNA sequencing. Each electroretinographic group was analyzed for the presence of disease causing changes using exact tests of binomial proportions corrected for multiple comparisons by Bonferroni method. Quantitative polymerase chain reaction (QPCR) was performed on patients who were homozygous for disease causing changes in the ABCA4 gene to rule out the possibility of deletions.
Results: Overall, 56 of 76 patients (and 77 of 152 alleles) exhibited coding sequence variations that were compatible with high-penetrance disease-causing mutations. The most common of these were His423Arg (9), frameshift mutations (7), Ala1038Val (7), and Pro1380Leu (6). Although no patients with His423Arg presented with normal ERGs, no significant correlation was observed between specific sequence variations and the electroretinographic characteristics or fundus appearance. However, a significantly greater fraction of patients with normal ERG studies failed to exhibit detectable disease-causing coding sequence variations in the ABCA4 gene identified on either allele (P = 0.0006).
Conclusion: STGD/FF patients in our cohort exhibit a wide range of electroretinographic abnormalities, some of which are more prevalent than previously suspected. No direct correlation between clinical appearance, electrophysiologic characteristics and specific ABCA4 alleles could be identified, although a significantly lower number of our cohort with a normal ERG exhibited detectable coding sequence variations in the ABCA4 gene. However, four patients with ERG dysfunction were homozygous for a His423Arg change proven by QPCR not to be an artifact of a deletion. The presence of electrophysiologic dysfunction is not uncommon in our cohort of patients with STGD. Thus, the ERG provides clinically important information of retinal function for STGD/FF and, as such, is still indicated as part of the evaluation of these patients.