We also made observations on the ultrastructural appearance of the macular RPE in eyes from two distinct genetic forms of Best disease (T6R and Y227N). The patient with the T6R mutation was the father of a 28-year-old eye donor described in a previous report,
32 who was found to have remarkable accumulations of lipofuscin in the RPE. The donor in the current report with a T6R mutation similarly exhibited substantial accumulations of lipofuscin within the RPE
(Fig. 3) . In contrast, transmission electron microscopy of an eye from a donor with the Y227N mutation, who had clinical manifestations of Best disease with peripheral flecks after the sixth decade of life, showed typical lipofuscin accumulation in the RPE of a 93-year-old donor
(Fig. 3) . There are at least two explanations for the discrepancy in the appearance of these samples. First, it is possible that different mutations in the
VMD2/Best1 gene result in different phenotypes. For example, different mutations in the
rds/peripherin gene can alternatively cause peripheral or macular retinal degeneration.
48 Similarly, in the case of
VMD2, numerous missense mutations have been described in patients with Best disease, whereas mutations that affect splicing are associated with ADVIRC.
42 It is also possible that the physiological effects of distinct mutations that cause Best disease may be variable. This possibility is borne out by in vitro studies in which the W93C and the R218C mutations had distinct effects on the kinetics of calcium-channel activation and inactivation, though with a similar outcome.
23 The second possible explanation for the dissimilarity in the lipofuscin accumulations among patients with different bestrophin changes is that one or more modifier genes are likely to greatly affect the expressivity of Best disease. For example, some patients with a specific bestrophin mutation within a family have vitelliform lesions from a very young age, whereas in others with the same mutation, vitelliform lesions may develop only with advanced age.
49 50 Hence, the presence of modifier genes that affect the expressivity of specific bestrophin mutations is inferred. The identification of these modifier genes and the mechanism(s) by which they affect Best disease expressivity will lead to better understanding of the disease process and to improved diagnosis and treatment.