, together with BEST2, BEST3
, and BEST4
, are part of a closely related gene family characterized by several transmembrane spanning domains and an invariant arginine-phenylalanine-proline (RFP) motif. These four human genes are believed to be orthologous to a gene in C.elegans
that shares a highly conserved 26-amino acid sequence beginning at position 289.48
Three lines of evidence support the idea that the novel bestrophin variation reported here is disease-causing. First, the glutamic acid residue normally present at position 292 is highly conserved evolutionarily. Second, glutamic acid is negatively charged at neutral pH while the lysine residue found in affected individuals is positively charged. This is the most extreme charge difference possible for a point mutation. Finally, the fact that the mutation was observed in three unrelated families with different bestrophin haplotypes suggests that the variation arose more than once. This makes it very likely that Glu292Lys is the disease-causing variation in the gene and not simply a non-disease-causing polymorphism in linkage disequilibrium with a true disease causing mutation nearby. Mutations are common in the region of the human BEST1
gene encoding amino acids 290–316, suggesting that this portion of the protein is critical to its function.
In the present study, we performed a detailed clinical and electrophysiological evaluation of five subjects from three unrelated families that share a previously undescribed missense mutation in BEST1
, a change from glutamic acid to lysine at amino acid position 292. We observed variable expressivity in our cohort of patients. Only the reduced light-peak on the EOG was completely penetrant. Hyperopia was also found in all probands but was distinctly absent in carriers. Though it is tempting to attribute this to reduced axial length from the elevated fundus lesion, hyperopia was also found in our patients with flat, atrophic retinas, as demonstrated on OCT. Moreover, similar degrees of hyperopia were seen in proband 2 despite atrophy in one eye and a vitelliform lesion in the other. Hyperopia is a common finding in patients with BMD22, 49
and is found in other conditions caused by mutations in BEST1
such as autosomal dominant vitreoretinochoroidopathy50
and autosomal recessive bestrophinopathy.51
Findings of hyperopia in probands may be related to genetic modifiers such as microphthalmia-associated transcription factor (MITF
Interactions between MITF
have not been fully explored. While it is unclear whether our probands were hyperopic before the development of lesions, it would be interesting to compare the prevalence of hyperopia in probands and carriers and determine whether hyperopia is a predictive factor for developing vitelliform lesions. Such a finding would have implications for prognosis and counseling of patients and families. The lack of hyperopia among carriers in our study could indicate a favorable prognostic sign in patients with this mutation.
While proband 1 had mildly diminished visual acuity despite the presence of vitelliform lesions, both his mother and his 57-year-old maternal grandfather had relatively normal fundi despite diminished EOG and presence of the Glu292Lys variation. In addition to emmetropia, normal visual acuity, and lack of fundus lesions, both carriers also had normal ffERG and mfERG responses. This appears to be the first demonstration of non-penetrant fundus findings in two consecutive generations with a confirmed BEST1 mutation. Given the early age at which large vitelliform lesions are typically observed, it seems more likely that the variable clinical findings among patients carrying the same BEST1 variation are due to modifying genes rather than to environmental factors.
The results of OCT and AF performed in our patients allow us to speculate on the nature of the vitelliform lesion, for which histopathology in a human eye has not been performed. Models of BEST1
mutations in dogs54
are not adequate for this purpose as these animals do not develop typical vitelliform lesions. Human eyes with the ‘scrambled egg’ appearance have abnormally high amounts of lipofuscin.17
Studies in elderly individuals homozygous or heterozygous for mutations in BEST1
have demonstrated modestly elevated levels of A2E compared to controls.18
However, it is unknown whether A2E is a by-product of the latter stages of disease or whether other components of lipofuscin contribute to the hyperautofluorescent appearance in early stages. Intense hyperautofluorescence corresponding to the vitelliform lesion was seen in the left eye of proband 2, but a circumscribed patch of decreased AF corresponded to the atrophic lesion with speckled increased AF. In light of histopathological studies of late BMD,15, 16
this loss of AF is likely due to irregularities in the RPE cell monolayer with secondary loss of photoreceptor outer segment turnover correlating with the poor level of vision in this eye. Relatively preserved visual acuity was seen in probands 1 and 2 with vitelliform lesions causing RPE elevations. Notably, OCT revealed no evidence of serous detachment in our patients, as recently reported in BMD patients studied with spectral-domain OCT.55
This contrasts with other studies of autofluorescent vitelliform lesions that showed fluid between the RPE and outer retina.21, 56
One hypothesis accounting for our findings is that mutated bestrophin in RPE, which likely contains lipofuscin in the vitelliform stage, causes impaired transport of fluid to the choroid, resulting in separation of RPE from choroid. Progression causes defective pumping of subretinal fluid to RPE, resulting in the detachment of the outer retina from RPE that was shown in previous studies of eyes with more advanced disease.
Patients with Glu292Lys variation in BEST1 exhibit intra- and interfamilial phenotypic variability. Thus, it is important for clinicians to realize that the identification of the variation by genetic testing does not always portend the eventual development of macular disease. Our findings support the idea that the portion of the protein consisting of amino acids 290–316 may be critical to the function of bestrophin. Relatively good visual acuity with vitelliform lesions can be explained by preservation of the outer retina demonstrated by OCT.