Novel insights into the molecular pathogenesis ofCYP4V2‐associated Bietti's retinal dystrophy

Abstract

Bietti's crystalline dystrophy (BCD) is a rare, autosomal recessive retinal degenerative disease associated with mutations in CYP4V2. In this study, we describe the genetic and clinical findings in 19 unrelated BCD patients recruited from five international retinal dystrophy clinics. Patients underwent ophthalmic examinations and were screened for CYP4V2 mutations by Sanger sequencing and quantitative polymerase chain reaction (qPCR) copy number variation screening. Eight CYP4V2 mutations were found in 10/19 patients, including three patients in whom only monoallelic mutations were detected. Four novel mutations were identified: c.604G>A; p.(Glu202Lys), c.242C>G; p.(Thr81Arg), c.604+4A>G; p.(?), and c.1249dup; p.(Thr417Asnfs*2). In addition, we identified a heterozygous paternally inherited genomic deletion of at least 3.8 Mb, encompassing the complete CYP4V2 gene and several other genes, which is novel. Clinically, patients demonstrated phenotypic variability, predominantly showing choroidal sclerosis, attenuated vessels, and crystalline deposits of varying degrees of severity. To our knowledge, our study reports the first heterozygous CYP4V2 deletion and hence a novel mutational mechanism underlying BCD. Our results emphasize the importance of copy number screening in BCD. Finally, the identification of CYP4V2-negative patients with indistinguishable phenotypes from CYP4V2-positive patients might suggest the presence of mutations outside the coding regions of CYP4V2, or locus heterogeneity, which is unreported so far.

Detailed functional and structural phenotype of Bietti crystalline dystrophy associated with mutations in CYP4V2 complicated by choroidal neovascularization

“…CYP4V2 -associated BCD shows a remarkably consistent phenotype despite reported variability in disease severity. 11, 21-24 In addition to the nearly pathognomonic presence of retinal crystals, other features shared amongst patients include the presence of very fine corneal crystals, nyctalopia and pericentral scotomas with relative preservation of foveal vision and foveal centers reflective of the underlying early regional predilection of the disease to the parafovea/perifovea with extension into the peripapillary and midperipheral retina. 8, 21, 25-31 We found RPE depigmentation on fundus exam limited to the retina immediately nasal to the fovea.…”

Genetics of Bietti Crystalline Dystrophy

“…49,55 Interestingly, clinical symptoms of BCD remain only in the eyes. The link between altered 32 Yin et al, 36 Meng et al, 59 Manzouri et al, 64 Shan et al 60 15 3 c.332T>C p.I111T Missense Li et al, 55 Astuti et al, 63 Rossi et al, 61 García-García et al, 65 Haddad et al, 66 Rossi et al 67 10 Lin et al, 11 Li et al, 17 Gocho et al, 27 Halford et al, 32 Yin et al, 36 Li et al, 55 Xiao et al, 57 Meng et al, 59 Astuti et al, 63 Nakamura et al, 68 Lee et al, 69 Chung et al, 70 Gekka et al, 71 Jin et al, 72 Liu et al, 73 Shan et al, 60 Tian et al, 62 Wada et al, 74 10 Li et al, 17 Li et al, 55 Xiao et al, 57 Meng et al, 59 36 Meng et al, 59 Mamatha et al 77 43 9 c.1091-2A>G Exon9del Splice site Li et al, 17 Yin et al, 36 Li et al, 55 Xiao et al, 57 Meng et al, 59 Shan et a...…”

“…The positively charged R400, located in the β-strand after K helix, is considered a hotspot for gene mutations, as substitutions for C and H have been reported in this residue and they have been predicted to alter heme coordination. 10,26,57,60,62,63 In addition, the D324V substitution in the central I helix neutralizes the negative charge in this residue and will alter the local isoelectric point (pI) around the active site. Furthermore, insertion or deletion of a proline residue has been predicted to influence the stability of the protein heme coordination-dependent secondary structures, which explains why H331P (in the central I helix) and P396L (in the β-strand after K helix) are the causative mutations in BCD.…”

“…Furthermore, insertion or deletion of a proline residue has been predicted to influence the stability of the protein heme coordination-dependent secondary structures, which explains why H331P (in the central I helix) and P396L (in the β-strand after K helix) are the causative mutations in BCD. 10 Astuti et al 63 49 9 c.1199G>A p.R400H Missense Halford et al, 32 Xiao et al, 57 Shan et al, 60 Tian et al 62 Despite the similar aromaticity and steric hindrance between tyrosine and histidine, Y219H could possibly influence the interaction of F helix with other secondary structures by introducing a charge from histidine into this position, depending on the local pI.…”

“…10,32,36,61,63,69 Our group has previously evaluated the genotype-phenotype correlation in a group of 18 Chinese patients in 13 families and showed that BCD with homozygous IVS6-8del17bp/insGC (c.802-8del17bp/insGC) or compound heterozygous IVS6-8del17bp/insGC and IVS8-2A_G (c1091-2A>G) mutations seemed to have a more severe disease phenotype based on electrophysiological testing. 10 Deletion at the exon 7 splice acceptor site causes an in-frame deletion of exon 7, resulting in the expression of a truncated protein.…”