Biallelic mutation of protocadherin-21 (PCDH21) causes retinal degeneration in humans

Abstract

Purpose: To describe the clinical findings and mutations in affected members of two families with an autosomal recessive retinal dystrophy associated with mutations in the protocadherin-21 (PCDH21) gene.

Methods: A full genome scan of members of two consanguineous families segregating an autosomal recessive retinal dystrophy was performed and regions identical by descent identified. Positional candidate genes were identified and sequenced. All patients had a detailed ophthalmic examination, including electroretinography and retinal imaging.

Results: Affected members of both families showed identical homozygosity for an overlapping region of chromosome 10q. Sequencing of a candidate gene, PCDH21, showed two separate homozygous single-base deletions, c.337delG (p.G113AfsX1) and c.1459delG (p.G487GfsX20), which were not detected in 282 control chromosomes. Affected members of the two families first reported nyctalopia in late teenage years and retained good central vision until their late 30s. No color vision was detected in any proband. The fundus appearance included the later development of characteristic circular patches of pigment epithelial atrophy at the macula and in the peripheral retina.

Conclusions: Biallelic mutations in the photoreceptor-specific gene PCDH21 cause recessive retinal degeneration in humans.

Figure 1

The pedigrees of the two families that took part in the study (denoted family 1 and 2 in the text) are displayed. Open and closed symbols denote unaffected and affected individuals, respectively. Deceased family members are denoted by diagonal lines; arrows indicate the probands in each family. A horizontal double line between parents indicates consanguinity. Panel A is a pedigree drawing of family 1, a first cousin consanguineous middle-eastern family, with four affected siblings. Panel B shows the nuclear family who took part in the study, and is part of a larger consanguineous pedigree not displayed or available for the study; the parents illustrated are first cousins.

Figure 2

Electropherograms of the mutations detected in Protocadherin-21 (PCDH21). Panel A illustrates the index c.338delG mutation in family 1; the wild-type (WT) sequence is displayed on the top row. The middle row shows the proband in family 1 (IV-2) with a homozygous (HM) c.338delG change, illustrated with a dash for the missing nucleotide when aligned with the wildtype sequence. The bottom row displays an unaffected parent of family 1 (III-3) with the c.338delG change in the heterozygous state (Het): the latter section of the heterozygous electropherogram shows two superimposed sequences due to the synchronous addition of nucleotides due to two distinct DNA templates derived from the wild type and the shorter mutant alleles of the heterozygote. Panel B illustrates the second mutation that was identified in PCDH21, in family 2. The top row displays the control individual with the wildtype (WT) allele; while the bottom row displays the affected proband (II-1) with a homozygous (HM) c.1463delG variant.

Figure 3

Co-segregation study of members of family 1 using the HaeIII restriction enzyme. Lane 1 is an undigested PCR product (286 bp). Lane 2 is a completely digested PCR product representing the wild-type allele (two products 197 and 56 bp). Affected individuals (lanes 4, 8, 9, and 10) show the homozygous mutant allele (one product 253 bp). Unaffected individuals, carriers (lanes 3, 5, 6, and 7) display the mutant allele in a heterozygous state (two products 253 and 197 bp). M represents the ϕX174RF DNA HaeIII marker.

Figure 4

These are illustrations of both the exon structure and a schematic of the Protocadherin-21 (PCDH21) protein with indications of the location of the two deletions. Sequence alignments demonstrate the conservation of both amino-acid positions across several species. Panel A: PCDH21 exon structure as derived from ensembl: the exons are colored according to domain structure, and, as illustrated in Panel B, with asterisks to denote the location of the novel deletions. Panel B is a schematic of the cellular domain structure derived from UniProt (created using Adobe Illustrator-Adobe Systems Inc.) with the protein numbers corresponding to each of the structural domains. Panel C is an sequence alignment created using PipeAlign for PCDH21 mutation p.G113fsX1, that was identified in family 1, and shows that there is a high level of conservation at this amino acid position-except for species Danio rerio. Panel D is a further alignment around the site of the PCDH21 deletion p.G487fsX20, identified in family 2, demonstrating a high level of conservation at this amino acid position and therefore providing evidence of the likely deleterious effect of this variant.

Figure 5

Color fundus composite photographs of affected individuals from families 1 and 2 and Goldmann visual fields from the proband in family 2. Panel A shows a fundus composite of the right eye from 34-year-old male (IV:6) from family 1; his visual acuity (VA) was 0.1 LogMAR with an electroretinogram (ERG) that was consistent with a “mild cone–rod dystrophy.” The image shows evidence of vessel attenuation and early peripheral retinal atrophy with few bone spicules. Panel B shows a fundus composite of the right eye in 36-year-old female (IV:5) from family 1 whose VA was 0.3 LogMAR, and who had absent color vision; her ERG was consistent with a diagnosis of retinitis pigmentosa. The fundus image shows macular depigmentation, dense peripheral bone spicules, and vessel attenuation. Panel C displays the color fundus composite from the left eye of 42-year-old male (IV:3) from family 1. His VA was 1.3 LogMAR, and the image displays circular patches of RPE atrophy both at the macula and in the periphery with associated peripheral pigment migration. His ERG was described as consisted with “advanced RP.” Panel D shows a color fundus composite from the right eye of 44-year-old female (IV:2) from family 1; Her visual acuity was limited to Counting Fingers (CF) and the view of the posterior pole is obscured by dense asteroid hyalosis as seen; there is peripheral bone spicule pigmentation and circular pigment epithelial atrophy was observed in the far periphery. Panel E shows the left eye color fundus composite of 46-year-old male (II:1) from family 2. His VA was recorded as Hand Movements (HM) at his most recent examination. No color vision was ever detected; the fundus image shows an atrophic retina, with vessel attenuation, RPE atrophy around the disc, at the macula and in the periphery, and associated peripheral pigment migration. His ERG when tested at age 32 years was undetectable. Panel F displays the Goldmann visual fields for patient II:1 with the V4e isopter in green and III4e in red: there are peripheral islands of residual vision and no response centrally to the largest and brightest target (V4e) in the right eye and small areas of response in the left eye.