In the present report, we describe the results obtained from the mutation screening of the PCDH15
gene in a series of 19 Spanish patients with USH in whom MYO7A
had been previously discarded as disease-causative. This study includes the analysis of the additional coding exons as well as the results of the screening of large rearrangements involving PCDH15
previously reported [22
Seven different point mutations were detected, two in the homozygous state. Including the large deletion and duplication detected in our patients with Multiplex Ligation-dependent Probe Amplification (MLPA) and Comparative Genomic Hybidization (CGH) array [22
], seven patients were carriers of at least one pathologic variant (36.8%). Thirteen pathologic PCDH15
alleles out of the 38 screened alleles were detected (34.2%).
All detected mutations have a clear pathogenic effect with the exception of the missense variant p.R134Q. It has been considered probable pathogenic because it was not detected in control samples and because it was found in trans together with a large PCDH15
duplication. Mutation p.R134G was described as responsible for non-syndromic recessive deafness in two consanguineous families from Pakistan [13
]. This may indicate an important role for the amino acid R134 in the protein.
Two missense variants, p.N174S and p.R1273S, were detected in very low frequency (see Appendix 2
). The novel variant p.N174S was identified in one allele of patient RP-1321, also a carrier of variant p.Q1496H (classified as pathogenic) located in the CDH23
]. Both genes are located in the same chromosome (PCDH15
in 10q21.1 and CDH23
in 10q22.3). Segregation analysis was performed in this family, and we observed that both missense variants were located in cis. These results do not support a possible digenic inheritance between CDH23
in this family. The PCDH15
change was not detected in control chromosomes, but bioinformatic tools do not suggest a pathologic effect of this variant. In view of these results, we considered the PCDH15
variant p.N174S probably non-pathogenic.
Regarding variant p.R1273S, it was identified in the heterozygous state in patient RP-1387. Bioinformatic analyses showed different results, classifying this change as neutral (PMut
), tolerated (SIFT
), and probably damaging (PolyPhen
). The change was not found in 100 control chromosomes, and the loss of a positive charge at position 1273 of the protein could affect the structure and interactions of protocadherin-15. However, a second PCDH15
mutation was not detected. Bonnet et al. [25
] reported this variant in the heterozygous state in one patient with USH1 with the second mutated allele remaining undetected. The authors classified p.R1273S as presumably pathogenic. However, we considered this change a presumed non-pathogenic variant.
Pathogenic variants were not detected in the additional PCDH15
exons. Two missense changes were identified, p.E1611A (located in exon 35, CD2) and p.Q1654P (exon 36, CD3). Whereas the protocadherin-15-CD1 has a restricted expression profile, a wider pattern of expression was observed in protocadherin-15-CD2 and -CD3 [17
]. To date, no mutation has been reported in the additional exons. This could be due to the small sample size screened for mutations or because mutations in these isoforms do not cause Usher syndrome but perhaps other disorders.
No mutational hotspots or recurrent mutations have been identified in the PCDH15
gene, with the exception of p.R245X, a founder mutation responsible for about 50%–60% of Usher syndrome type I cases in the Ashkenazi Jewish population [24
Despite the exhaustive screening of PCDH15
, including the direct sequencing of all coding exons from known isoforms and including a search for large rearrangements, only one heterozygous mutation, c.1304_1305insC, was identified in patient RP-1286. Technical errors or hidden mutations in gene regions such as introns or regulatory sequences may account for this case. This patient may also be a carrier of a variant in PCDH15
, and two causative mutations may be located in some other USH gene. However, we calculated that there was only a 0.0046 probability that this case is a happenstance carrier [27
mutations were detected in seven patients. All were referred to as USH1, although detailed clinical data were obtained from only six, supporting their clinical diagnosis. Mutations in PCDH15
are responsible for Usher syndrome type I (USH1F) and DFNB23. Previous studies suggest an association of hypomorphic alleles with DFNB23 due to the conservation of residual function sufficient for normal vision but not for hearing, while severe mutations are USH1 causative [13
]. In our cohort of patients, most of the detected pathologic variants were truncating mutations. Only in patient RP-367 was a missense variant identified in trans together with a large duplication, in whom a clear USH1 diagnosis was established. Ahmed et al. [13
] described DFNB23 families homozygous for variant p.R134G. The phenotypic differences could be due to the second mutation (a rearrangement leading to a truncated protein in our patient with USH1), or that the missense variants had different effects on protein.
A better understanding of the consequences of PCDH15 mutations within the different isoforms and tissues where the mutations are expressed will help shed light on which functions are conserved or damaged causing different phenotypes as USH1 or DFNB23. This knowledge will be helpful in developing therapies for these diseases.