Sequence variants in COL4A1 and COL4A2 genes in Ecuadorian families with keratoconus.

PURPOSE: Keratoconus (KTCN) is a non-inflammatory, usually bilateral disorder of the eye which results in the conical shape and the progressive thinning of the cornea. Several studies have suggested that genetic factors play a role in the etiology of the disease. Several loci were previously described as possible candidate regions for familial KTCN; however, no causative mutations in any genes have been identified for any of these loci. The purpose of this study was to evaluate role of the collagen genes collagen type IV, alpha-1 (COL4A1) and collagen type IV, alpha-2 (COL4A2) in KTCN in Ecuadorian families. METHODS: COL4A1 and COL4A2 in 15 Ecuadorian KTCN families were examined with polymerase chain reaction amplification, and direct sequencing of all exons, promoter and intron-exon junctions was performed. RESULTS: Screening of COL4A1 and COL4A2 revealed numerous alterations in coding and non-coding regions of both genes. We detected three missense substitutions in COL4A1: c.19G>C (Val7Leu), c.1663A>C (Thr555Pro), and c.4002A>C (Gln1334His). Five non-synonymous variants were identified in COL4A2: c.574G>T (Val192Phe), c.1550G>A (Arg517Lys), c.2048G>C (Gly683Ala), c.2102A>G (Lys701Arg), and c.2152C>T (Pro718Ser). None of the identified sequence variants completely segregated with the affected phenotype. The Gln1334His variant was possibly damaging to protein function and structure. CONCLUSIONS: This is the first mutation screening of COL4A1 and COL4A2 genes in families with KTCN and linkage to a locus close to these genes. Analysis of COL4A1 and COL4A2 revealed no mutations indicating that other genes are involved in KTCN causation in Ecuadorian families.

Deletions and duplications of developmental pathway genes in 5q31 contribute to abnormal phenotypes.

Although copy number changes of 5q31 have been rarely reported, deletions have been associated with some common characteristics, such as short stature, failure to thrive, developmental delay (DD)/intellectual disability (ID), club feet, dislocated hips, and dysmorphic features. We report on three individuals with deletions and two individuals with duplications at 5q31, ranging from 3.6 Mb to 8.1 Mb and 830 kb to 3.4 Mb in size, respectively. All five copy number changes are apparently de novo and involve several genes that are important in developmental pathways, including PITX1, SMAD5, and WNT8A. The individuals with deletions have characteristic features including DD, short stature, club feet, cleft or high palate, dysmorphic features, and skeletal anomalies. Haploinsufficiency of PITX1, a transcription factor important for limb development, is likely the cause for the club feet, skeletal anomalies, and cleft/high palate, while additional genes, including SMAD5 and WNT8A, may also contribute to additional phenotypic features. Two patients with deletions also presented with corneal anomalies. To identify a causative gene for the corneal anomalies, we sequenced candidate genes in a family with apparent autosomal dominant keratoconus with suggestive linkage to 5q31, but no mutations in candidate genes were found. The duplications are smaller than the deletions, and the patients with duplications have nonspecific features. Although development is likely affected by increased dosage of the genes in the region, the developmental disruption appears less severe than that seen with deletion.

Variants in SKP1, PROB1, and IL17B genes at keratoconus 5q31.1-q35.3 susceptibility locus identified by whole-exome sequencing.

Keratoconus (KTCN) is a protrusion and thinning of the cornea, resulting in impairment of visual function. The extreme genetic heterogeneity makes it difficult to discover factors unambiguously influencing the KTCN phenotype. In this study, we used whole-exome sequencing (WES) and Sanger sequencing to reduce the number of candidate genes at the 5q31.1-q35.3 locus and to prioritize other potentially relevant variants in an Ecuadorian family with KTCN. We applied WES in two affected KTCN individuals from the Ecuadorian family that showed a suggestive linkage between the KTCN phenotype and the 5q31.1-q35.3 locus. Putative variants identified by WES were further evaluated in this family using Sanger sequencing. Exome capture discovered a total of 173 rare (minor allele frequency <0.001 in control population) nonsynonymous variants in both affected individuals. Among them, 16 SNVs were selected for further evaluation. Segregation analysis revealed that variants c.475T>G in SKP1, c.671G>A in PROB1, and c.527G>A in IL17B in the 5q31.1-q35.3 linkage region, and c.850G>A in HKDC1 in the 10q22 locus completely segregated with the phenotype in the studied KTCN family. We demonstrate that a combination of various techniques significantly narrowed the studied genomic region and reduced the list of the putative exonic variants. Moreover, since this locus overlapped two other chromosomal regions previously recognized in distinct KTCN studies, our findings suggest that this 5q31.1-q35.3 locus might be linked with KTCN.

Variant c.2262A>C in DOCK9 Leads to Exon Skipping in Keratoconus Family.

PURPOSE: Keratoconus (KTCN) is a degenerative disorder of the eye that is characterized by a conical shape and thinning of the cornea, resulting in impaired visual function. Previously, we identified heterozygous single base-pair substitutions in DOCK9, IPO5, and STK24, showing concurrent 100% segregation with the affected phenotype in an Ecuadorian family. As the pathogenic consequences of these variants were not obvious, we performed in vitro splicing analyses to determine their functional significance. METHODS: We generated expression constructs using patient DNA as a template corresponding to the wild-type and mutant alleles of DOCK9, IPO5, and STK24. After transfecting HeLa cells with each construct, total RNA samples were extracted, reverse transcribed, and amplified using specific primers. RESULTS: In vitro splicing analysis revealed that only c.2262A>C in exon 20 of DOCK9 led to aberrant splicing, resulting in the changed ratio between two protein isoforms: a normal transcript and a transcript with exon skipping. The exon skipping causes a premature stop codon, disrupting the functional domains of DOCK9 protein, which may alter the biological role of DOCK9 as a Cdc42 activator. CONCLUSIONS: Based on in vitro results, we demonstrated that c.2262A>C substitution in DOCK9, previously identified in KTCN-affected members of an Ecuadorian family, leads to a splicing aberration. However, because the mutation effect was observed in vitro, a definitive relationship between DOCK9 and KTCN phenotype could not be established. Our results indicate that further elucidation of the causes of KTCN is needed.

Substitution at IL1RN and deletion at SLC4A11 segregating with phenotype in familial keratoconus.

PURPOSE: Keratoconus (KTCN) is a thinning and anterior protrusion of the cornea that results in altered refractive powers and loss of visual acuity. Despite numerous studies, the reasons for development and progression of KTCN remain unknown. Genetic studies have led to identification of several loci linked with KTCN, including a locus in one multigenerational Ecuadorian family. The purpose of this study was to identify sequence variants in candidate genes segregating with the KTCN phenotype in another Ecuadorian family. METHODS: Nonparametric linkage analysis was performed in Ecuadorian family KTCN-019. Candidate genes IL1A, IL1B, IL1RN, and SLC4A11 were selected and examined in this family by direct sequencing of all exons, promoters, and intron-exon junctions. RESULTS: Two novel suggestive loci were identified in 2q13-q14.3 and 20p13-p12.2. Screening of the candidate genes revealed 66 sequence variants, including five novel variants, in both coding and noncoding regions. The substitution c.214+242C > T in the IL1RN gene was observed in all affected individuals and three apparently unaffected family members. The novel deletion of 54 nucleotides in position c.2558+149_2558+203 in SLC4A11 was observed in all patients but one, as well as two healthy individuals and one person with an unknown phenotype. CONCLUSIONS: The analyses of selected genes have led to identification of numerous sequence variants in the examined Ecuadorian family. Both substitution c.214+242C > T in IL1RN and novel deletion c.2558+149_2558+203del54 in SLC4A11 were observed significantly more frequently in family members with KTCN (P = 0.004525 and P = 0.00761, respectively), suggesting involvement of these two genes in KTCN etiology in the studied family.

Localization of a gene for keratoconus to a 5.6-Mb interval on 13q32.

PURPOSE: Keratoconus (KTCN) is a noninflammatory thinning and anterior protrusion of the cornea that results in steepening and distortion of the cornea, altered refractive powers, and reduced visual acuity. Several loci responsible for a familial form of KTCN have been mapped, however; no mutations in any genes have been identified for any of these loci. There is also evidence that VSX1 and SOD1 may be involved in the etiology of KTCN. The purpose of this study was to verify the available data and to identify a new keratoconus susceptibility locus. METHODS: KTCN without other ocular or systemic features was diagnosed in 18 families. VSX1 and SOD1 sequencing was performed on affected individuals and control subjects. Genomewide linkage analysis was then performed in all families using polymorphic microsatellite markers with an average spacing of 5 cM. Next, single-nucleotide polymorphism (SNP) arrays, fluorescence in situ hybridization (FISH) analysis, and a comparative genomic hybridization array were used in one family to assess a candidate region on 13q32. RESULTS: All previously reported KTCN loci were excluded. VSX1 and SOD1 were sequenced, and no potentially functional variants were found. One KTCN family yielded a maximum multipoint parametric LOD score of 4.1 and multipoint nonparametric linkage (NPL) LOD score of 3.2. Multipoint linkage and haplotype analysis narrowed the locus to a 5.6-Mb region between the SNPs rs9516572 and rs3825523 on 13q32. CONCLUSIONS: The results exclude VSX1 and SOD1 as potential disease-causing genes in these families and localize a novel gene for keratoconus to a 5.6-Mb interval on 13q32.