Outcome of ABCA4 disease-associated alleles in autosomal recessive retinal dystrophies: retrospective analysis in 420 Spanish families.

OBJECTIVE: To provide a comprehensive overview of all detected mutations in the ABCA4 gene in Spanish families with autosomal recessive retinal disorders, including Stargardt's disease (arSTGD), cone-rod dystrophy (arCRD), and retinitis pigmentosa (arRP), and to assess genotype-phenotype correlation and disease progression in 10 years by considering the type of variants and age at onset. DESIGN: Case series. PARTICIPANTS: A total of 420 unrelated Spanish families: 259 arSTGD, 86 arCRD, and 75 arRP. METHODS: Spanish families were analyzed through a combination of ABCR400 genotyping microarray, denaturing high-performance liquid chromatography, and high-resolution melting scanning. Direct sequencing was used as a confirmation technique for the identified variants. Screening by multiple ligation probe analysis was used to detect possible large deletions or insertions in the ABCA4 gene. Selected families were analyzed further by next generation sequencing. MAIN OUTCOME MEASURES: DNA sequence variants, mutation detection rates, haplotypes, age at onset, central or peripheral vision loss, and night blindness. RESULTS: Overall, we detected 70.5% and 36.6% of all expected ABCA4 mutations in arSTGD and arCRD patient cohorts, respectively. In the fraction of the cohort where the ABCA4 gene was sequenced completely, the detection rates reached 73.6% for arSTGD and 66.7% for arCRD. However, the frequency of possibly pathogenic ABCA4 alleles in arRP families was only slightly higher than that in the general population. Moreover, in some families, mutations in other known arRP genes segregated with the disease phenotype. CONCLUSIONS: An increasing understanding of causal ABCA4 alleles in arSTGD and arCRD facilitates disease diagnosis and prognosis and also is paramount in selecting patients for emerging clinical trials of therapeutic interventions. Because ABCA4-associated diseases are evolving retinal dystrophies, assessment of age at onset, accurate clinical diagnosis, and genetic testing are crucial. We suggest that ABCA4 mutations may be associated with a retinitis pigmentosa-like phenotype often as a consequence of severe (null) mutations, in cases of long-term, advanced disease, or both. Patients with classical arRP phenotypes, especially from the onset of the disease, should be screened first for mutations in known arRP genes and not ABCA4.

Mutation analysis of 272 Spanish families affected by autosomal recessive retinitis pigmentosa using a genotyping microarray.

PURPOSE: Retinitis pigmentosa (RP) is a genetically heterogeneous disorder characterized by progressive loss of vision. The aim of this study was to identify the causative mutations in 272 Spanish families using a genotyping microarray. METHODS: 272 unrelated Spanish families, 107 with autosomal recessive RP (arRP) and 165 with sporadic RP (sRP), were studied using the APEX genotyping microarray. The families were also classified by clinical criteria: 86 juveniles and 186 typical RP families. Haplotype and sequence analysis were performed to identify the second mutated allele. RESULTS: At least one-gene variant was found in 14% and 16% of the juvenile and typical RP groups respectively. Further study identified four new mutations, providing both causative changes in 11% of the families. Retinol Dehydrogenase 12 (RDH12) was the most frequently mutated gene in the juvenile RP group, and Usher Syndrome 2A (USH2A) and Ceramide Kinase-Like (CERKL) were the most frequently mutated genes in the typical RP group. The only variant found in CERKL was p.Arg257Stop, the most frequent mutation. CONCLUSIONS: The genotyping microarray combined with segregation and sequence analysis allowed us to identify the causative mutations in 11% of the families. Due to the low number of characterized families, this approach should be used in tandem with other techniques.

Genotype-Phenotype Correlations in a Spanish Cohort of 506 Families With Biallelic ABCA4 Pathogenic Variants.

PURPOSE: To define genotype-phenotype correlations in the largest cohort study worldwide of patients with biallelic ABCA4 variants, including 434 patients with Stargardt disease (STGD1) and 72 with cone-rod dystrophy (CRD). DESIGN: Cohort study. METHODS: We characterized 506 patients with ABCA4 variants using conventional genetic tools and next-generation sequencing technologies. Medical history and ophthalmologic data were obtained from 372 patients. Genotype-phenotype correlation studies were carried out for the following variables: variant type, age at symptom onset (AO), and clinical phenotype. RESULTS: A total of 228 different pathogenic variants were identified in 506 ABCA4 patients, 50 of which were novel. Genotype-phenotype correlations showed that most of the patients with biallelic truncating variants presented with CRD and that these cases had a significantly earlier AO than patients with STGD1. Three missense variants are associated with CRD for the first time (c.1804C>T; p.[Arg602Trp], c.3056C>T; p.[Thr1019Met], and c.6320G>C; p.[Arg2107Pro]). Analysis of the most prevalent ABCA4 variant in Spain, c.3386G>T; p.(Arg1129Leu), revealed that is correlated to STGD1, later AO, and foveal sparing. CONCLUSIONS: Our study, conducted in the largest ABCA4-associated disease cohort reported to date, updates the genotype-phenotype model established for ABCA4 variants and broadens the mutational spectrum of the gene. According to our observations, patients with ABCA4 presenting with 2 truncating variants may first present features of STGD1 but eventually develop rod dysfunction, and specific missense variants may be associated with a different phenotype, underscoring the importance of an accurate genetic diagnosis. Also, it is a prerequisite for enrollment in clinical trials, and to date, no other treatment has been approved for STGD1.

ABCA4 mutations in Portuguese Stargardt patients: identification of new mutations and their phenotypic analysis.

PURPOSE: To resolve the spectrum of causative retina-specific ATP-binding cassette transporter gene (ABCA4) gene mutations in Portuguese Stargardt (STGD) patients and compare allele frequencies obtained in this cohort with those of previous population surveys. METHODS: Using a microarray technique (ABCR400 gene chip), we screened all previously reported ABCA4 gene mutations in the genomic DNA of 27 patients from 21 unrelated Stargardt families whose phenotypes had been clinically evaluated using psychophysics and electrophysiological measurements. Furthermore, we performed denaturing high performance liquid chromatography whenever one or both mutant alleles failed to be detected using the ABCR gene chip. RESULTS: A total of 36 mutant alleles (out of the 54 tested) were identified in STGD patients, resulting in a detection rate of 67%. Two mutant alleles were present in 12 out of 21 STGD families (57%), whereas in four out of 21 (19%) of the families, only one mutant allele was found. We report the presence of 22 putative pathogenic alterations, including two sequence changes not found in other populations, c.2T>C (p.Met1Thr) and c.4036_4037delAC (p.Thr1346fs), and two novel disease-associated variants, c.400C>T (p.Gln134X) and c.4720G>T (p.Glu1574X). The great majority of the mutations were missense (72.7%). Seven frameshift variants (19.4%), three nonsense mutations (8.3%), and one splicing sequence change (2.7%) were also found in STGD chromosomes. The most prevalent pathologic variant was the missense mutation p.Leu11Pro. Present in 19% of the families, this mutation represents a quite high prevalence in comparison to other European populations. In addition, 23 polymorphisms were also identified, including four novel intronic sequence variants. CONCLUSIONS: To our knowledge, this study represents the first report of ABCA4 mutations in Portuguese STGD patients and provides further evidence of different mutation frequency across populations. Phenotypic characterization of novel putative mutations was addressed.

CERKL mutations and associated phenotypes in seven Spanish families with autosomal recessive retinitis pigmentosa.

PURPOSE: Retinitis pigmentosa (RP) is a genetically heterogeneous group of inherited retinopathies. Up to now, 39 genes and loci have been implicated in nonsyndromic RP, yet the genetic bases of >50% of the cases, particularly of the recessive forms, remain unknown. A novel gene (CERKL) has been described as associated with RP26. It encodes a ceramide kinase that is assumed to be involved in sphingolipid-mediated apoptosis in the retina. This is a report of the phenotypes and genotypes of persons carrying disease-causing mutations in CERKL. METHODS: Two hundred ten unrelated Spanish families with nonsyndromic autosomal recessive RP were analyzed for sequence variations. Seven of these families presented a mutation in CERKL. Nine affected persons of these families were clinically investigated, including visual field, electrophysiology, and fundus examination. RESULTS: The mutation p.Arg257ter was identified in the homozygous state in all seven affected families. The patients with this variation in CERKL presented a common phenotype with characteristic macular and peripheral lesions. CONCLUSIONS: This study presents the first genotype-phenotype correlation for persons carrying p.Arg257ter mutation and provides clues for a characteristic phenotype of these mutations among persons with autosomal recessive cases.

Molecular analysis of ABCA4 and CRB1 genes in a Spanish family segregating both Stargardt disease and autosomal recessive retinitis pigmentosa.

PURPOSE: Stargardt disease (STGD), characterized by central visual impairment, is the most common juvenile macular dystrophy. All recessively inherited cases are thought to be due to mutations in the ABCA4 gene. Early-onset autosomal recessive retinitis pigmentosa (arRP) is a severe retinal degeneration that presents before the patient is ten years old. It has been associated with mutations in different genes, including CRB1. The aim of this study was to determine the genetic causes for two different retinal dystrophies, STGD and early-onset arRP, both segregating in one Spanish family. METHODS: Mutational analyses were performed using the ABCR400 and Leber congenital amaurosis (LCA) genotyping microarrays. Additional scanning for mutations was conducted by denaturing high performance liquid chromatography (dHPLC); results were confirmed by direct sequencing. RESULTS: A patient, who exhibited a STGD phenotype, was found to be homozygous for the p.Asn1805Asp (c.5413A>G) mutation in ABCA4. However, his affected sister, who had the arRP phenotype, was found to be heterozygous for this allele; no other sequence change could be found in ABCA4. Analysis using the LCA chip revealed the p.Cys948Tyr mutation in CRB1 in heterozygous state. A second mutation (p.Trp822ter) was found in the CRB1 gene in the affected female by denaturing high performance liquid chromatography (dHPLC) and direct sequencing. CONCLUSIONS: Two distinct retinal dystrophies with mutations affecting two different genes cosegregated in this family. The presence of two different phenotypes associated with mutations in two distinct genes in one single family must be considered especially when dealing with retinal dystrophies which bear high carrier frequencies in general population.

Spectrum of the ABCA4 gene mutations implicated in severe retinopathies in Spanish patients.

PURPOSE: The purpose of this study is to describe the spectrum of mutations in the ABCA4 gene found in Spanish patients affected with several retinal dystrophies. METHODS: Sixty Spanish families with different retinal dystrophies were studied. Samples were analyzed for variants in all 50 exons of the ABCA4 gene by screening with the ABCR400 microarray, and results were confirmed by direct sequencing. Haplotype analyses were also performed. For those families with only one mutation detected by the microarray, denaturing (d)HPLC was performed to complete the mutational screening of the ABCA4 gene. RESULTS: The sequence analysis of the ABCA4 gene led to the identification of 33 (27.5%) potential disease-associated alleles among the 60 patients. These comprised 16 distinct sequence variants in 25 of the 60 subjects investigated. For autosomal recessive cone-rod dystrophy (arCRD), we found that 50% of the CRD families with the mutation had two recurrent changes (2888delG and R943Q). For retinitis pigmentosa (RP) and autosomal dominant macular dystrophy (adMD), one putative disease-associated allele was identified in 9 of the 27 and 3 of the 7 families, respectively. CONCLUSIONS: In the population studied, ABCA4 plays an important role in the pathogenesis of arCRD. However, mutations in this gene are less frequently identified in other retinal dystrophies, like RP or adMD, and therefore it is still not clear whether ABCA4 is involved as a modifying factor or the relationship is a fortuitous association.

Mutation screening of 299 Spanish families with retinal dystrophies by Leber congenital amaurosis genotyping microarray.

PURPOSE: Leber Congenital Amaurosis (LCA) is one of the most severe inherited retinal dystrophies with the earliest age of onset. This study was a mutational analysis of eight genes (AIPL1, CRB1, CRX, GUCY2D, RPE65, RPGRIP1, MERTK, and LRAT) in 299 unrelated Spanish families, containing 42 patients with initial diagnosis of LCA: 107 with early-onset autosomal recessive retinitis pigmentosa (ARRP; onset <10 years of age) and 150 with non-early-onset ARRP (onset, >10 years of age). METHODS: Samples were studied by using a genotyping microarray (Asper Biotech, Ltd., Tartu, Estonia) followed by a family study in cases with potential digenism/triallelism. RESULTS: The frequencies of alleles carrying disease-causing mutations found in the authors'cohort using the chip were 23.8% (20/84) for LCA with 13 families carrying mutations, 6.1% (13/214) for early-onset ARRP with 12 families carrying mutations, and 4.3% (13/300) for non-early-onset ARRP with 12 families carrying mutations. CRB1 was the most frequently found mutated gene in affected Spanish families. Five families with anticipated digenism or triallelism were further studied in depth. Digenism could be discarded in all these cases; however, triallelism could not be ruled out. CONCLUSIONS: CRB1 is the main gene responsible for LCA in the Spanish population. Sequence changes p.Asp1114Gly (RPGRIP1), p.Pro701Ser (GUCY2D), and p.Tyr134Phe (AIPL1) were found at similar frequencies in patients and control subjects. The authors therefore suggest that these changes be considered as polymorphism or modifier alleles, rather than as disease-causing mutations. The LCA microarray is a quick and reasonably low-cost first step in the molecular diagnosis of LCA. The diagnosis should be completed by conventional laboratory analysis as a second step. This stepwise proceeding permits detection of novel disease-causing mutations and identification of cases involving potential digenism/triallelism. Previous accurate ophthalmic diagnosis was found to be indispensable.

Frequency of CEP290 c.2991_1655A>G mutation in 175 Spanish families affected with Leber congenital amaurosis and early-onset retinitis pigmentosa.

PURPOSE: Leber congenital amaurosis (LCA) is the most severe inherited retinopathy with the earliest age of onset. To date, eleven genes have been reported to cause the non-syndromic LCA phenotype. The CEP290 gene has been shown to account for Joubert and Senior-Loken syndromes and to represent a frequent cause of non-syndromic LCA. The aim of the present study was to establish the prevalence of CEP290 c.2991_1655A>G in non-syndromic Spanish patients having LCA or early-onset retinitis pigmentosa (RP). METHODS: We used automated sequencing to examine 49 non-syndromic Spanish families with LCA and 126 Spanish families with early-onset RP for the CEP290 c.2991_1655A>G mutation. As a control, we recruited 50 unrelated Spanish healthy individuals. RESULTS: The frequencies of mutated alleles were 6% in LCA cases and 0% in early-onset RP and healthy individual controls. These results were compared to other populations. CONCLUSIONS: The CEP290 c.2991_1655A>G mutation frequency in Spanish non-syndromic LCA families is lower than that of other countries.

A novel SLC6A8 mutation associated with motor dysfunction in a child exhibiting creatine transporter deficiency.

Creatine transporter (CT) deficiency is an X-linked disorder caused by mutations in the SLC6A8 gene. We describe a clinical, biochemical and molecular examination of a child with X-linked cerebral creatine deficiency. Increased urinary creatine/creatinine ratio, abnormal brain proton magnetic resonance spectroscopy and reduced creatine transport confirmed the clinical diagnosis. SLC6A8 analysis revealed a novel mutation that was hemizygous in the child and not detected in his mother. CT deficiency should be considered in children, especially males, with mental retardation.