Comparative immunogenetics of autism and schizophrenia.

Autism and schizophrenia are highly heritable neurodevelopmental disorders, each mediated by a diverse suite of genetic and environmental risk factors. Comorbidity and familial aggregation of such neurodevelopmental disorders with other disease-related conditions can provide important insights into their etiology. Epidemiological studies have documented reduced rates of rheumatoid arthritis, a systemic autoimmune condition, in schizophrenia, and recent work has shown increased rates of rheumatoid arthritis in first-degree relatives of autistic individuals, especially mothers. Advances in understanding the genetic basis of rheumatoid arthritis have shown that much of the genetic liability to this condition is due to risk and protective alleles at the HLA DRB1 locus. These data allow robust testing of the hypotheses that allelic variation at DRB1 pleiotropically modulates risk of rheumatoid arthritis, autism and schizophrenia. Systematic review of the literature indicates that reported associations of DRB1 variants with these three conditions are congruent with a pleiotropic model: DRB1*04 alleles have been associated with increased risk of rheumatoid arthritis and autism but decreased risk of schizophrenia, and DRB1*13 alleles have been associated with protection from rheumatoid arthritis and autism but higher risk of schizophrenia. These convergent findings from genetics and epidemiology imply that a subset of autism and schizophrenia cases may be underlain by genetically based neuroimmune alterations, and that analyses of the causes of risk and protective effects from DRB1 variants may provide new approaches to therapy.

Fine mapping of ZNF804A and genome-wide significant evidence for its involvement in schizophrenia and bipolar disorder.

A recent genome-wide association study (GWAS) reported evidence for association between rs1344706 within ZNF804A (encoding zinc-finger protein 804A) and schizophrenia (P=1.61 × 10(-7)), and stronger evidence when the phenotype was broadened to include bipolar disorder (P=9.96 × 10(-9)). In this study we provide additional evidence for association through meta-analysis of a larger data set (schizophrenia/schizoaffective disorder N=18 945, schizophrenia plus bipolar disorder N=21 274 and controls N=38 675). We also sought to better localize the association signal using a combination of de novo polymorphism discovery in exons, pooled de novo polymorphism discovery spanning the genomic sequence of the locus and high-density linkage disequilibrium (LD) mapping. The meta-analysis provided evidence for association between rs1344706 that surpasses widely accepted benchmarks of significance by several orders of magnitude for both schizophrenia (P=2.5 × 10(-11), odds ratio (OR) 1.10, 95% confidence interval 1.07-1.14) and schizophrenia and bipolar disorder combined (P=4.1 × 10(-13), OR 1.11, 95% confidence interval 1.07-1.14). After de novo polymorphism discovery and detailed association analysis, rs1344706 remained the most strongly associated marker in the gene. The allelic association at the ZNF804A locus is now one of the most compelling in schizophrenia to date, and supports the accumulating data suggesting overlapping genetic risk between schizophrenia and bipolar disorder.

Association study of SNAP25 and schizophrenia in Irish family and case-control samples.

SNAP25 occurs on chromosome 20p12.2, which has been linked to schizophrenia in some samples, and recently linked to latent classes of psychotic illness in our sample. SNAP25 is crucial to synaptic functioning, may be involved in axonal growth and dendritic sprouting, and its expression may be decreased in schizophrenia. We genotyped 18 haplotype-tagging SNPs in SNAP25 in a sample of 270 Irish high-density families. Single marker and haplotype analyses were performed in FBAT and PDT. We adjusted for multiple testing by computing q values. Association was followed up in an independent sample of 657 cases and 411 controls. We tested for allelic effects on the clinical phenotype by using the method of sequential addition and 5 factor-derived scores of the OPCRIT. Nine of 18 SNPs had P values <0.05 in either FBAT or PDT for one or more definitions of illness. Several two-marker haplotypes were also associated. Subjects inheriting the risk alleles of the most significantly associated two-marker haplotype were likely to have higher levels of hallucinations and delusions. The most significantly associated marker, rs6039820, was observed to perturb 12 transcription-factor binding sites in in silico analyses. An attempt to replicate association findings in the case-control sample resulted in no SNPs being significantly associated. We observed robust association in both single marker and haplotype-based analyses between SNAP25 and schizophrenia in an Irish family sample. Although we failed to replicate this in an independent sample, this gene should be further tested in other samples.

Replication of association between schizophrenia and ZNF804A in the Irish Case-Control Study of Schizophrenia sample.

A recent genome-wide association study reported association between schizophrenia and the ZNF804A gene on chromosome 2q32.1. We attempted to replicate these findings in our Irish Case-Control Study of Schizophrenia (ICCSS) sample (N=1021 cases, 626 controls). Following consultation with the original investigators, we genotyped three of the most promising single-nucleotide polymorphisms (SNPs) from the Cardiff study. We replicate association with rs1344706 (trend test one-tailed P=0.0113 with the previously associated A allele) in ZNF804A. We detect no evidence of association with rs6490121 in NOS1 (one-tailed P=0.21), and only a trend with rs9922369 in RGRIP1L (one-tailed P=0.0515). On the basis of these results, we completed genotyping of 11 additional linkage disequilibrium-tagging SNPs in ZNF804A. Of 12 SNPs genotyped, 11 pass quality control criteria and 4 are nominally associated, with our most significant evidence of association at rs7597593 (P=0.0013) followed by rs1344706. We observe no evidence of differential association in ZNF804A on the basis of family history or sex of case. The associated SNP rs1344706 lies in approximately 30 bp of conserved mammalian sequence, and the associated A allele is predicted to maintain binding sites for the brain-expressed transcription factors MYT1l and POU3F1/OCT-6. In controls, expression is significantly increased from the A allele of rs1344706 compared with the C allele. Expression is increased in schizophrenic cases compared with controls, but this difference does not achieve statistical significance. This study replicates the original reported association of ZNF804A with schizophrenia and suggests that there is a consistent link between the A allele of rs1344706, increased expression of ZNF804A and risk for schizophrenia.

Association analysis of the PIP4K2A gene on chromosome 10p12 and schizophrenia in the Irish study of high density schizophrenia families (ISHDSF) and the Irish case-control study of schizophrenia (ICCSS).

Molecular studies support pharmacological evidence that phosphoinositide signaling is perturbed in schizophrenia and bipolar disorder. The phosphatidylinositol-4-phosphate-5-kinase type-II alpha (PIP4K2A) gene is located on chromosome 10p12. This region has been implicated in both diseases by linkage, and PIP4K2A directly by association. Given linkage evidence in the Irish Study of High Density Schizophrenia Families (ISHDSF) to a region including 10p12, we performed an association study between genetic variants at PIP4K2A and disease. No association was detected through single-marker or haplotype analysis of the whole sample. However, stratification into families positive and negative for the ISHDSF schizophrenia high-risk haplotype (HRH) in the DTNBP1 gene and re-analysis for linkage showed reduced amplitude of the 10p12 linkage peak in the DTNBP1 HRH positive families. Association analysis of the stratified sample showed a trend toward association of PIP4K2A SNPs rs1417374 and rs1409395 with schizophrenia in the DTNBP1 HRH positive families. Despite this apparent paradox, our data may therefore suggest involvement of PIP4K2A in schizophrenia in those families for whom genetic variation in DTNBP1 appears also to be a risk factor. This trend appears to arise from under-transmission of common alleles to female cases. Follow-up association analysis in a large Irish schizophrenia case-control sample (ICCSS) showed significant association with disease of a haplotype comprising these same SNPs rs1417374-rs1409395, again more so in affected females, and in cases with negative family history of the disease. This study supports a minor role for PIP4K2A in schizophrenia etiology in the Irish population.

The dystrobrevin-binding protein 1 gene: features and networks.

The dystrobrevin-binding protein 1 (DTNBP1) gene has been one of the most studied and promising schizophrenia susceptibility genes since it was first reported to be associated with schizophrenia in the Irish Study of High Density Schizophrenia Families (ISHDSF). Although many studies have been performed both at the functional level and in association with psychiatric disorders, there has been no systematic review of the features of the DTNBP1 gene, protein or the relationship between function and phenotype. Using a bioinformatics approach, we identified the DTNBP1 gene in 13 vertebrate species. The comparison of these genes revealed a conserved gene structure, protein-coding sequence and dysbindin domain, but a diverse noncoding sequence. The molecular evolutionary analysis suggests the DTNBP1 gene probably originated in chordates and matured in vertebrates. No signature of recent positive selection was seen in any primate lineage. The DTNBP1 gene likely has many more alternative transcripts than the current three major isoforms annotated in the NCBI database. Our examination of risk haplotypes revealed that, although the frequency of a single nucleotide polymorphism (SNP) or haplotype might be significantly different in cases from controls, difference between major geographic populations was even larger. Finally, we constructed the first DTNBP1 interactome and explored its network features. Besides the biogenesis of lysosome-related organelles complex 1 and dystrophin-associated protein complex, several molecules in the DTNBP1 network likely provide insight into the role of DTNBP1 in biological systems: retinoic acid, beta-estradiol, calmodulin and tumour necrosis factor. Studies of these subnetworks and pathways may provide opportunities to deepen our understanding of the mechanisms of action of DTNBP1 variants.

A region of 35 kb containing the trace amine associate receptor 6 (TAAR6) gene is associated with schizophrenia in the Irish study of high-density schizophrenia families.

The TAAR6 gene has been previously associated with schizophrenia in 192 pedigrees of European and African ancestry. To replicate these findings we performed an association study of TAAR6 in 265 pedigrees of the Irish Study of High-Density Schizophrenia Families (ISHDSF). Of the 24 genotyped single-nucleotide polymorphisms only rs12189813 and rs9389011 provided single-marker evidence for association (0.0094

Alcohol dependence is associated with the ZNF699 gene, a human locus related to Drosophila hangover, in the Irish Affected Sib Pair Study of Alcohol Dependence (IASPSAD) sample.

Because tolerance is an important aspect of alcohol dependence (AD) in humans, recent evidence showing that the Drosophila gene hang is critically involved in the development of alcohol tolerance in the fly suggests that variation in related human loci might be important in the etiology of alcohol-related disorders. The orthology of hang in mammals is complex, but a number of human gene products (including ZNF699) with similar levels of amino-acid identity (18-26%) and similarity (30-41%), are consistently identified as the best matches with the translated hang sequence. We tested for association between the dichotomous clinical phenotype of alcohol dependence and seven single nucleotide polymorphisms (SNPs) in ZNF699 in our sample of 565 genetically independent cases and 496 siblings diagnosed with AD, and 609 controls. In analyses of genetically independent cases and controls, four of the seven single markers show strong evidence for association with AD (0.00003

Genomewide linkage study in the Irish affected sib pair study of alcohol dependence: evidence for a susceptibility region for symptoms of alcohol dependence on chromosome 4.

Alcoholism is a relatively common, chronic, disabling and often treatment-resistant disorder. Evidence from twin and adoption studies indicates a substantial genetic influence, with heritability estimates of 50-60%. We conducted a genome scan in the Irish Affected Sib Pair Study of Alcohol Dependence (IASPSAD). Most probands were ascertained through alcoholism treatment settings and were severely affected. Probands, affected siblings and parents were evaluated by structured interview. A 4 cM genome scan was conducted using 474 families of which most (96%) were comprised by affected sib pairs. Nonparametric and quantitative linkage analyses were conducted using DSM-IV alcohol dependence (AD) and number of DSM-IV AD symptoms (ADSX). Quantitative results indicate strong linkage for number of AD criteria to a broad region of chromosome 4, ranging from 4q22 to 4q32 (peak multipoint LOD=4.59, P=2.1 x 10(-6), at D4S1611). Follow-up analyses suggest that the linkage may be due to variation in the symptoms of tolerance and out of control drinking. There was evidence of weak linkage (LODs of 1.0-2.0) to several other regions, including 1q44, 13q31, and 22q11 for AD along with 2q37, 9q21, 9q34 and 18p11 for ADSX. The location of the chromosome 4 peak is consistent with results from prior linkage studies and includes the alcohol dehydrogenase gene cluster. The results of this study suggest the importance of genetic variation in chromosome 4 in the etiology and severity of alcoholism in Caucasian populations.

Association of the NRG1 gene and schizophrenia: a meta-analysis.

We investigated the association of the NRG1 gene and schizophrenia using meta-analytic techniques, combining all published data while restricting our analysis to studies investigating the most commonly reported single marker (SNP8NRG221533). We also investigated whether ancestry (European vs East Asian) and study design (family-based vs case-control) moderated any association. We found no evidence for an association of SNP8NRG221533 with schizophrenia, and significant between-study heterogeneity, which persisted when family-based studies were combined separately. However, when haplotype-based P-values were combined, there was evidence in support of an association of NRG1 with schizophrenia, and no evidence of between-study heterogeneity. Our meta-analysis provides support for the association of NRG1 with schizophrenia, but indicates that firmly establishing the role of NRG1 gene in schizophrenia by genetic association requires much larger sample sizes than have hitherto been reported. Association analyses and replications should take place at the level of the gene, rather than at the level of SNP, haplotype, or functional variant. Meta-analysis would then be carried out on the basis of the combination of P-values.

No evidence for linkage or association of neuregulin-1 (NRG1) with disease in the Irish study of high-density schizophrenia families (ISHDSF).

The neuregulin-1 gene (NRG1) at chromosome 8p21-22 has been implicated as a schizophrenia susceptibility gene in Icelandic, Scottish, Irish and mixed UK populations. The shared ancestry between these populations led us to investigate the NRG1 polymorphisms and appropriate marker haplotypes for linkage and/or association to schizophrenia in the Irish study of high-density schizophrenia families (ISHDSF). Neither single-point nor multi-point linkage analysis of NRG1 markers gave evidence for linkage independent of our pre-existing findings telomeric on 8p. Analysis of linkage disequilibrium (LD) across the 252 kb interval encompassing the 7 marker core Icelandic/Scottish NRG1 haplotype revealed two separate regions of modest LD, comprising markers SNP8NRG255133, SNP8NRG249130 and SNP8NRG243177 (telomeric) and microsatellites 478B14-428, 420M9-1395, D8S1810 and 420M9-116I12 (centromeric). From single marker analysis by TRANSMIT and FBAT we found no evidence for association with schizophrenia for any marker. Haplotype analysis for the three SNPs in LD region 1 and, separately, the four microsatellites in LD region 2 (analyzed in overlapping 2-marker windows), showed no evidence for overtransmission of specific haplotypes to affected individuals. We therefore conclude that if NRG1 does contain susceptibility alleles for schizophrenia, they impact quite weakly on risk in the ISHDSF.

Optic disc morphology of patients with OPA1 autosomal dominant optic atrophy.

BACKGROUND/AIMS: Patients with autosomal dominant optic atrophy (ADOA) are genetically heterogeneous, but all have disc pallor. A degree of cupping in ADOA can make the distinction from normal tension glaucoma (NTG) clinically difficult. This study aimed to clarify the features of the optic nerve of patients with ADOA at the OPA1 locus. METHODS: 29 patients (58 eyes), from 12 families, were identified in a prospective observational study of patients with ADOA examined by a single observer between 1995 and 1998, in whom genetic analysis showed either evidence for linkage to chromosome 3q28 or mutations in the ADOA gene, OPA1. All of the patients had disc and fundal photographs available for retrospective analysis. Clinical data collected included disc appearance, intraocular pressure, Snellen visual acuity, Hardy-Rand-Rittler colour vision plates, and Humphrey 30-2 visual fields. RESULTS: Mean age at time of examination was 37 years and mean visual acuity was 6/24. Disc morphology showed temporal disc pallor in 30 eyes (52%) and total disc pallor in 28 eyes (48%). At least one disc showed a cup to disc ratio of more than 0.5 in 18 patients (28 discs, 48%). The temporal neuroretinal rim always showed pallor and shallow shelving (or saucerisation) was seen in 46 eyes (79%). Only 12 discs (21%) had deep excavation and baring of blood vessels. All of the patients had normal intraocular pressure and no family history of glaucoma. There was a temporal grey, pigmentary crescent in 12 patients (18 eyes, 31%) and peripapillary atrophy in 20 patients (40 eyes, 69%), but disc margin haemorrhages were not seen. There was no maculopathy or retinopathy. CONCLUSION: The optic disc morphology, described for the first time in this genetically homogeneous population of patients with OPA1 ADOA, shows a distinctive absence of a healthy neuroretinal rim and shallow saucerisation of the optic disc cup, with frequent peripapillary atrophy.

A frameshift mutation in exon 28 of the OPA1 gene explains the high prevalence of dominant optic atrophy in the Danish population: evidence for a founder effect.

Dominant optic atrophy (DOA) is a hereditary optic neuropathy characterised by decreased visual acuity, colour vision deficits, centro-coecal scotoma and optic nerve pallor. The gene OPA1, encoding a dynamin-related GTPase, has recently been identified within the genetic linkage interval for the major locus for DOA on chromosome 3q28 and shown to harbour genetic aberrations segregating with disease in DOA families. The prevalence of the disorder in Denmark is reported to be the highest of any geographical location, suggestive of a founder effect. In order to establish the genetic basis of disease in a sample of 33 apparently unrelated Danish families, we screened DNA from affected members for OPA1 gene mutations by heteroduplex analysis and direct sequencing. A novel identical mutation in exon 28 (2826delT) was associated with DOA in 14 pedigrees and led to a frameshift and abnormal OPA1 protein -COOH terminus. Haplotype analysis of a region of approximately 1 Mb flanking the OPA1 gene using eight polymorphic markers revealed a common haplotype shared by all 14 patients; this haplotype was markedly over-represented compared with ethnically matched controls. Statistical analysis confirmed significant linkage disequilibrium with DOA over approximately 600 kb encompassing the disease mutation. We have therefore demonstrated that the relatively high frequency of DOA in Denmark is attributable to a founder mutation responsible for approximately 42% of the examined families and suggest that presymptomatic screening for the (2826delT) mutation may facilitate diagnosis and genetic counselling in a significant proportion of DOA patients of Danish ancestry.

OPA1, encoding a dynamin-related GTPase, is mutated in autosomal dominant optic atrophy linked to chromosome 3q28.

Autosomal dominant optic atrophy (ADOA) is the most prevalent hereditary optic neuropathy resulting in progressive loss of visual acuity, centrocoecal scotoma and bilateral temporal atrophy of the optic nerve with an onset within the first two decades of life. The predominant locus for this disorder (OPA1; MIM 165500) has been mapped to a 1.4-cM interval on chromosome 3q28-q29 flanked by markers D3S3669 and D3S3562 (ref. 3). We established a PAC contig covering the entire OPA1 candidate region of approximately 1 Mb and a sequence skimming approach allowed us to identify a gene encoding a polypeptide of 960 amino acids with homology to dynamin-related GTPases. The gene comprises 28 coding exons and spans more than 40 kb of genomic sequence. Upon sequence analysis, we identified mutations in seven independent families with ADOA. The mutations include missense and nonsense alterations, deletions and insertions, which all segregate with the disease in these families. Because most mutations probably represent null alleles, dominant inheritance of the disease may result from haploinsufficiency of OPA1. OPA1 is widely expressed and is most abundant in the retina. The presence of consensus signal peptide sequences suggests that the product of the gene OPA1 is targeted to mitochondria and may exert its function in mitochondrial biogenesis and stabilization of mitochondrial membrane integrity.

Sequence variation within the RPGR gene: evidence for a founder complex allele.

In our study of sequence variation within the RPGR gene associated with X-linked retinitis pigmentosa, we and others have observed a high rate of new mutation within this gene, as all reported mutations are unique or uncommon. In this article we report the identification in a single family of a complex allele of 7 sequence variants in linkage disequilibrium, of which four result in amino-acid alterations (Arg425Lys, DGlu, Thr533Met and Gly566Glu). This complex allele was initially found in a family with XLRP. However, further study revealed an estimated prevalence of 4.3% (15/344 chromosomes) with this complex allele in the European population indicating the non-pathogenic nature of this allele and, along with previously reported polymorphisms, further supporting a high level of human protein diversity for RPGR. This common complex allele may have been established in the population as a founder effect. Complete gene sequencing identified a potential pathogenic sequence variant in the family described (IVS6+5G>A). This study emphasises the need to create a more complete picture of the allelic variation within a gene, suggests cautious interpretation of a phenotypic association with variant sequences, and highlights the potential problems associated with interpreting genetic studies for diagnostic purposes.

Evidence for a new locus for X-linked retinitis pigmentosa (RP23).

PURPOSE: X-linked retinitis pigmentosa (XLRP) is a degenerative disease of the retina characterized in the early stages of disease by night blindness as a result of rod photoreceptor loss, progressing to severe disease with loss of central vision by the third decade in affected males. XLRP displays exceptional genetic heterogeneity, with five reported loci on the human X-chromosome. To investigate the level of heterogeneity for XLRP in the patient pool in the current study, extensive haplotype analysis, linkage analysis, and mutation screening were performed. METHODS: Haplotype analysis of a family with diagnosed XLRP was scored with more than 34 polymorphic markers spanning the entire X-chromosome, including regions already identified as harboring XLRP genes and retina-specific genes. Two-point and multipoint lod scores were calculated. Affected male DNA was amplified with primers specific for the retinoschisis gene (XLRS1), and the products were screened for nucleic acid alterations by direct automated sequencing. RESULTS: In this article haplotype and linkage data are presented identifying a new locus for XLRP on the short arm of the X-chromosome, distinct from previously reported gene localizations for XLRP. The phenotype is atypical, in that the onset of vision loss in the male members of this family is unusually early, and female obligate carriers have normal fundi and waveforms. Informative recombination events in this family define a locus for XLRP (RP23) on Xp22 between the markers DXS1223 and DXS7161, spanning approximately 15 cM. A maximum lod score of 2.1 was calculated for the locus order DXS7103-8 cM-(RP23/DXS1224)-4 cM-DXS999. This new locus (RP23) encompasses the retinoschisis disease gene; therefore, XLRS1 was screened for a mutation. No sequence alteration was identified indicating that mutations in the coding region of the gene responsible for retinoschisis do not cause RP23. CONCLUSIONS: The results describe evidence for a new locus for XLRP (RP23), adding to the established genetic heterogeneity for this disease and the number of genes expressed in ocular tissue residing on the X-chromosome.

Novel frameshift mutations in the RP2 gene and polymorphic variants.

Mutations in the RP2 gene located on Xp11.23 are associated with X-linked retinitis pigmentosa (XLRP), a severe form of progressive retinal degeneration which leads to complete loss of vision in affected males. To date, 14 different mutations in the RP2 gene have been reported to cause XLRP, the majority of which lead to a coding frameshift within the gene and predicted truncation of the protein product. We here report two novel frameshift mutations in RP2 identified in XLRP families by PCR-SSCP and direct sequencing, namely 723delT and 796-799del. Four single nucleotide polymorphisms (SNPs) within the coding region of RP2 are also described (105A>T, 597T>C, 844C>T, 1012G>T), the first polymorphisms to be reported within this gene of unknown function, two of which alter the amino acid sequence. The current study extends the XLRP mutation profile of RP2 and highlights non-pathogenic coding sequence variations which may facilitate both functional studies of the gene and analysis of intragenic allelic contribution to the phenotype.

Difference between RP2 and RP3 phenotypes in X linked retinitis pigmentosa.

AIM: X linked retinitis pigmentosa (XLRP) has two genetic loci known as "RP2" and "RP3". Clinical features reported to differentiate RP2 from RP3 include a higher prevalence of myopia and primary cone dysfunction in RP2, and late onset night blindness and tapetal reflex in RP3. Members from 14 XLRP families were examined in an attempt to verify these differences. METHODS: 16 affected males and 37 females from 14 XLRP families assigned as either RP2 or RP3 by haplotype analysis and/or by heterogeneity analysis were examined. Members of all 14 families who were willing to participate but unavailable for examination were contacted and detailed interviews carried out. RESULTS: No clear phenotypic differences were found that could be used to reliably differentiate RP2 from RP3 with respect to myopia and onset of night blindness. The tapetal reflex was also found to be present in carriers of both RP2 and RP3. CONCLUSIONS: XLRP is a heterogeneous class of rod degenerative disorders with no clear phenotypic differentiation between the two genetic loci RP2 and RP3. There is a continuum of clinical presentations which can be seen in both RP2 and RP3, but the features within a given family tend to be consistent. However, interfamilial variability is prevalent leading to a wide range of clinical presentations and more than one abnormal allele at each gene locus cannot be excluded.

Identification of novel RPGR (retinitis pigmentosa GTPase regulator) mutations in a subset of X-linked retinitis pigmentosa families segregating with the RP3 locus.

The X-linked form of retinitis pigmentosa (XLRP) is a severe disease of the retina, characterised by night blindness and visual field constriction in a degenerative process, culminating with complete loss of sight within the third decade of life. Genetic mapping studies have identified two major loci for XLRP: RP3 (70%-75% of XLRP) and RP2 (20%-25% of XLRP). The RPGR (retinitis pigmentosa GTPase regulator) gene has been cloned within the RP3 genomic interval and it has been shown that 10%-20% of XLRP families have mutations in this gene. Here, we describe a single-strand conformational polymorphism-based mutation screening of RPGR in a pool of 29 XLRP families for which the disease segregates with the RP3 locus, in order to investigate the proportion of RP3 families with RPGR mutations and to relate the results to previous reports. Five different new mutations have been identified: two splice site mutations for exon 1 and three frameshift mutations in exons 7, 10 and 11. The percentage of RPGR mutations identified is 17% (5/29) in our genetically well-defined population. This figure is comparable to the percentage of RP2 gene mutations that we have detected in our entire XLRP patient pool (10%-15%). A correlation of RPGR mutations with phenotype in the families described in this study and the biochemical characterisation of reported mutations may provide insights into the function of the protein.