Understanding the implications of the PAX9 gene in tooth development.

AIM: Tooth agenesis is characterised by the congenital absence of one or more teeth. The Pax9 gene has been associated with nonsyndromic forms. MATERIALS AND METHODS: To investigate the molecular mechanisms, we evaluated specific haplotypes frequency in exon 3 of the Pax9 gene in 26 patients and 21 controls, using an Italian population. RESULTS: Presence of His239His and the Ala240Pro were confirmed in exon 3 of the Pax9 gene. A frequency of 20.2€ of the T allele at position 717 and a C frequency of 33€ of Ala240Pro polymorphism, that reached 40.5€ in the control group, were observed. The 39 C/C-240 C/C or G/Chaplotype which we defined Pax9hapl a had a proportion of 61.9€ in control individuals. The frequency of Pax9hapl a tested in the patients was different from controls, being 81.3€ in normalcy and 18.8€ in oligodontia (p<0.05). CONCLUSION: Our observations suggest that Pax9hapl a may have a protective effect against sporadic oligodontia.

Novel mutations of the RPGR gene in RP3 families.

X-linked retinitis pigmentosa is a severe retinal degeneration characterized by night blindness and visual field constriction, leading to complete blindness within the third decade of life. Mutations in the RPGR gene (retinitis pigmentosa GTPase regulator), located on Xp21.1 in the RP3 region, have been associated with an RP phenotype. Further to our previous mutation screening of RPGR in families segregating with the RP3 locus, we have expanded this study to include other 8 RP3 pedigrees. Here we report the results of this expanded study and the identification of five mutations in RPGR, four of which are novel (IVS6+5 G>A, 950-951delAA, 963 T>C, EX8del) and one of which occurs in the donor splice site of intron 1 (IVS1+1 G>A). These findings bring the proportion of "RP3 genotypes" with a mutation in this gene to 27% (10/37). Hum Mutat 15:386, 2000.

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.

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.

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.

Genotype-phenotype correlation in British families with X linked congenital stationary night blindness.

AIM: To correlate the phenotype of X linked congenital stationary night blindness (CSNBX) with genotype. METHODS: 11 CSNB families were diagnosed with the X linked form of the disease by clinical evaluation and mutation detection in either the NYX or CACNA1F gene. Phenotype of the CSNBX patients was defined by clinical examination, psychophysical, and standardised electrophysiological testing. RESULTS: Comprehensive mutation screening identified NYX gene mutations in eight families and CACNA1F gene mutations in three families. Electrophysiological and psychophysical evidence of a functioning but impaired rod system was present in subjects from each genotype group, although the responses tended to be more severely affected in subjects with NYX gene mutations. Scotopic oscillatory potentials were absent in all subjects with NYX gene mutations while subnormal OFF responses were specific to subjects with CACNA1F gene mutations. CONCLUSIONS: NYX gene mutations were a more frequent cause of CSNBX than CACNA1F gene mutations in the 11 British families studied. As evidence of a functioning rod system was identified in the majority of subjects tested, the clinical phenotypes "complete" and "incomplete" do not correlate with genotype. Instead, electrophysiological indicators of inner retinal function, specifically the characteristics of scotopic oscillatory potentials, 30 Hz flicker and the OFF response, may prove more discriminatory.

Is optic nerve fibre mis-routing a feature of congenital stationary night blindness?

PURPOSE: To determine whether patients with congenital stationary night blindness (CSNB) have electrophysiological evidence of optic nerve fibre mis-routing similar to that found in patients with ocular albinism (OA). METHOD: We recorded the Pattern Onset VEP using a protocol optimised to detect mis-routing of optic nerve fibres in older children and adults. We tested 20 patients (age 15-69 yrs) with X-linked or autosomal recessive CSNB, 14 patients (age 9-56 yrs) with OA and 13 normally pigmented volunteers (age 21-66 yrs). We measured the amplitude and latency of the CI component at the occipital midline and over left and right occipital hemispheres. We also assessed the computed inter-hemispheric "difference" signal. Subjects with CSNB were classified as having the "complete" or "incomplete" phenotype on the basis of their ERG characteristics. Members of X-linked CSNB pedigrees underwent mutation screening of the NYX and CACNA1F genes. RESULTS: CI was significantly smaller over the ipsilateral hemisphere and more prominent over the contralateral hemisphere in OA patients compared with both controls and CSNB patients. In CSNB patients CI response amplitudes were not significantly different from controls but peak latency was prolonged at all three electrodes compared with controls. The inter-hemispheric "difference" signal was abnormal for the OA group but not for the CSNB group. Contralateral dominance of CI could be identified in the majority of OA patients and the "difference" signal was opposite in polarity for left compared with right eye stimulation in every patient in this group. Only 3 of 20 patients with CSNB showed significant inter-hemispheric asymmetry similar to that seen in the OA patients. All 3 CSNB patients with evidence for optic nerve fibre mis-routing had X-linked pedigrees: 2 had an identified mutation in the NYX gene but no mutation in either the NYX or CACNA1F genes was identified in the third. VEP evidence of optic nerve fibre mis-routing was present in 3 of the 11 subjects with "complete" phenotype and none of the 9 patients with "incomplete" phenotype CSNB. CONCLUSION: Mis-routing of optic nerve fibres does occur in CSNB but we found evidence of it in only 15% of our patients.

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.