Localization of non-specific X-linked mental retardation gene (MRX73) to Xp22.2

Clinical and molecular studies are reported on a family (MRX73) of five males with non-specific X-linked mental retardation (XLMR). A total of 33 microsatellite and RFLP markers was typed. The gene for this XLMR condition was been linked to DXS1195, with a lod score of 2.36 at theta = 0. The haplotype and multipoint linkage analyses suggest localization of the MRX73 locus to an interval of 2 cM defined by markers DXS8019 and DXS365, in Xp22.2. This interval contains the gene of Coffin-Lowry syndrome (RSK2), where a missense mutation has been associated with a form of non-specific mental retardation. Therefore, a search for RSK2 mutations was performed in the MRX73 family, but no causal mutation was found. We hypothesize that another unidentified XLMR gene is located near RSK2.     

A truncating mutation in the IL1RAPL1 gene is responsible for X-linked mental retardation in the MRX21 family

X-linked mental retardation (XLMR) is a genetically heterogeneous condition, due to mutations in at least 50 genes, involved in functioning of the central nervous system and located on the X chromosome. Nonspecific XLMR (MRX) is characterized essentially by mental retardation transmitted by X-linked inheritance. More than 80 extended MRX pedigrees have been reported to date, which have been distinguished exclusively by physical position of the corresponding gene on the X chromosome, established by linkage analysis. One such family, MRX21, which was described by us in 1993 and localized to Xp11.4-pter, has now been reanalyzed with additional markers and after one more affected individual had became available. This extra information allowed a significant reduction of the linkage interval and, eventually, identification of the mutant gene. A stop mutation in exon 10 of the IL1RAPL1 gene (in Xp21) was found in the four affected males and in obligate carriers, allowing conclusive counseling of other family members of uncertain carrier status. The W487X mutation results in the production of a truncated IL1RAPL protein, comprised of the extracellular Ig-like domain and transmembrane tract, but lacking the last 210 aminoacids of the cytoplasmic domain. MRX21 is the first extended MRX family with a point mutation in IL1RAPL1 and the second with a stop mutation, which had been previously found only in a small family. Our report confirms the role of the IL1RAPL1 gene in causing nonspecific mental retardation in males and underlines the importance of detailed linkage analysis before candidate gene mutational screening.     

Localization of a non-syndromic X-linked mental retardation gene (MRX80) to Xq22-q24

Isolated mental retardation is clinically and genetically heterogenous and may be inherited in an autosomal dominant, autosomal recessive, or X-linked manner. We report here a linkage analysis in a large family including 15 members, 6 of whom presenting X-linked non-syndromic mental retardation (MRX). Two-point linkage analysis using 23 polymorphic markers covering the entire X chromosome demonstrated significant linkage between the causative gene and DXS8055 with a maximum LOD score of 2.98 at theta = 0.00. Haplotype analysis indicated location for the disease gene in a 23.1 cM interval between DXS1106 and DXS8067. This MRX localization overlaps with 7 XLMR loci (MRX23, MRX27, MRX30, MRX35, MRX47, MRX53, and MRX63). This interval contains two genes associated with non-syndromic mental retardation (NSMR), namely the PAK3 gene, encoding a p21-activated kinase (MRX30 and MRX47) and the FACL4 gene encoding a fatty acyl-CoA ligase (MRX63). As skewed X-inactivation, an apparently constant feature in FACL4 carrier females was not observed in an obligate carrier belonging to the MRX family presented here, the PAK3 gene should be considered as the strongest candidate for this MRX locus.     

Localization of two X-linked mental retardation (XLMR) genes to Xp: MRX37 gene at Xp22.31-p22.32 and a putative MRX gene on Xp22.11-p22.2

MRX genes of 2 families with X-linked mental retardation (XLMR) were localized by linkage analysis. In family A, the gene was mapped to Xp22.31–p22.32, with significant LOD scores to various Xp22 markers within a distance of 6 Mb between DXS1223 and DXS1224. The MRX gene of this family was designated MRX37. In a mentally retarded female who is a carrier of the MRX37 gene, a random pattern of X inactivation was demonstrated. In family B, a positive LOD score, although not significant (< + 2), was found with the marker DXS1202 at Xp22.11–p22.2. © 1996 Wiley-Liss, Inc.     

X-linked mental retardation with neonatal hypotonia in a French family (MRX15): Gene assignment to Xp11.22-Xp21.1

Linkage analysis was performed in a family with non-specific X-linked mental retardation (MRX 15). Hypotonia in infancy was the most remarkable physical manifestation. The severity of mental deficiency was variable among the patients, but all of them had poor or absent speech. Significant lod scores at a recombination fraction of zero were detected with the marker loci DXS1126, DXS255, and DXS573 (Zmax = 2.01) and recombination was observed with the two flanking loci DXS164 (Xp21.1) and DXS988 (Xp11.22), identifying a 17 cM interval. This result suggests a new gene localization in the proximal Xp region. In numerous families with non-specific X-linked mental retardation (MRX), the corresponding gene has been localized to the paracentromeric region in which a low recombination rate impairs the precision of mapping. © 1996 Wiley-Liss, Inc.     

Regional localization of a nonspecific X-linked mental retardation gene (MRX59) to Xp21.2-p22.2.

Linkage analysis was performed on a four-generation family with nonspecific mental retardation (MRX59). The five affected males, ranging in age from 2 years to 52 years, have a normal facial appearance and mild to severe mental impairment. Four obligate carriers are physically normal and not retarded. A maximum LOD score of 2.41 at straight theta = 0.00 was observed with the microsatellite markers, DMD45 in Xp21.2, DXS989 in Xp22.1, and DXS207 in Xp22.2. Recombinations were detected within the dystrophin gene (DMD) in one of the affected males and between DXS207 and DXS987 in Xp22.2 in one of the carriers. These recombinants define the proximal and distal boundaries of a candidate gene region. Genetic localization of this familial condition made prenatal diagnosis informative for one of the obligate carriers.     

Linkage mapping of a nonspecific form of X-linked mental retardation (MRX53) in a large Pakistani family

Nonspecific X-linked mental retardation is a nonprogressive, genetically heterogeneous condition that affects cognitive function in the absence of other distinctive clinical manifestations. We report here linkage data on a large Pakistani family affected by a form of X-linked nonspecific mental retardation. X chromosome genotyping of family members and linkage analysis allowed the identification of a new disease locus, MRX53. The defined critical region spans approximately 15 cM between DXS1210 and DXS1047 in Xq22.2-26. A LOD score value of 3.34 at no recombination was obtained with markers DXS1072 and DXS8081.     

Localization of a gene for X-linked nonspecific mental retardation (MRX24) in Xp22.2–p22.3

Nonspecific X-linked mental retardation (MRX) includes several distinct genetic entities in which mental retardation is not associated with additional distinguishing physical changes. We report linkage data in a Spanish family with MRX, using polymorphic DNA markers distributed over the X chromosome. Two-point linkage analysis demonstrated close linkage between the MRX locus and DXS85 in Xp22.3 with a peak lod score of 2.28 at a Ø = 0.00. Analysis of multiple informative meioses suggested a localization of the MRX locus (MRX24) between DXS278 and DXS207. Multipoint linkage analysis resulted in a maximum LOD score of 2.45 at 3 cM proximal to DXS85, and allowed us to reject a localization of the MRX24 gene in all other regions from Xp21–Xqter. These findings localize the MRX24 gene in the chromosomal region Xp22.2–p22.3. © 1995 Wiley-Liss, Inc.     

A novel nonsense mutation in TUSC3 is responsible for non-syndromic autosomal recessive mental retardation in a consanguineous Iranian family

The genetic basis of autosomal recessive mental retardation (ARMR) is extremely heterogeneous, and there is reason to suspect that the number of underlying gene defects may well go beyond 1,000. To date, however, only less than 10 genes have been implicated in non‐specific/non‐syndromic ARMR (NS‐ARMR). As part of an ongoing systematic study aiming to identify further ARMR genes, we investigated a consanguineous family with three patients with NS‐ARMR. By linkage analysis and subsequent mutation screening we identified a novel nonsense mutation (c.163C > T [p.Q55X]) in the second exon of the TUSC3 gene. This is the third MR causing defect in TUSC3 to be described and the second independent mutation in this gene in a cohort of more than 200 ARMR families from the Iranian population. This argues for a more prominent role of TUSC3 in the etiology of this genetically heterogeneous disorder as compared to most of the other so far identified ARMR genes. © 2011 Wiley‐Liss, Inc.     

Refined 2.7 centimorgan locus in Xp21.3-22.1 for a nonspecific X-linked mental retardation gene (MRX54).

Nonspecific X-linked mental retardation (MRX) is a heterogeneous condition in which mental retardation (MR) appears to be the only consistent manifestation. A large genetic interval of assignment obtained on individual families by linkage analysis, genetic, heterogeneity, and phenotypic variability usually are major obstacles to fine-map and identify the related disease genes. Here we report on a large Tunisian family (MRX54) with an MRX condition. X-linked recessive inheritance is strongly suggested by the segregation of MR through seven unaffected carrier females to 14 affected males in two generations. Two-point linkage analysis demonstrated significant linkage between the disorder and several markers in Xp21.3-22.1 (maximum LOD score Zmax = 3.56, recombination fraction 0 = 0 at DXS1202), which was confirmed by multipoint linkage analyses. Recombinant events observed with the flanking markers DXS989 and DXS1218 delineate a refined locus of approximately 2.7 cM in accordance with the physical distance between these two markers. The small interval of assignment observed in this family overlaps not only with nine large MRX loci previously reported in Xp21.3-22.1 but also with two inherited microdeletions in Xp21.3-22.1 involved in nonspecific MR. Although the involvement of several genes located in the Xp21.3-22.1 region cannot be ruled out, data reported in this study could be used as a starting point for the search of such gene(s).     

Localization of non‐specific X‐linked mental retardation gene (MRX73) to Xp22.2

Clinical and molecular studies are reported on a family (MRX73) of five males with non‐specific X‐linked mental retardation (XLMR). A total of 33 microsatellite and RFLP markers was typed. The gene for this XLMR condition was been linked to DXS1195, with a lod score of 2.36 at theta = 0. The haplotype and multipoint linkage analyses suggest localization of the MRX73 locus to an interval of 2 cM defined by markers DXS8019 and DXS365, in Xp22.2. This interval contains the gene of Coffin‐Lowry syndrome (RSK2), where a missense mutation has been associated with a form of non‐specific mental retardation. Therefore, a search for RSK2 mutations was performed in the MRX73 family, but no causal mutation was found. We hypothesize that another unidentified XLMR gene is located near RSK2. © 2001 Wiley‐Liss, Inc.     

Non-syndromic X-linked mental retardation associated with a missense mutation (P312L) in the FGD1 gene

Three brothers with non-syndromal X-linked mental retardation were found to have a novel missense mutation in FGD1, the gene associated with the Aarskog syndrome. Although the brothers have short stature and small feet, they lack distinct craniofacial, skeletal or genital findings suggestive of Aarskog syndrome. Their mother, the only obligate carrier available for testing, has the FGD1 mutation. The mutation, a C934T base change in exon 4, results in the proline at position 312 to be substituted with a leucine. This missense mutation is predicted to eliminate a beta-turn, creating an extra-long stretch of coiled sequence which may affect the orientations of an SH3 (Src homology 3) binding domain and the first structural conserved region. A new molecular defect associated with non-syndromal X-linked mental retardation affords an opportunity to seek specific diagnosis in males with previously unexplained developmental delays and this opens further predictive tests in families at risk.     

Linkage mapping of a nonspecific form of X‐linked mental retardation (MRX53) in a large Pakistani family

Nonspecific X‐linked mental retardation is a nonprogressive, genetically heterogeneous condition that affects cognitive function in the absence of other distinctive clinical manifestations. We report here linkage data on a large Pakistani family affected by a form of X‐linked nonspecific mental retardation. X chromosome genotyping of family members and linkage analysis allowed the identification of a new disease locus, MRX53. The defined critical region spans approximately 15 cM between DXS1210 and DXS1047 in Xq22.2–26. A LOD score value of 3.34 at no recombination was obtained with markers DXS1072 and DXS8081. © 2001 Wiley‐Liss, Inc.