Regional localization of an X-linked mental retardation gene to Xp21.1-Xp22.13 (MRX38)

Is there a Mendelian transmission ratio distortion of the c.429_452dup(24bp) polyalanine tract ARX mutation?

Intellectual disability is common. Aristaless-related homeobox (ARX) gene is one of the most frequently mutated and pleiotropic genes, implicated in 10 different phenotypes. More than half of ~100 reported cases with ARX mutations are due to a recurrent duplication of 24 bp, c.429_452dup, which leads to polyalanine tract expansion. The excess of affected males among the offspring of the obligate carrier females raised the possibility of transmission ratio distortion for the c.429_452dup mutation. We found a significant deviation from the expected Mendelian 1:1 ratio of transmission in favour of the c.429_452dup ARX mutation. We hypothesise that the preferential transmission of the c.429_452dup mutation may be due to asymmetry of meiosis in the oocyte. Our findings may have implications for genetic counselling of families segregating the c.429_452dup mutation and allude to putative role of ARX in oocyte biology.

XLMR in MRX families 29, 32, 33 and 38 results from the dup24 mutation in the ARX (Aristaless related homeobox) gene

Background

X-linked mental retardation (XLMR) is the leading cause of mental retardation in males. Mutations in the ARX gene in Xp22.1 have been found in numerous families with both nonsyndromic and syndromic XLMR. The most frequent mutation in this gene is a 24 bp duplication in exon 2. Based on this fact, a panel of XLMR families linked to Xp22 was tested for this particular ARX mutation.

Methods

Genomic DNA from XLMR families linked to Xp22.1 was amplified for exon 2 in ARX using a Cy5 labeled primer pair. The resulting amplicons were sized using the ALFexpress automated sequencer.

Results

A panel of 11 families with X-linked mental retardation was screened for the ARX 24dup mutation. Four nonsyndromic XLMR families – MRX29, MRX32, MRX33 and MRX38 – were found to have this particular gene mutation.

Conclusion

We have identified 4 additional XLMR families with the ARX dup24 mutation from a panel of 11 XLMR families linked to Xp22.1. This finding makes the ARX dup24 mutation the most common mutation in nonsyndromic XLMR families linked to Xp22.1. As this mutation can be readily tested for using an automated sequencer, screening should be considered for any male with nonsyndromic MR of unknown etiology.

X-linked mental retardation, short stature, microcephaly and hypogonadism maps to Xp22.1-p21.3 in a Belgian family

X-linked mental retardation (XLMR) is a heterogeneous disorder that can be classified as either non-specific (MRX), when mental retardation is the only feature, or as syndromic mental retardation (MRXS). Genetic defects underlying XLMR are being identified at a rapid pace, often starting from X-chromosomal aberrations and XLMR families with a well-defined linkage interval. Here, we present a new family with a syndromic form of XLMR, including mild mental retardation, short stature, microcephaly and hypogonadism. Two-point linkage analysis with 24 polymorphic markers spanning the entire X chromosome was carried out. We could assign the causative gene to a 6 cM interval in Xp22.1-p21.3, with a maximum LOD score of 2.61 for markers DXS989 and DXS1061 at theta = 0.00. No mutations were found in the presented family for two known MRX genes mapping to this interval, ARX and IL1RAPL-1. These data indicate that the interval Xp22.1-p21.3 contains at least one additional MRXS gene.

Dissection of an inverted X(p21.3q27.1) chromosome associated with mental retardation

In a 6 year old boy referred for mental retardation, fragile X syndrome was ruled out by cytogenetic and molecular analyses. Cytogenetic investigations revealed an inverted X chromosome (p21.3q27.1). A similar chromosomal rearrangement was detected in his mildly mentally retarded mother. Fluorescence in situ hybridization (FISH), using a panel of ordered YAC clones, allowed the identification of YACs spanning both the Xp21.3 and Xq27.1 breakpoints, where many non-specific mental retardation loci have been reported so far. Further investigations by FISH showed that the IL1RAPL1 gene at Xp21.3 was disrupted by the X chromosome inversion and therefore its inactivation may be related to the mental retardation observed in our patients.

Novel mental retardation-epilepsy syndrome linked to Xp21.1-p11.4

We evaluated a kindred with X-linked mental retardation and epilepsy. Seven affected males with mild to moderate mental retardation developed seizures (primarily generalized, tonic-clonic, and atonic) that began on average at 6.8 months of age (range, 4 to 14 months). These patients did not have a history of infantile spasms. There were no dysmorphic features. Other than mental retardation, the neurological examination was unremarkable, with exception of 2 affected subjects who had mild generalized rigidity and ataxia. We identified tight linkage to a group of markers on Xp21.1-p11.4. A maximum two-point LOD score of +3.83 at straight theta = 0 was obtained for markers DXS8090, DXS1069, DXS8102, and DXS8085. This locus spans 7.7cM between DXS1049 and DXS8054 and does not overlap the locus for X-linked West syndrome. The tetraspanin gene, implicated in nonspecific mental retardation, is mapped to this region. We sequenced the tetraspanin coding sequence in subjects with X-linked mental retardation and epilepsy and did not identify disease-specific mutations. The syndrome we describe, designated X-linked mental retardation and epilepsy, is clinically and genetically distinct from X-linked West syndrome and other X-linked mental retardation-epilepsy syndromes.

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).

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.

Confirmation of linkage in X-linked infantile spasms (West syndrome) and refinement of the disease locus to Xp21.3-Xp22.1

The syndrome of infantile spasms, hypsarrhythmia, and mental retardation (West syndrome) is a classical form of epilepsy, occurring in early infancy, which is etiologically heterogeneous. In rare families, West syndrome is an X-linked recessive condition, mapped to Xp11.4-Xpter (MIM 308350). We have identified a multi-generation family from Western Canada with this rare syndrome of infantile spasms, seen exclusively in male offspring from asymptomatic mothers, thereby confirming segregation as an X-linked recessive trait. Using highly polymorphic microsatellite CA-repeat probes evenly distributed over the entire X chromosome, linkage to markers DXS7110, DXS989, DXS1202, and DXS7106 was confirmed, with a maximum LOD score of 3.97 at a theta of 0.0. The identification of key recombinants refined the disease-containing interval between markers DXS1226 and the adrenal hypoplasia locus (AHC). This now maps the X-linked infantile spasms gene locus to chromosome Xp21.3-Xp22.1 and refines the interval containing the candidate gene to 7.0 cM. Furthermore, this interval overlaps several loci previously linked with either syndromic or non-syndromic X-linked mental retardation (XLMR), including one recognized locus implicated in neuroaxonal processing (radixin, RDXP2). Collectively, these studies lend strong support for the presence of one or more genes intrinsic to brain development and function, occurring within the critical interval defined between Xp21.3-Xp22.1.

Inherited microdeletion in Xp21.3-22.1 involved in non-specific mental retardation

X-linked mental retardation (XLMR) is a genetically and clinically heterogeneous common disorder. A cumulative frequency of about 1/600 male births was estimated by different authors, including the fragile X syndrome, which affects 1/4000 males. Given this very high cumulative frequency, identification of genes and molecular mechanisms involved in other XLMRs, represents a challenging task of considerable medical importance. In this report we describe clinical and molecular investigations in the family of a mentally retarded boy for whom a microdeletion in Xp21.3-22.1 was detected within the frame of a previously reported systematic search for deletion using STS-PCR screening. Thorough clinical investigation of the sibling showed that two affected brothers exhibit a moderate non-specific mental retardation without any additional neurological impairment, statural growth deficiency or characteristic dysmorphy. Molecular analysis revealed that the microdeletion observed in this family is an inherited defect which cosegregates with mental retardation as an X-linked recessive condition, since both non-deleted boys and transmitting mother are normal. These results and the inherited microdeletion detected within the same region associated with non-specific MR, reported by Raeymaekers et al., suggest that Xp21.3 MR locus is prone to deletions. Therefore, search for microdeletions in the eight families assigned by linkage analysis to this region might allow a better definition of the critical region and an identification of the gene involved in this X-linked mental retardation.

XLMR genes: update 1996

A current list of all known forms of X-linked mental retardation (XLMR) and a slightly revised classification are presented. The number of known disorders has not increased because 6 disorders have been combined based on new molecular data or on clinical grounds and only 6 newly described XLMR disorders have been reported. Of the current 105 XLMR disorders, 34 have been mapped, and 18 disorders and 1 nonspecific XLMR (FRAXE) have been cloned. The number of families with nonspecific XLMR with a LOD score of > or = 2.0 has more than doubled, with 42 (including FRAXE) now being known. a summary of the localization of presumed nonspecific mental retardation (MR) genes from well-studied X-chromosomal translocations and deletions is also included. Only 10-12 nonoverlapping loci are required to explain all localizations of nonspecific MR from both approaches. These new trends mark the beginning of a significantly improved understanding of the role of genes on the X chromosome in producing MR. Continued close collaboration between clinical and molecular investigators will be required to complete the process.