Non-skewed X-inactivation may cause mental retardation in a female carrier of X-linked alpha-thalassemia/mental retardation syndrome (ATR-X): X-inactivation study of nine female carriers of ATR-X

X-linked alpha-thalassemia/mental retardation syndrome (ATR-X) is a syndromic form of X-linked mental retardation. We investigated the X-inactivation status of nine female ATR-X carriers by methylation-specific PCR of the HUMARA gene. Six carriers demonstrated a skewed X-inactivation pattern (>90:10) and one showed a non-skewed pattern (72:28), while two were uninformative because of homozygosity for the CAG repeat polymorphic alleles in the HUMARA. Only the carrier mother who showed non-skewed X-inactivation had moderate mental retardation. These findings suggest that mutations in ATRX may cause mental retardation in females, if the X chromosome carrying mutated ATRX is not properly inactivated.     

X-linked mental retardation syndrome with characteristic "coarse" facial appearance, brachydactyly, and short stature maps to proximal Xq.

We describe a three-generation family in which X-linked mental retardation (XLMR) is associated with minor facial anomalies and brachydactyly. Two brothers and four nephews have "coarse" facial appearance, brachydactyly with widening of the distal phalanges, short stature, and moderate mental retardation. The three obligate carrier women have normal intelligence and normal physical findings. The results of linkage analysis carried out in 1988 using restriction fragment length polymorphisms (RFLPs) were suggestive of linkage to DXYS1 and DXS101 in proximal Xq (Zmax = 1.63 at straight thetamax = 0.0) [Carpenter et al., 1988: Am J Med Genet 43:A139]. The family was restudied with 16 microsatellite loci from Xp11.4 through Xq24. Linkage analysis demonstrated significant linkage to DXS1003, ALAS2, AR, DXS986, DXS990, DXS454, DXS1106, DXS1105, and DXS1220 from Xp11.3 to Xq23 (Zmax = 2.53 at straight thetamax = 0.0). Recombinations detected between MAOB and DXS1055 and between DXS1220 and DXS1001 place the disease locus between Xp11.3 and Xq23. Among the genes known to map to this region is the XNP gene for the alpha-thalassemia/mental retardation syndrome (ATR-X). This fact, along with the phenotypic similarity between our patients and ATR-X males, led us to consider XNP as a candidate gene for this family. X-inactivation studies provided further evidence for the involvement of XNP by showing completely skewed X-inactivation patterns in the three obligate carrier females, a pattern characteristic of carriers of XNP mutations.     

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.     

Börjeson-Forssman-Lehmann syndrome in a woman with skewed X-chromosome inactivation

B?rjeson-Forssman-Lehmann (BFL) syndrome is an X-linked recessive disorder characterized by minor facial anomalies, obesity, epilepsy, and severe mental retardation. The phenotype of male patients is usually severe, whereas that of carriers is less severe, suggesting X-linked incompletely recessive inheritance. A recent linkage study mapped the BFL syndrome gene to Xq26-q27. The etiology of the condition in female patients with full manifestations is not known, although nonrandom X-chromosome inactivation has been considered. We recently developed an assay for X-inactivation studies based on the methylation-specific polymerase chain reaction (PCR) technique. Using the methylation-specific PCR assay, a woman with typical findings of this syndrome was shown to have an extremely skewed X-inactivation pattern. This finding suggests that the full manifestations of the BFL syndrome in carriers may be caused by skewed X inactivation with a high proportion of cells in which the X chromosome with a normal gene be inactivated, leaving the X chromosome with a mutant gene active.     

Monogenic X-linked mental retardation: is it as frequent as currently estimated? The paradox of the ARX (Aristaless X) mutations

Mental retardation affects 30 to 50% more males than females, and X-linked mental retardation (XLMR) is thought to account for the major part of this sex bias. Nonsyndromic XLMR is very heterogeneous, with more than 15 genes identified to date, each of them accounting for a very small proportion of nonsyndromic families. The Aristaless X (ARX) gene is an exception since it was found mutated in 11 of 136 such families, with a highly recurrent mutation (dup24) leading to an expansion of a polyalanine tract in the protein. The rather high frequency of dup24 reported in families with clear X-linked MR (6.6%) contrasts with the very low prevalence of this mutation observed in sporadic male MR (0.13%). We conclude that monogenic XLMR has much lower prevalence in male MR (< 10%) than the 23% that would be required to account for a 30% male excess of mental retardation.     

Mutation frequencies of X-linked mental retardation genes in families from the EuroMRX consortium

The EuroMRX family cohort consists of about 400 families with non-syndromic and 200 families with syndromic X-linked mental retardation (XLMR). After exclusion of Fragile X (Fra X) syndrome, probands from these families were tested for mutations in the coding sequence of 90 known and candidate XLMR genes. In total, 73 causative mutations were identified in 21 genes. For 42% of the families with obligate female carriers, the mental retardation phenotype could be explained by a mutation. There was no difference between families with (lod score >2) or without (lod score <2) significant linkage to the X chromosome. For families with two to five affected brothers (brother pair=BP families) only 17% of the MR could be explained. This is significantly lower (P=0.0067) than in families with obligate carrier females and indicates that the MR in about 40% (17/42) of the BP families is due to a single genetic defect on the X chromosome. The mutation frequency of XLMR genes in BP families is lower than can be expected on basis of the male to female ratio of patients with MR or observed recurrence risks. This might be explained by genetic risk factors on the X chromosome, resulting in a more complex etiology in a substantial portion of XLMR patients. The EuroMRX effort is the first attempt to unravel the molecular basis of cognitive dysfunction by large-scale approaches in a large patient cohort. Our results show that it is now possible to identify 42% of the genetic defects in non-syndromic and syndromic XLMR families with obligate female carriers.     

X chromosome inactivation and X-linked mental retardation

The expression of X-linked genes in females heterozygous for X-linked defects can be modulated by epigenetic control mechanisms that constitute the X chromosome inactivation pathway. At least four different effects have been found to influence, in females, the phenotypic expression of genes responsible for X-linked mental retardation (XLMR). First, non-random X inactivation, due either to stochastic or genetic factors, can result in tissues in which one cell type (for example, that in which the X chromosome carrying a mutant XLMR gene is active) dominates, instead of the normal mosaic cell population expected as a result of random X inactivation. Second, skewed inactivation of the normal X in individuals carrying a deletion of part of the X chromosome has been documented in a number of mentally retarded females. Third, functional disomy of X-linked genes that are expressed inappropriately due to the absence of X inactivation has been found in mentally retarded females with structurally abnormal X chromosomes that do not contain the X inactivation center. And fourth, dose-dependent overexpression of X-linked genes that normally “escape” X inactivation may account for the mental and developmental delay associated with increasing numbers of otherwise inactive X chromosomes in individuals with X chromosome aneuploidy. © 1996 Wiley-Liss, Inc.     

Psychometric assessment of families with X-linked mental retardation

As part of an integrated approach to DNA-linkage analysis in X-linked mental retardation (XLMR), 29 members of five families suspected of having XLMR underwent psychometric assessment. Mental retardation was confirmed in all participants. The range of mental retardation varied from mild to profound within and between families. In addition, these preliminary results indicated family-specific cognitive profiles in MRX45 and MRX46. The fact that two non-overlapping loci were involved provides strong evidence that specific cognitive profiles are linked to specific loci (genes) in mental retardation. We therefore recommend the application of standardised psychometric tests for the assessment of XLMR.     

Novel JARID1C/SMCX mutations in patients with X-linked mental retardation

X-linked mental retardation (XLMR) is a heterogeneous disorder that affects approximately 2 in 1000 males. JARID1C/SMCX is relatively new among the known XLMR genes, and seven different mutations have been identified previously in this gene [Jensen LR et al., Am. J. Hum. Genet. 76:227-236, 2005]. Here, we report five novel JARID1C mutations in five XLMR families. The changes comprise one nonsense mutation (p.Arg332X) and four missense mutations (p.Asp87Gly; p.Phe642Leu; p.Arg750Trp; p.Tyr751Cys) affecting evolutionarily conserved amino acids. The degree of mental retardation in the affected males ranged from mild to severe, and some patients suffered from additional disorders such as epilepsy, short stature, or behavioral problems. This study brings the total number of reported JARID1C mutations to twelve. In contrast to other XLMR genes in which mutations were found only in single or very few families, JARID1C appears to be one of the more frequently mutated genes in this disorder.     

Linkage analysis in three families with nonspecific X-linked mental retardation

Nonspecific X-linked mental retardation (XLMR) is a common disorder. The number of genes involved in this condition is not known, but it is estimated to be more than 10. We present a clinical and linkage study on 3 families with XLMR. All families were analyzed using highly polymorphic markers covering the X chromosome; screening for the fragile X mutation was negative. The first family (MRX 36) consisted of 1 female and 4 male patients in 3 generations and 7 healthy individuals. Considering the female as an expressing heterozygous carrier, a maximum LOD score of 3.41 was reached in region Xp21.2–Xp22.1. Considering her phenotype to be unknown, a LOD_max of 1.97 was reached in the same region. The second family consisted of 5 affected and 6 healthy males with mild to borderline mental retardation. Linkage analysis using an X-linked recessive model with full penetrance and no phenocopies excluded linkage over almost the entire X chromosome. Using alternative models, including an affecteds-only analysis, a LOD_max of 1.49 was found in region Xq24–28. The third family, consisting of 4 male patients with moderate mental retardation in 1 generation yielded a LOD_max of 0.9 in region Xp22.13–11.3. However, even in this small pedigree, exclusion mapping was able to exclude very large parts of the X chromosome and in this way identify a likely candidate region. © 1996 Wiley-Liss, Inc.     

Expanding phenotype of XNP mutations: mild to moderate mental retardation

Mutations in the XNP gene have been reported in alpha thalassemia/mental retardation (MR) syndrome (ATR-X) and other severe X-linked MR conditions with facial dysmorphisms. In this report, we describe a missense mutation in exon 18 in a family with borderline to moderate MR. Like other disorders associated with an XNP mutation, skewed X-inactivation was found in all carrier females in this family. Only retrospective examination revealed childhood facial hypotonia and HbH inclusions in some of the affected males. These results expand the spectrum of clinical phenotypes known to be due to mutations in the XNP gene, and indicate that XNP mutation analysis should not be restricted to patients with severe MR and characteristic facial features.     

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.     

New X-linked syndrome of mental retardation, short stature, and hypertelorism

We present a three-generation family with five retarded, abnormally appearing males and two abnormally appearing females (presumably manifesting carriers). The phenotype of the patients is different from that of all other previously described types of X-linked mental retardation (XLMR). The patients had prominent forehead, frontal bossing, hypertelorism, broad nasal tip, and anteverted nares. Chromosomes were normal including fragile X analysis. Skeletal roentgenograms were normal.     

De novo MECP2 duplication in two females with random X-inactivation and moderate mental retardation

Xq28 duplications including MECP2 are a well-known cause of severe mental retardation in males with seizures, muscular hypotonia, progressive spasticity, poor speech and recurrent infections that often lead to early death. Female carriers usually show a normal intellectual performance due to skewed X-inactivation (XCI). We report on two female patients with a de novo MECP2 duplication associated with moderate mental retardation. In both patients, the de novo duplication occurred on the paternal allele, and both patients show a random XCI, which can be assumed as the triggering factor for the phenotype. Furthermore, we describe the phenotype that might be restricted to unspecific mild-to -moderate mental retardation with neurological features in early adulthood.     

Familial skewed X-chromosome inactivation linked to a component of the cohesin complex, SA2

The gene dosage inequality between females with two X-chromosomes and males with one is compensated for by X-chromosome inactivation (XCI), which ensures the silencing of one X in every somatic cell of female mammals. XCI in humans results in a mosaic of two cell populations: those expressing the maternal X-chromosome and those expressing the paternal X-chromosome. We have previously shown that the degree of mosaicism (the X-inactivation pattern) in a Canadian family is directly related to disease severity in female carriers of the X-linked recessive bleeding disorder, haemophilia A. The distribution of X-inactivation patterns in this family was consistent with a genetic trait having a co-dominant mode of inheritance, suggesting that XCI choice may not be completely random. To identify genetic elements that could be responsible for biased XCI choice, a linkage analysis was undertaken using an approach tailored to accommodate the continuous nature of the X-inactivation pattern phenotype in the Canadian family. Several X-linked regions were identified, one of which overlaps with a region previously found to be linked to familial skewed XCI. SA2, a component of the cohesin complex is identified as a candidate gene that could participate in XCI through its association with CTCF.     

Clinical aspects of the MASA syndrome in a large family, including expressing females

We have evaluated, both clinically and by linkage analysis, a large family with 22 known affected males with the MASA syndrome (McKusick 303300). Clinical findings varied widely amongst the affected family members, with some appearing initially to have the MASA syndrome and others to have X-linked hydrocephalus (HSAS) (McKusick 307000). Important findings included the presence of adducted thumbs in two obligate carriers, learning problems or mild mental retardation in three females, two of whom were obligate carriers, and hydrocephalus with neonatal death in three females born to obligate carriers. X-inactivation analysis in lymphocytes from the two women with adducted thumbs revealed preferential inactivation of one X chromosome, suggesting that nonrandom X-inactivation may be responsible for clinical expression in females. The presence of HSAS in some individuals of this family and the MASA syndrome in others further supports the hypothesis that these two conditions are the result of a mutation in the same gene.     

Cognitive function in Coffin-Lowry syndrome

Coffin-Lowry syndrome (CLS) is an X-linked disorder associated with mental retardation, distinctive facies and hands, hypotonia, and skeletal abnormalities. The syndrome results from mutations in the RSK2 gene located in Xp22.2. Although the syndrome has been elucidated clinically, few, if any, studies have focused on the cognitive deficits of the affected males or carrier females. The subjects of the present study were selected from two African-American families who have the same missense mutation (C340T) in RSK2. The subjects included six affected males, seven carrier females, three normal males and three non-carrier (normal) females. Normal family members served as contrast/comparison cohorts to control for socio-economic, sociocultural and genetic variables which would impinge on intellectual abilities. Analysis of cognitive function, as measured by the Stanford-Binet Intelligence Scale, 4th edn, demonstrated a distinct hierarchy of abilities from normal to carrier to affected patients. The mean composite IQs of the cohorts were 90.8, 65.0 and 43.2 for normal, carrier and affected individuals, respectively. These findings lend support to the clinical concept of negative intellectual effects in carriers of certain X-linked mental retardation conditions. X-inactivation studies showed that carrier females had mild to significant skewing. Normal females in the family did not demonstrate skewing. The correlation coefficient between IQ and X-inactivation status among carriers was not significant.