A study of mental retardation in children in the Island of Hawaii

Mutations in the BRWD3 gene cause X-linked mental retardation associated with macrocephaly

In the course of systematic screening of the X-chromosome coding sequences in 250 families with nonsyndromic X-linked mental retardation (XLMR), two families were identified with truncating mutations in BRWD3, a gene encoding a bromodomain and WD-repeat domain-containing protein. In both families, the mutation segregates with the phenotype in affected males. Affected males have macrocephaly with a prominent forehead, large cupped ears, and mild-to-moderate intellectual disability. No truncating variants were found in 520 control X chromosomes. BRWD3 is therefore a new gene implicated in the etiology of XLMR associated with macrocephaly and may cause disease by altering intracellular signaling pathways affecting cellular proliferation.

Diagnostic investigations in individuals with mental retardation: a systematic literature review of their usefulness

There are no guidelines available for diagnostic studies in patients with mental retardation (MR) established in an evidence-based manner. Here we report such study, based on information from original studies on the results with respect to detected significant anomalies (yield) of six major diagnostic investigations, and evaluate whether the yield differs depending on setting, MR severity, and gender. Results for cytogenetic studies showed the mean yield of chromosome aberrations in classical cytogenetics to be 9.5% (variation: 5.4% in school populations to 13.3% in institute populations; 4.1% in borderline-mild MR to 13.3% in moderate-profound MR; more frequent structural anomalies in females). The median yield of subtelomeric studies was 4.4% (also showing female predominance). For fragile X screening, yields were 5.4% (cytogenetic studies) and 2.0% (molecular studies) (higher yield in moderate-profound MR; checklist use useful). In metabolic investigations, the mean yield of all studies was 1.0% (results depending on neonatal screening programmes; in individual populations higher yield for specific metabolic disorders). Studies on neurological examination all showed a high yield (mean 42.9%; irrespective of setting, degree of MR, and gender). The yield of neuroimaging studies for abnormalities was 30.0% (higher yield if performed on an indicated basis) and the yield for finding a diagnosis based on neuroradiological studies only was 1.3% (no data available on value of negative findings). A very high yield was found for dysmorphologic examination (variation 39-81%). The data from this review allow conclusions for most types of diagnostic investigations in MR patients. Recommendations for further studies are provided.

Cytogenetic diagnosis of fragile X syndrome: study of 305 suspected cases in Saudi Arabia

BACKGROUND

Fragile X syndrome is the most common cause of inherited mental retardation. Patients with fragile X syndrome show variable mental disability, typical long and narrow facial appearance with large ears and prominent fontanelle and frequent macro-orchidism. It is generally associated with a fragile site at Xq 27.3, which can be observed in the metaphase chromosome following selective culture conditions. At the molecular level, the fragile X syndrome is associated with an amplification of CGG repeat sequence of the FMR1 gene. The prevalence estimates are reported as one per 1500 males and one per 2500 females. Estimated prevalence rates of fragile X syndrome in different ethnic groups range from 0.4-0.8 per 1000 in males and 0.2-0.6 per 1000 in females. In this study, we have determined the frequency of fragile X-positive cases in 305 preselected patients.

MATERIALS AND METHODS

Three hundred and five Saudi patients with mental retardation/developmental delay/clinical suspicion of fragile X syndrome were screened for fragile X chromosome by cytogenetic methods. The majority of patients (95.59%) screened were under the age of 20 years.

RESULTS

Two hundred and ninety-nine patients (98.03%) were in the category of mild to moderate mental retardation. Twenty-four males (7.86%) and two females (0.65%) were found to express fragile X site at q27.3. The frequency of fragile X-positive cells in males ranged between 7% and 58% (mean 26+/-13.11), while in the females it was between 14% and 21% (mean 12.5+/-35), respectively.

CONCLUSION

The frequency of fragile X positive cases found in this study is similar to other reports of fragile X syndrome in preselected patients.

Nonsyndromic X-linked mental retardation: review and mapping of MRX29 to Xp21

The gene responsible for nonsyndromic mental retardation in a family with 7 affected males has been localized to Xp21. The maximal two-point lod score was 3.31 for tight linkage to marker DXS1202 in Xp21.3-p22.3 with crossovers between the 3' portion of the DMD gene (DXS1234) proximally and locus DXS989 distally. The XLMR gene in this family has been assigned the designation MRX29. The localization overlaps with at least six other MRX entities linked to the distal short arm of the X chromosome.

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

A gene responsible for X-linked mental retardation with macrocephaly and seizures (MRX38) in a family with five affected males in three generations was localized to Xp21.1-p22.13 by linkage analysis. Recombination events placed the gene between DXS1226 distally and DXS1238 proximally, defining an interval of approximately 14 cM. A peak lod score of 2.71 was found with several loci in Xp21.1 (DXS992, DXS1236, DXS997, and DXS1036) at a recombination fraction of zero. The map intervals of 5 X-linked mental retardation loci, MRX2 (Xp22.1-p22.2), MRX19 (Xp22), MRX21 (Xp21.1-p22.3), MRX29 (Xp21.2-p22.1), and MRX32 (Xp21.2-p22.1), and two syndromal mental retardation loci, Partington syndrome (PRTS; Xp22) and Coffin-Lowry syndrome (CLS; Xp22.13-p22.2), overlap this region. As none of these display the same phenotype seen in the family reported here, this X-linked mental retardation locus may represent a new entity.

No apparent involvement of the FMR1 gene in five patients with phenotypic manifestations of the fragile X syndrome

Most fragile X patients have a significant increase in the number of CGG repeats in the FMR1 gene. Two patients were described with a deletion and one patient with a point mutation in the FMR1 gene. We describe 5 patients with a fragile X or Martin-Bell phenotype. Two brothers were discordant for the region containing the FMR1 gene; if there is a common cause for the mental retardation this is not located in the FMR1 gene. In the other 3 patients the expression of the FMR1 gene was found to be normal and no abnormalities were noted in the FMR1 mRNA. No amplification was found in the GCC repeat which is associated with the fragile site FRAXE. We conclude that the Martin-Bell phenotype can also be caused by mutations outside the FMR1 gene.

XLMR genes: update 1994

We provide a comprehensive list of all known forms of X-linked mental retardation. It comprises 127 entries, subdivided into 5 categories (syndromes, dominant disorders, metabolic disorders, neuromuscular disorders, and nonspecific mental retardation). Map location of 69 putative loci demonstrates several overlaps, which will only be resolved by more refined mapping or cloning of the respective genes. The ultimate goal of identifying all the genes on the X chromosome whose mutations cause mental retardation will require a concerted effort between clinical and molecular investigators.

Pericentromeric genes for non-specific X-linked mental retardation (MRX)

Extensive linkage analyses in three families with non-specific X-linked mental retardation (MRX) have localized the gene in each family to the pericentromeric region of the chromosome. The MRX17 gene is localized with a peak lod of 2.41 (theta = 0.0) with the trinucleotide repeat polymorphism at the androgen receptor (AR) gene locus. This gene lies in the interval between the markers DXS255 and DXS990, as defined by recombinants. The MRX18 gene maps to the interval between the markers DXS538 and DXS1126, with a peak lod score of 2.01 (theta = 0.0) at the PFC gene locus. In the third family (Family E) with insufficient informative meioses for assignment of an MRX acronym, the maximum lod score is 1.8 at a recombination fraction of zero for several marker loci between DXS207 and DXS426. Exclusions from the regions of marker loci spanning Xq support the localization of the MRX gene in Family E to the pericentromeric region. Localizations of these and other MRX genes have determined that MRX2 and MRX19 map to distal Xp, MRX3, and MRX6 map to distal Xq, whilst the majority cluster in the pericentromeric region. In addition, we confirm that there are at least two distinct MRX genes near the centromere as delineated by the non-overlapping regional localizations of MRX17 and MRX18. Determination of these non-overlapping localizations is currently the only means of classifying non-syndromal forms of mental retardation and determining the minimum number of MRX loci.

Non-specific X-linked mental retardation: linkage analysis in MRX2 and MRX4 families revisited

We have previously reported linkage analysis in 3 families with non-specific X-linked mental retardation (XLMR). This used RFLPs and was limited by the relatively low informativeness and density of markers available. We have performed a new linkage analysis using microsatellites (including new Genethon markers) in the two most informative families. In the MRX2 family, a lod score of 2.61 at theta = 0.05 had previously been obtained with DXS85 in Xp22.2. We now report a tighter linkage with AFM 135xe7 (DXS989, z = 4.62 at theta = 0.00) and established the order DXS85-DXS207-DXS999 (AFM234 y12)-MRX2, DXS365, DXS1052 (AFM 163yh2), DXS989-DXS1065 (AFM224zf2), DMD 3'. The localization of MRX2 in Xp22.2-p22.1 is thus clearly different from the more distal MRX gene defined by patients with contiguous gene syndromes. In the MRX4 family, a maximum lod score of 2.53 at theta = 0.00 had been obtained with DXS159 in Xq13. Our present study did not show recombination from ALAS2 in Xp11.21 to DXS441 in Xq13.3 (z = 3.38 at theta = 0.00 for the latter marker) and the closest flanking markers are DXS255 in Xp11.22 and DXYS1 in Xq21.3. Reduced recombination around the centromere prevents precise mapping. The localisation of MRX4 overlaps with that of several other MRX families.

Regional localisation of a non-specific X-linked mental retardation gene (MRX19) to Xp22

A gene responsible for a non-specific form of X-linked mental retardation (MRX19) was localised by linkage analysis. Exclusions and regional localisation were made using 21 highly informative PCR-based markers along the X chromosome. Significant lod scores at a recombination fraction of zero were detected with the marker loci DXS207, DXS987 (Zmax = 3.58) and DXS999 (Zmax = 3.28) indicating that this gene is localised to the proximal portion of Xp22. Recombination between MRX19 and the flanking loci KAL and DXS989 was observed. The multipoint CEPH background map, with map distances in cM, is DXS996-1.8-KAL-19.0-DXS207-0.9-[DXS987,DXS443 ]-4.3-DXS999-3.5-DXS365-14.0-DXS989. Two other MRX disorders and two syndromal mental retardations, Coffin-Lowry syndrome and Partington syndrome, have been mapped to this region. There is a possibility that the 3 MRX disorders are the same entity. Most MRX disorders remain clustered around the pericentromeric region.

XLMR genes: update 1992

Up to now, we have identified 77 X-linked conditions in which mental retardation is the primary or a major component manifestation. These conditions were subdivided into 2 categories, designated respectively "X-linked mental retardation syndromes" and "Non-specific X-linked mental retardation". Forty genes have been regionally mapped onto the X chromosome. However, in several instances the data were derived from a single family and most lod scores were less than 3.0.

Non-specific X linked mental retardation

Non-specific X linked mental retardation (MRX) is mental retardation in persons of normal physical appearance who have no recognisable features apart from a characteristic pedigree. Review of published reports shows that there is clinical variability in the degree of mental retardation within families and genetic heterogeneity, based on gene localisation, between families. We propose a classification based on genetic localisation and a set of minimal clinical features that should be recorded in the hope of identifying possible specific phenotypes.

XLMR genes: update 1990

We have identified 39 X-linked conditions in which mental retardation seems to be the primary characteristic, although pathogenesis is unknown. These conditions can be subdivided into syndromal and non-syndromal, depending on the existence of a recognizable pattern of minor anomalies and/or malformations, or lack thereof. Seventeen genes have been regionally mapped onto the X chromosome. However, in 14 instances the data were derived from a single family and most lod scores were less than 3.0.

Fragile X frequency in a mentally retarded population in Brazil

Seventy-five male and 50 female students from 2 special schools for mildly, moderately retarded, or borderline individuals were screened clinically and cytogenetically in order to estimate the contribution of fragile X [fra(X)] syndrome to the cause of mental retardation in Brazil. We found 6 males (8%) from 4 families and 2 unrelated females (4%) with fra(X) chromosomes. One male and one female were isolated cases. The estimated frequency of Martin-Bell [fra(X)] syndrome among mentally impaired individuals in Brazil was similar to that previously reported in other countries.

Linkage analysis suggests at least two loci for X-linked non-specific mental retardation

Epidemiological studies have suggested that non-specific X-linked mental retardation (XLMR) might be at least as frequent as the fragile X syndrome. The identification of all mutations causing XLMR would thus appear of prime importance. In the absence of other clinical signs the problem of genetic heterogeneity is acute. This can be partly overcome by the analysis of large families. We have been able to perform linkage analysis in 3 such families. The condition in family 1 was described as clinically resembling the fra (X) syndrome by Proops et al [1983]: the kindred includes 7 affected males in 3 sibships. Family 2 from Denmark has affected males in 4 generations; however, several affected relatives in this extended pedigree are deceased. Family 3 from France counts 6 affected males in two sibships. The families were analysed with about 25 X-linked markers. Linkage with markers in Xp22.2-p22.3 was found in family 1: z(theta) = 2.62 at theta = 0.06 for DXS85 (probe 782). Suggestion of linkage was found in family 2 with both the Duchenne muscular dystrophy region (DXS164 in Xp21.2) and with DXS1 (Xq11-q12). In family 3, DXS159 (Xq12-q13) gave a lod score of 2.53 at theta = 0; results were compatible with localisation of the putative XLMR locus in this family proximal to DXYS1 (Xq21). These data suggest that at least two non-specific XLMR loci could exist, one in Xp22 and the other in the q12-q13 region.

A cytogenetic study of a population of retarded females with special reference to the fragile (X) syndrome

A cytogenetic survey of a population of 278 mentally retarded females on community placement is described. Thirty-five females had an aneuploid chromosome constitution and a single female was found to have the fra(X) syndrome. The frequency of the fra(X) syndrome among female retardates is discussed together with the apparent absence of de novo mutants among this class of fra(X) probands.

The marker (X) syndrome: a cytogenetic and genetic analysis

The results of a cytogenetic and segregation analysis of 110 pedigrees of the mar (X) syndrome are reported. The cytogenetic study indicated an inverse relationship between IQ and the mar(X) frequency in females but not in males. A small but significant effect of age on mar(X) frequency was observed in both males and females, but in females it was restricted to those of normal intelligence, retarded females showing no significant effect. Classical segregation analysis was performed using the program SEGRAN, analysing sexes separately. A 20% deficit of affected males was observed, the most plausible explanation for the majority of these cases being incomplete penetrance. Since this was an unexpected result, the data were scrutinized for possible biases; however, correction of these had little effect on the estimate. The penetrance of mental impairment in carrier females was estimated to be 30% and of mental impairment and/or mar(X) expression to be 56%. Thus 44% of carriers cannot be detected with our definition of affection. No evidence for sporadic cases among affected males was found. Complex segregation analysis was performed using the sex-linked version of POINTER, analysing sexes together. This was done in order to test the results from classical segregation analysis, to test for family resemblance and to estimate mutation rates. It was confirmed that there was a 20% deficit of affected males, that penetrance of mental impairment in females was approximately 30% and that there was no evidence for sporadic males. Thus all males with the gene appear to have received it from their carrier mothers and all mutations must occur in sperm. The mutation rate in sperm was estimated to be as high as 7.2 X 10(-4), implying that over one-half of random carrier females are fresh mutants. Our results have important implications for genetic counseling as they imply that all mothers of isolated affected males are carriers, that normal brothers of affected males have a 17% chance of carrying the gene and transmitting it to all their daughters, and that normal sisters of affected males have, at most, a 30% chance of being carriers. Since there are biases in the data due to the testing of particular individuals, these probabilities must be considered approximations until they are independently confirmed.