X-linked non-specific mental retardation

Non-specific mental retardation is a very common and genetically heterogeneous disorder but, to date, only six genes related to this condition have been identified. Five of these six have been found in the past two years, through positional-cloning efforts of mapped X-linked families. The characteristics of the newly identified genes are providing insights into the molecular mechanisms of mental impairment and the development of cognitive functions.     

细胞信号转导与X连锁的非特异性精神发育迟滞

非特异性精神发育迟滞是患者仅表现出一般或特殊认知功能障碍的一种病症。对相关的基因及其生理功能进行研究,不仅对弄清非特异性精神发育迟滞的遗传基础有重要意义,还能揭示人类认知功能的分子遗传机理。文章对一些涉及X连锁的非特异精神发育迟滞的基因,其表达产物同时参与细胞信号转导的信号分子,如跨膜受体、鸟苷酸相关蛋白和激酶的生理功能及其研究现状进行了阐述,揭示了细胞信号转导与人类认知活动之间的密切关系,为精神发育迟滞的治疗或预防提供新思路。     

FACL4基因与X连锁非特异性智力低下的关系

对X 连锁非特异性智力低下相关基因的功能研究越来越引起研究者们的注意,目前已见报道的与X 连锁非特异性智力低下相关的基因有24个,FACL4基因是第一个被发现的参与脂肪酸代谢的智力低下相关基因.本文阐述了FACL4基因的结构和功能,以及该基因的研究进展及其与X连锁非特异性智力低下发生的可能机制.     

X-linked mental retardation

X-linked mental retardation (XLMR) affects 1.8 per thousand male births and is usually categorized as "syndromic" (MRXS) or "non-specific" (MRX) forms according to the presence or absence of specific signs in addition to the MR. Up to 60 genes have been implicated in XLMR and certain mutations can alternatively lead to MRXS or MRX. Indeed the extreme phenotypic and allelic heterogeneity of XLMR makes the classification of most genes difficult. Therefore, following identification of new genes, accurate retrospective clinical evaluation of patients and their families is necessary to aid the molecular diagnosis and the classification of this heterogeneous group of disorders. Analyses of the protein products corresponding to XLMR genes show a great diversity of cellular pathways involved in MR. Common mechanisms are beginning to emerge : a first group of proteins belongs to the Rho and Rab GTPase signaling pathways involved in neuronal differentiation and synaptic plasticity and a second group is related to the regulation of gene expression. In this review, we illustrate the complexity of XLMR conditions and present recent data about the FMR1, ARX and Oligophrenin 1 genes.     

MECP2 mutation in male patients with non-specific X-linked mental retardation

In contrast to the preponderance of affected males in families with X-linked mental retardation, Rett syndrome (RTT) is a neurological disorder occurring almost exclusively in females. The near complete absence of affected males in RTT families has been explained by the lethal effect of an X-linked gene mutation in hemizygous affected males. We report here on a novel mutation (A140V) in the MECP2 gene detected in one female with mild mental retardation. In a family study, the A140V mutation was found to segregate in the affected daughter and in four adult sons with severe mental retardation. These results indicate that MECP2 mutations are not necessarily lethal in males and that they can be causative of non-specific X-linked mental retardation.     

X-linked mental retardation

Genetic factors have an important role in the aetiology of mental retardation. However, their contribution is often underestimated because in developed countries, severely affected patients are mainly sporadic cases and familial cases are rare. X-chromosomal mental retardation is the exception to this rule, and this is one of the reasons why research into the genetic and molecular causes of mental retardation has focused almost entirely on the X-chromosome. Here, we review the remarkable recent progress in this field, its promise for understanding neural function, learning and memory, and the implications of this research for health care.     

X-linked recessively inherited non-specific mental retardation. Report of a large family.

A large kindred is described in which 22 males and 3 females show non-specific mental retardation with impaired speech. An X-linked recessive is the most likely mode of inheritance of this condition. Similar families have been described in the literature, characteristic physical abnormalities are absent and performance I.Q. tends to be higher than verbal I.Q. This possible heterogenous condition may be a major individual cause of mental deficiency in males, and may account for the excess of male retardates in the population.     

X-linked mental retardation and epigenetics

The search for the genetic defects in constitutional diseases has so far been restricted to direct methods for the identification of genetic mutations in the patients' genome. Traditional methods such as karyotyping, FISH, mutation screening, positional cloning and CGH, have been complemented with newer methods including array-CGH and PCR-based approaches (MLPA, qPCR). These methods have revealed a high number of genetic or genomic aberrations that result in an altered expression or reduced functional activity of key proteins. For a significant percentage of patients with congenital disease however, the underlying cause has not been resolved strongly suggesting that yet other mechanisms could play important roles in their etiology. Alterations of the 'native' epigenetic imprint might constitute such a novel mechanism. Epigenetics, heritable changes that do not rely on the nucleotide sequence, has already been shown to play a determining role in embryonic development, X-inactivation, and cell differentiation in mammals. Recent progress in the development of techniques to study these processes on full genome scale has stimulated researchers to investigate the role of epigenetic modifications in cancer as well as in constitutional diseases. We will focus on mental impairment because of the growing evidence for the contribution of epigenetics in memory formation and cognition. Disturbance of the epigenetic profile due to direct alterations at genomic regions, or failure of the epigenetic machinery due to genetic mutations in one of its components, has been demonstrated in cognitive derangements in a number of neurological disorders now. It is therefore tempting to speculate that the cognitive deficit in a significant percentage of patients with unexplained mental retardation results from epigenetic modifications.     

Monogenic causes of X-linked mental retardation

Mutations in X-linked genes are likely to account for the observation that more males than females are affected by mental retardation. Causative mutations have recently been identified in both syndromic X-linked mental retardation (XLMR) and in the genetically heterogeneous 'nonspecific' forms of XLMR, for which cognitive impairment is the only defining clinical feature. Proteins that function in chromatin remodelling are affected in three important syndromic forms of XLMR. In nonspecific forms of the disorder, defects have been found in signal-transduction pathways that are believed to function during neuronal maturation. These findings provide important insights into the molecular and cellular defects that underlie mental retardation.     

The frequency of X-linked mental retardation

600 mentally retarded children, 339 boys and 261 girls aged 5-9 years were examined. Boys to girls ratio (B:G) was found in the total group to be 1.3. Among 257 children with mental retardation of confirmed genetic origin B:G = 1.4, in 129 children with confirmed exogenous defects B:G = 1.04. The significant prevalence of boys over girls was characteristic of children with monogenic forms. The frequency of X-linked mental retardation in the total group, in all mentally retarded boys and in boys with genetic forms was 12.5 +/- 1.3%, 22.1 +/- 2.2% and 28.5 +/- 2.8%, respectively. The frequency of X-linked mental retardation was higher in boys with genetic forms of imbecility.     

Further delineation of X-linked mental retardation

Summary Chromosomal, clinical, and psychological data are presented on members of six families with X-linked mental retardation. Affected males in three of these families express the fra(X)(q28) marker, while the retarded males in the other three do not. Similar variable physical and psychological charateristics, such as lop ears, large testes, and perseverative speech, are present in affected males in all six families. Preliminary analysis of the psychological data also shows that males with and without marker expression cannot be differentiated with certainty. On this basis we suggest that there is a type of X-linked mental retardation with many phenotypic features of marker-X mental retardation but without expression of the X chromosome fragile site.     

Making an impression on X-linked mental retardation

Oligophrenin-1 encodes a rhoGAP protein involved in X-linked mental retardationBilluart, P. (1998)Nature 392, 923–926PAK3 mutation in nonsyndromic X-linked mental retardationAllen, K.M. et al. (1998)Nat. Genet. 20, 25–30Mutations in GDI1 are responsible for X-linked non-specific mental retardationD'Adamo, P. et al. (1998)Nat. Genet. 19, 134–139Non-specific X-linked semidominant mental retardation by mutations in a Rab GDP-dissociation inhibitorBienvenu, T. et al. (1998)Hum. Mol. Genet. 7, 1311–1315     

Chromosome X-linked mental retardation and marfanoid syndrome

In the present paper we report two pairs of slightly to moderately mentally retarded brothers with Marfanoid habitus and similar craniofacial changes. They had a long and narrow face, small mandible, high-arched palate and hypernasal voice, as previously reported by Lujan et al. (1984) in 4 mentally retarded males of a large kindred. The present data suggest the existence of a specific type of X-linked mental retardation with Marfanoid habitus.     

Fragile X chromosome and X-linked mental retardation.

A family is described in which three normal females transmitted to seven males X-linked mental retardation associated with macro-orchidism and a fragile site on the long arm of the X chromosome -- fra(X)(q27). The affected males also had minor clinical features in common: a large forehead, long face, large ears, a long upper lip and large extremities.     

Some problems in the genetics of X-linked mental retardation

X-linked mental retardation has recently become one of the most interesting genetic anomalies. Studying this group of conditions has led to many insights into the mechanisms involved in normal and abnormal gene actions in humans. Since the early 1980s, the number of disease entities for which the responsible genes could be localized on the X chromosome has increased from year to year; at the Ninth International Workshop on Fragile-X-Syndrome and X-linked Mental Retardation, 199 such disease units were counted (Hamel, 1999). Conventionally, these units were subdivided into two groups: syndromal and non-syndromal types. The syndro- mal types are characterized by external features, neurological signs, and/or metabolic anomalies. The non-syndromal types do not show such specific features; here, the X-linked mode of inheritance is the only indicator. Due to the reduced reproduction of mentally severely retarded males, a relatively high fraction of new mutants among cases of a specific type must be expected. It cannot be the purpose of the present short article to review sufficiently well the entire field; this would require a complete book. Rather, it is our intention to point to some open problems and possible ways for their solution.     

X-linked mental retardation with macro-orchidism and marker X chromosomes

Summary A family with X-linked mental retardation and a marker X chromosome was ascertained by the presence of macro-orchidism in the three institutionalized probands. Verbal evaluation revealed a generalized language disability with commonly occurring articulation errors. The heterozygous females in this family exhibited some reduction in mental ability; the marker X chromosome was demonstrated in both sexes.     

Dissociation between mental retardation and fragile site expression in a family with fragile X-linked mental retardation

Summary We report an extended family in which two brothers with a fragile X chromosome are mentally retarded while a third brother with the fragile site is both phenotypically and mentally normal. The study of six probes detecting restriction fragment length polymorphisms on either sides of the fragile site Xq27 confirmed that the fragile X regions inherited by these three brothers were identical from DXS 102 to the telomere. These data highlight the heterogeneity of the fragile X syndrome, which is discussed in the framework of the different hypotheses previously proposed.     

Linkage mapping of a severe X-linked mental retardation syndrome.

A four-generation Swedish family with a new type of X-linked mental retardation syndrome was recently reported by Gustavson et al. The complex syndrome includes microcephaly, severe mental retardation, optical atrophy with decreased vision or blindness, severe hearing defect, characteristic facial features, spasticity, seizures, and restricted joint motility. The patients die during infancy or early in childhood. Twenty-one family members, including two affected males, were available for study. Linkage analysis was conducted in the family by using 11 RFLP markers and 10 VNTR markers spread along the X chromosome. A hypervariable short tandem repeat of DXS294 at Xq26 showed a peak two-point lod score of 3.35 at zero recombination fraction. Calculations using the same markers revealed a multipoint peak lod score of 3.65 at DXS294. Crossover events with the centromeric marker DXS424 and the telomeric marker DXS297 delimit a probable region for the gene localization. It is noteworthy that hte disease loci of two other syndromes with overlapping clinical manifestations recently were shown by Turner et al. and Pettigrew et al. to be linked to markers at Xq26.     

Identification of an mRNA-decapping regulator implicated in X-linked mental retardation

Two major decapping enzymes are involved in the decay of eukaryotic mRNA, Dcp2 and DcpS. Despite the detection of robust DcpS decapping activity in cell extract, minimal to no decapping is detected from human Dcp2 (hDcp2) in extract. We now demonstrate that one reason for the lack of detectable hDcp2 activity in extract is due to the presence of inhibitory trans factor(s). Furthermore, we demonstrate that a previously identified testis-specific protein of unknown function implicated in nonspecific X-linked mental retardation, VCX-A, can function as an inhibitor of hDcp2 decapping in vitro and in cells. VCX-A is a noncanonical cap-binding protein that binds to capped RNA but not cap structure lacking an RNA. Its cap association is enhanced by hDcp2 to further augment the ability of VCX-A to inhibit decapping. Our data demonstrate that VCX-A can regulate mRNA stability and that it is an example of a tissue-specific decapping regulator.