Deletion including the oligophrenin-1 gene associated with enlarged cerebral ventricles, cerebellar hypoplasia, seizures and ataxia

Non-specific X-linked mental retardation is a heterogeneous group of disorders with an incidence of approximately 1 in 500 males. A recently identified gene in Xq12, encoding a Rho-GTPase-activating protein, was found to be mutated in individuals with mental retardation. We describe here two sisters with a 46,XY karyotype and a microdeletion of the oligophrenin-1 gene and 1.1 Mb of flanking DNA. We have characterised the molecular interval defining this microdeletion syndrome with the fibre-FISH technique. A visual physical map of 1.2 Mb was constructed which spans the oligophrenin-1 gene and the androgen receptor gene. The analysis of the patients revealed a deletion which extended from the 5' end of the AR gene to a region approximately 80 kb proximal to the EPLG2 gene. The clinical manifestations of the two sisters include psychomotor retardation, seizures, ataxia, hypotonia and complete androgen insensitivity. Cranial MRI scans show enlargement of the cerebral ventricles and cerebellar hypoplasia. Our findings give further support for the involvement of the oligophrenin-1 gene in specific morphological abnormalities of the brain which is of importance in the investigation of male patients presenting with mental retardation. In combination with our results from physical mapping we suggest that a region around the oligophrenin-1 locus is relatively bereft of vital genes.

Subcortical laminar heterotopia in two sisters and their mother: MRI, clinical findings and pathogenesis

MR imaging, clinical data and underlying pathogenesis of subcortical laminar heterotopia (SCLH), also known as band heterotopia, in two sisters and their mother are presented. On MR imaging a different degree of SCLH was found in all three affected family-members. The inversion recovery sequence was considered most useful in the demonstration of the heterotopic band of gray matter and the assessment of cortical thickness. The younger sister presented with epileptic seizures at the age of five months and a delayed achievement of developmental milestones. The older sister of seven years had epileptic seizures since the age of one year, and developmental delay. Their mother has only had one seizure-like episode at the age of 39. Her psychomotor development had been normal. Investigation of DNA samples of the three female family-members revealed a mutation in the X-linked doublecortin gene. Within families with band heterotopia, this gene has also been related to male family members with lissencephaly.

Doublecortin is a developmentally regulated, microtubule-associated protein expressed in migrating and differentiating neurons

Recently, we and others reported that the doublecortin gene is responsible for X-linked lissencephaly and subcortical laminar heterotopia. Here, we show that Doublecortin is expressed in the brain throughout the period of corticogenesis in migrating and differentiating neurons. Immunohistochemical studies show its localization in the soma and leading processes of tangentially migrating neurons, and a strong axonal labeling is observed in differentiating neurons. In cultured neurons, Doublecortin expression is highest in the distal parts of developing processes. We demonstrate by sedimentation and microscopy studies that Doublecortin is associated with microtubules (MTs) and postulate that it is a novel MAP. Our data suggest that the cortical dysgeneses associated with the loss of Doublecortin function might result from abnormal cytoskeletal dynamics in neuronal cell development.

X-linked nonspecific mental retardation (MRX16) mapping to distal Xq28: linkage study and neuropsychological data in a large family

A genetic linkage study was performed on a large four-generation family with variable nonspecific X-linked mental retardation (MRX16), speech abnormalities, and retardation of all milestones. Significant linkage was found in the Xq28 region with loci DXS52, DXS15, BGN, and DXS1108 with maximum LOD scores of 4.86, 4.01, 4.83, and 5.43, respectively, at theta = 0.00. Recombination was observed at the locus DXS1113, thus mapping the gene in an 8-Mb interval between this marker and the Xq telomere. Linkage intervals of three other MRX families overlap with this interval in Xq28 where the RABGDIA gene, mutated in the MRX41 and MRX48 families, is also located. In MRX3, MRX28, but also in MRX16, no alteration of RABGDIA has been found, thus suggesting the existence of at least two MRX genes in distal Xq28.

Non-specific X-linked semidominant mental retardation by mutations in a Rab GDP-dissociation inhibitor

Non-specific X-linked mental retardation (MRX) is a very common disorder which affects approximately 1 in 600 males. Despite this high frequency, little is known about the molecular defects underlying this disorder, mainly because of the clinical and genetic heterogeneity which is evident from linkage studies. Recently, a collaborative study using the candidate gene approach demonstrated the presence of mutations in GDIalpha, a Rab GDP-dissociation inhibitor encoded by a gene localized in Xq28, associated with non-specific mental retardation. GDIalpha is mainly a brain-specific protein that plays a critical role in the recycling of Rab GTPases involved in membrane vesicular transport. The study presented here was designed to assess the prevalence of mutations in the GDIalpha in mentally retarded patients and to discuss the clinical phenotypes observed in affected individuals. Mutation screening of the whole coding region of the GDIalpha gene, using a combination of denaturing gradient gel electrophoresis and direct sequencing, was carried out in 164 patients found negative for expansions across the FRAXA GCC repeat. In addition to the nonsense mutation recently reported in MRX48, we have identified a novel missense mutation in exon 11 of the GDIalpha gene in one familial form of non-specific mental retardation. In this family (family R), all affected males show moderate to severe mental retardation, and the X-linked semidominant inheritance is strongly suggested by the severe phenotypes in males with respect to mildly affected females or unaffected obligatory carriers. This study showed that the prevalence of GDIalpha mutations in non-specific mental retardation could be estimated to be 0.5-1%, and molecular diagnosis and genetic counselling in some cases of non-specific mental handicap can now be provided.

A Golgi localization signal identified in the Menkes recombinant protein

Menkes disease arises from a genetic impairment in copper transport. The gene responsible for the phenotype has been identified as a copper transporting ATPase ( ATP7A ). Recently, the protein encoded by the ATP7A gene has been localized to the Golgi complex. In order to investigate the role of the Menkes disease protein in copper transport, recombinant constructs containing both the full-length open reading frame and an alternatively spliced form have been successfully expressed and localized in mammalian cells. Other studies of a patient with occipital horn syndrome, an allelic variant of Menkes disease, have demonstrated that only this alternatively spliced isoform and not the full-length form is expressed in this patient. The milder form of this patient's phenotype suggests that the alternatively spliced isoform has some functional role in copper transport. In the present study the full-length recombinant Menkes protein was shown by immunofluorescence to localize to the Golgi apparatus and the alternatively spliced form, lacking sequences for transmembrane domains 3 and 4 encoded by exon 10, was shown to localize to the endoplasmic reticulum. Using sequences from exon 10 fused to a non-Golgi reporter molecule, a 38 amino acid sequence containing transmembrane domain 3 of the Menkes protein was found to be sufficient for localization to the Golgi complex. Therefore, the protein sequence encoded by exon 10 may be responsible for this differential localization and both isoforms may be required for comprehensive transport of copper within the cell.

Somatic mutations of the beta-catenin gene are frequent in mouse and human hepatocellular carcinomas

Hepatocellular carcinoma (HCC) is the major primary malignant tumor in the human liver, but the molecular changes leading to liver cell transformation remain largely unknown. The Wnt-beta-catenin pathway is activated in colon cancers and some melanoma cell lines, but has not yet been investigated in HCC. We have examined the status of the beta-catenin gene in different transgenic mouse lines of HCC obtained with the oncogenes c-myc or H-ras. Fifty percent of the hepatic tumors in these transgenic mice had activating somatic mutations within the beta-catenin gene similar to those found in colon cancers and melanomas. These alterations in the beta-catenin gene (point mutations or deletions) lead to a disregulation of the signaling function of beta-catenin and thus to carcinogenesis. We then analyzed human HCCs and found similar mutations in eight of 31 (26%) human liver tumors tested and in HepG2 and HuH6 hepatoma cells. The mutations led to the accumulation of beta-catenin in the nucleus. Thus alterations in the beta-catenin gene frequently are selected for during liver tumorigenesis and suggest that disregulation of the Wnt-beta-catenin pathway is a major event in the development of HCC in humans and mice.

doublecortin is the major gene causing X-linked subcortical laminar heterotopia (SCLH)

Subcortical laminar heterotopia (SCLH), or 'double cortex', is a cortical dysgenesis disorder associated with a defect in neuronal migration. Clinical manifestations are epilepsy and mental retardation. This disorder, which mainly affects females, can be inherited in a single pedigree with lissencephaly, a more severe disease which affects the male individuals. This clinical entity has been described as X-SCLH/LIS syndrome. Recently we have demonstrated that the doublecortin gene, which is localized on the X chromosome, is implicated in this disorder. We have now performed a systematic mutation analysis of doublecortin in 11 unrelated females with SCLH (one familial and 10 sporadic cases) and have identified mutations in 10/11 cases. The sequence differences include nonsense, splice site and missense mutations and these were found throughout the gene. These results provide strong evidence that loss of function of doublecortin is the major cause of SCLH. The absence of phenotype-genotype correlations suggests that X-inactivation patterns of neuronal precursor cells are likely to contribute to the variable clinical severity of this disorder in females.

Mutations in GDI1 are responsible for X-linked non-specific mental retardation

Rab GDP-dissociation inhibitors (GDI) are evolutionarily conserved proteins that play an essential role in the recycling of Rab GTPases required for vesicular transport through the secretory pathway. We have found mutations in the GDI1 gene (which encodes uGDI) in two families affected with X-linked non-specific mental retardation. One of the mutations caused a non-conservative substitution (L92P) which reduced binding and recycling of RAB3A, the second was a null mutation. Our results show that both functional and developmental alterations in the neuron may account for the severe impairment of learning abilities as a consequence of mutations in GDI1, emphasizing its critical role in development of human intellectual and learning abilities.

A gene for non-specific X-linked mental retardation (MRX55) is located in Xp11

A new family with a non-specific X-linked mental retardation (MRX55) is described. An X-linked recessive inheritance is suggested by the segregation from two healthy transmitting females of moderate mental retardation in three males, without any specific clinical, radiological or biological features. Two point linkage analysis demonstrated significant linkage between the disorder and several markers in Xp11 (Zmax = 2.11, theta = 0); multipoint linkage analyses confirmed the significant linkage with a maximum lod score (Z = 2.11 at theta = 0, at DXS8012). Recombination events observed with the flanking markers DXS1068 and DXS1275 delineate a 34 centimorgan interval in the pericentromeric region. The interval of assignment pointed out in this family overlaps with several MRX loci previously reported in Xp11 which are reviewed here in.

Oligophrenin-1 encodes a rhoGAP protein involved in X-linked mental retardation

Primary or nonspecific X-linked mental retardation (MRX) is a heterogeneous condition in which affected patients do not have any distinctive clinical or biochemical features in common apart from cognitive impairment. Although it is present in approximately 0.15-0.3% of males, most of the genetic defects associated with MRX, which may involve more than ten different genes, remain unknown. Here we report the characterization of a new gene on the long arm of the X-chromosome (position Xq12) and the identification in unrelated individuals of different mutations that are predicted to cause a loss of function. This gene is highly expressed in fetal brain and encodes a protein of relative molecular mass 91K, named oligophrenin-1, which contains a domain typical of a Rho-GTPase-activating protein (rhoGAP). By enhancing their GTPase activity, GAP proteins inactivate small Rho and Ras proteins, so inactivation of rhoGAP proteins might cause constitutive activation of their GTPase targets. Such activation is known to affect cell migration and outgrowth of axons and dendrites in vivo. Our results demonstrate an association between cognitive impairment and a defect in a signalling pathway that depends on a Ras-like GTPase.

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.

A novel CNS gene required for neuronal migration and involved in X-linked subcortical laminar heterotopia and lissencephaly syndrome

X-SCLH/LIS syndrome is a neuronal migration disorder with disruption of the six-layered neocortex. It consists of subcortical laminar heterotopia (SCLH, band heterotopia, or double cortex) in females and lissencephaly (LIS) in males, leading to epilepsy and cognitive impairment. We report the characterization of a novel CNS gene encoding a 40 kDa predicted protein that we named Doublecortin and the identification of mutations in four unrelated X-SCLH/LIS cases. The predicted protein shares significant homology with the N-terminal segment of a protein containing a protein kinase domain at its C-terminal part. This novel gene is highly expressed during brain development, mainly in fetal neurons including precursors. The complete disorganization observed in lissencephaly and heterotopia thus seems to reflect a failure of early events associated with neuron dispersion.

Gene for nonspecific X-linked mental retardation (MRX 47) is located in Xq22.3-q24

We describe a large family with nonspecific X-linked mental retardation (MRX 47). An X-linked recessive transmission is suggested by the inheritance from the mothers in two generations of a moderate to severe form of mental retardation in six males, without any specific clinical findings. Two point linkage analysis demonstrated significant linkage between the disorder and two markers in Xq23 (Zmax = 3.75, theta = 0). Multipoint linkage analyses confirmed the significant linkage with a maximum lod score (Z = 3.96, theta = 0) at DXS1059. Recombination events observed with the flanking markers DXS1105 and DXS8067 delineate a 17 cM interval. This interval overlaps with several loci of XLMR disorders previously localized in Xq23-q24, which are reviewed herein.

A gene for dominant nonspecific X-linked mental retardation is located in Xq28

A large family (MRX48) with a nonspecific X-linked mental retardation condition is described. An X-linked semidominant inheritance is suggested by the segregation in three generations of a moderate to severe mental retardation in seven males and by a milder intellectual impairment in two females, without any specific clinical, radiological, or biological feature. Two-point linkage analysis demonstrated significant linkage between the disorder and several markers in Xq28 (maximum LOD score [Zmax] = 2.71 at recombination fraction [theta] = 0); multipoint linkage analyses confirmed the significant linkage with a Zmax of 3.3 at theta = 0, at DXS1684. A recombination event observed with the flanking marker DXS8011 delineates a locus between this marker and the telomere. The approximate length of this locus is 8-9 cM, corresponding to 5.5-6 Mb. In an attempt to explain the variable intellectual impairment in females, we examined X-chromosome inactivation in all females of the family. Inactivation patterns in lymphocytes were random or moderately skewed, and no correlation between the phenotypic status and a specific inactivation pattern was observed. The interval of assignment noted in this family overlaps with five MRX loci previously reported in Xq28.

Mapping of the X-breakpoint involved in a balanced X;12 translocation in a female with mild mental retardation

Balanced chromosomal abnormalities such as translocations and inversions have been identified in many genetic diseases. Cloning of the breakpoints involved in these abnormalities has led to the identification of the disease-related genes. Recent reports suggest the presence of a mental retardation locus at Xq11-12. We have identified a female patient with a balanced translocation t (X;12) (q11;q15) associated with mild mental retardation. We identified a yeast artificial chromosome spanning the X-chromosome breakpoint by using fluorescent in situ hybridization techniques. A cosmid library of this YAC has been constructed and the search for candidate genes is in progress.

Dominant X linked subcortical laminar heterotopia and lissencephaly syndrome (XSCLH/LIS): evidence for the occurrence of mutation in males and mapping of a potential locus in Xq22

X linked subcortical laminar heterotopia and lissencephaly syndrome (XSCLH/ LIS) is an intriguing disorder of cortical development, which causes classical lissencephaly with severe mental retardation and epilepsy in hemizygous males, and subcortical laminar heterotopia (SCLH) associated with milder mental retardation and epilepsy in heterozygous females. Here we report an exclusion mapping study carried out in three unrelated previously described families in which males are affected with lissencephaly and females with SCLH, using 38 microsatellite markers evenly distributed on the X chromosome. Most of the X chromosome was excluded and potential intervals of assignment in Xq22.3-q23 or in Xq27 are reported. Although the number of informative meioses did not allow a decision between these two loci, it is worth noting that the former interval is compatible with the mapping of a breakpoint involved in a de novo X;autosomal balanced translocation 46,XX,t(X;2)(q22;p25) previously described in a female with classical lissencephaly. In addition, haplotype inheritance in two families showed a grandpaternal origin of the mutation and suggested in one family the presence of mosaicism in germline cells of normal transmitting males.

X-linked neurodegenerative syndrome with congenital ataxia, late-onset progressive myoclonic encephalopathy and selective macular degeneration, linked to Xp22.33-pter

Linkage analysis was performed in a previously described family segregating for an X-linked progressive neurological disorder [Bertini et al., 1992]. In three generations, the disease was inherited from the mothers in seven affected males (Fig. 1). Five had severe congenital hypotonia and died during the first year of life. Two other boys (maternal cousins) were found to have severe congenital ataxia, late-onset progressive myoclonic encephalopathy, and selective macular degeneration; brain CT-scan showed moderate cerebellar vermis hypoplasia. Linkage analysis was carried out in 12 informative relatives using 35 microsatellite markers (Généthon) evenly distributed on the X chromosome. A multipoint analysis showed a significant linkage (Z > 2) between the disease and three markers in the Xp22.33 region: DYS403 (Z = 2.37, theta = 0) which maps in the pseudoautosomal region, DXS7099 (Z = 2.45, theta = 0), and DXS7100 (Z = 2.48, theta = 0). Further linkage analysis with more telomeric markers will refine the location of this severe X-linked encephalopathy.