A gene for nonspecific X-linked mental retardation (MRX41) is located in the distal segment of Xq28

Novel GDI1 mutation in a large family with nonsyndromic X-linked intellectual disability

X-linked intellectual disability (XLID) is a heterogeneous disorder, and mutations in more than 90 genes have been associated with XLID to date. We report on a large multi-generational German family in which the affected male family members had nonsyndromic intellectual disability, that is, they had neither abnormal body measurements nor any other significant clinical problems. Molecular genetic analysis revealed a frameshift mutation in GDI1 (c.1185_1186delAG; Ser396ProfsX15) that co-segregated with the disease. GDI1 encodes for the GDP-dissociation inhibitor alpha (αGDI), a protein involved in the regulation of the activity of Rab GTPases. Only three families with GDI1 mutations have been reported so far. The present family supports the lack of additional phenotypic features in patients with GDI1 mutations, rendering a clinical diagnosis of GDI1-associated XLID impossible. Thus, this family not only broadens the spectrum of GDI1 mutations but also emphasizes the need for parallel testing of all known genes associated with ID in patients with an unspecific phenotype.

Impairment of cerebello-thalamo-frontal pathway in Rab-GDI mutated patients with pure mental deficiency

BACKGROUND

Rab-GDI mutations are responsible for "pure" mental deficiency, without any specific clinical features or brain malformation. Therefore, screening for mutations in mentally retarded patients is not available on a routine basis. Moreover, neuronal networks involved in mental deficiency still remain largely unknown.

METHODS

We performed a fine neuropsychological and imaging study in five patients from two unrelated families, affected with mental deficiency due to a mutation in the Rab-GDI gene. High resolution 3D brain MRI of the five mentally retarded adult males (mean age 33 years) were compared to MRI of 14 healthy males (mean age 35 years) using a Voxel-Based Morphometric analysis (VBM).

RESULTS

All patients had isolated moderate mental retardation (WAIS-III IQ range, 41-50; mean 45) without specific morphological or behavioural features. No obvious brain abnormality was observed on visual inspection of individual scans. Using VBM analysis, Rab-GDI mutated patients' MRIs exhibited significant brain changes compared to normal subjects (p<0.05, corrected for multiple comparisons): increased grey matter density in left cerebellum and in left angular gyrus, decreased grey matter volume in thalami, decreased white matter density in prefrontal lobes, right fusiform occipito-temporal gyrus, and decreased white matter volume in cerebellar peduncles.

CONCLUSIONS

These morphological changes observed in Rab-GDI mutated patients, mainly localized in the cerebello-thalamo-prefrontal pathway, are consistent with the hypothesis that the cerebellum is one of the critical components of a global learning network. Our results open new avenues in the diagnosis of non-specific mental deficiency using gene-specific "brain maps" as endophenotypes.

Physical and genetic characterization reveals a pseudogene, an evolutionary junction, and unstable loci in distal Xq28

A large portion of human Xq28 has been completely characterized but the interval between G6PD and Xqter has remained poorly understood. Because of a lack of stable, high-density clone coverage in this region, we constructed a 1.6-Mb bacterial and P1 artificial chromosome (BAC and PAC, respectively) contig to expedite mapping, structural and evolutionary analysis, and sequencing. The contig helped to reposition previously mismapped genes and to characterize the XAP135 pseudogene near the int22h-2 repeat. BAC clones containing the distal int22h repeats also demonstrated spontaneous rearrangements and sparse coverage, which suggested that they were unstable. Because the int22h repeats are involved in genetic diseases, we examined them in great apes to see if they have always been unstable. Differences in copy number among the apes, due to duplications and deletions, indicated that they have been unstable throughout their evolution. Taking another approach toward understanding the genomic nature of distal Xq28, we examined the homologous mouse region and found an evolutionary junction near the distal int22h loci that separated the human distal Xq28 region into two segments on the mouse X chromosome. Finally, haplotype analysis showed that a segment within Xq28 has resisted excessive interchromosomal exchange through great ape evolution, potentially accounting for the linkage disequilibrium recently reported in this region. Collectively, these data highlight some interesting features of the genomic sequence in Xq28 and will be useful for positional cloning efforts, mouse mutagenesis studies, and further evolutionary analyses.

Mapping to distal Xq28 of nonspecific X-linked mental retardation MRX72: linkage analysis and clinical findings in a three-generation Sardinian family

Families with mentally retarded males found to be negative for FRAXA and FRAXE mutations are useful in understanding the genetic basis of X-linked mental retardation. According to the most recent data (updated to 1999), 69 MRX loci have been mapped and 6 genes cloned. Here we report on a linkage study performed on 20 subjects from a 4-generation Sardinian family segregating a non-specific X-linked recessive mental retardation (XLMR)(MRX72) associated with global delay of all psychomotor development. Five of 8 affected males have been tested for mental age, verbal and performance skills and behavioral anomalies; mental impairment ranged from mild to severe. Only minor anomalies were present in the affected subjects. Two-point linkage analysis based on 28 informative microsatellites spanning the whole X chromosome demonstrated linkage between the disorder and markers DXS1073 and F8c in Xq28 (maximum Lod score of 2. 71 at straight theta = 0.00). Multipoint linkage analysis confirmed the linkage with a Z(max) of 3.0 at straight theta = 0.00 at DXS1073 and F8c. Recombination in an affected male at DXS1073 and F8c allowed us to delimit centromerically and telomerically the region containing the putative candidate gene. The region, where MRX72 maps, overlaps that of another MRX families previously mapped to Xq28, two of which harbored mutations in GDI. Involvement of this gene was excluded in our family, suggesting another MRX might reside in Xq28.

X-linked mental retardation syndrome with short stature, small hands and feet, seizures, cleft palate, and glaucoma is linked to Xq28

Of the gene-rich regions of the human genome, Xq28 is the most densely mapped. Mutations of genes in this band are responsible for 10 syndromal forms of mental retardation and 5 nonsyndromal forms. Clinical and molecular studies reported here add an additional syndromic form of X-linked mental retardation (XLMR) to this region. The condition comprises short stature, small hands and feet, seizures, cleft palate, and glaucoma. One affected male died at age 19 years in status epilepticus, but others have survived to old age. Carrier females do not have somatic anomalies or mental impairment. The gene is localized to the terminal 8 Mb of Xq28 with markers distal to DXS8011 showing linkage to the disorder with a lod score of 2.11 at zero recombination.

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.

Renpenning syndrome maps to Xp11

Mutations in genes on the X chromosome are believed to be responsible for the excess of males among individuals with mental retardation. Such genes are numerous, certainly >100, and cause both syndromal and nonsyndromal types of mental retardation. Clinical and molecular studies have been conducted on the Mennonite family with X-linked mental retardation (XLMR) reported, in 1962, by Renpenning et al. The clinical phenotype includes severe mental retardation, microcephaly, up-slanting palpebral fissures, small testes, and stature shorter than that of nonaffected males. Major malformations, neuromuscular abnormalities, and behavioral disturbances were not seen. Longevity is not impaired. Carrier females do not show heterozygote manifestations. The syndrome maps to Xp11.2-p11.4, with a maximum LOD score of 3.21 (recombination fraction 0) for markers between DXS1039 and DXS1068. Renpenning syndrome (also known as "MRXS8"; gene RENS1, MIM 309500) shares phenotypic manifestations with several other XLMR syndromes, notably the Sutherland-Haan syndrome. In none of these entities has the responsible gene been isolated; hence, the possibility that two or more of them may be allelic cannot be excluded at present.

A new X linked recessive syndrome of mental retardation and mild dysmorphism maps to Xq28

Efforts to understand the genetic basis of mental retardation are greatly assisted by the identification of families with multiple relatives with mental retardation that clinical geneticists encounter in the routine practice of their profession. Here we describe a linkage study of a four generation family in which X linked recessive mental retardation (XLMR) is associated with minor dysmorphism and premature death of the affected males. Microsatellite based polymorphic loci evenly spaced over the entire X chromosome were used initially to detect linkage to Xq28. Further analysis identified a haplotype of Xq28 markers bounded proximally by locus DXS1113 and distally by DXS1108 that cosegregated with XLMR in this family. Two point lod scores > 3.0 provided strong evidence that the gene locus responsible for XLMR in this family is within this 7 Mb region of Xq28. The minor anomalies noted in some affected males were not distinctive enough to suggest a unique syndrome. None of our patients had features of the Waisman-Laxova syndrome or the PPM-X syndrome. The possibility of allelism with any of the five other non-specific XLMR syndromes (MRX3, MRX16, MRX25, MRX28, and MRX41) mapped to Xq28 could not be excluded. While the recognition of a gene responsible for this disorder needs much additional work, multiple female relatives at risk in this family benefit immediately from knowing their genotype and heterozygotes will have the opportunity to undergo prenatal diagnosis.

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.