X-linked Recessive Mental Retardation Research Articles

New SMS mutation leads to a striking reduction in spermine synthase protein function and a severe form of Snyder–Robinson X-linked recessive mental retardation syndrome

We report the identification of a novel mutation at a highly conserved residue within the N-terminal region of spermine synthase (SMS) in a second family with Snyder–Robinson X-linked mental retardation...

A novel X-linked recessive mental retardation syndrome comprising macrocephaly and ciliary dysfunction is allelic to oral–facial–digital type I syndrome

We report on a large family in which a novel X-linked recessive mental retardation (XLMR) syndrome comprising macrocephaly and ciliary dysfunction co-segregates with a frameshift mutation in the OFD1 gene. Mutations of OFD1 have been associated with oral-facial-digital type 1 syndrome (OFD1S) that is characterized by X-chromosomal dominant inheritance and lethality in males. In contrast, the carrier females of our family were clinically inconspicuous, and the affected males suffered from severe mental retardation, recurrent respiratory tract infections and macrocephaly. All but one of the affected males died from respiratory problems in infancy; and impaired ciliary motility was confirmed in the index patient by high-speed video microscopy examination of nasal epithelium. This family broadens the phenotypic spectrum of OFD1 mutations in an unexpected way and sheds light on the complexity of the underlying disease mechanisms.

Midline defects in FG syndrome: Does tethered spinal cord contribute to the phenotype?

FG syndrome is an X-linked recessive mental retardation syndrome with ano-rectal anomalies, constipation, and occasional urinary incontinence. Because tethered spinal cord syndrome (TCS) has similar symptoms, we evaluated imaging for TCS in patients with FG syndrome. Patients were recruited from the International FG Syndrome Support Group, and an FG Syndrome Consensus Group reviewed clinical histories, medical records, and photographs of each responding patient. Results of cranial and spinal imaging studies were available for 12 patients. Of 12 boys with FG syndrome, 6 had hypoplasia of the corpus callosum, and 3 of these had TCS (all with constipation and urinary symptoms). The other 9 did not have urinary symptoms. After surgical untethering, bowel and bladder symptoms improved. Tethered spinal cord syndrome occurred in 25% of patients with FG syndrome associated with hypoplasia of the corpus callosum and causing bowel and bladder incontinence. A high index of suspicion is necessary for early diagnosis, and timely intervention results in significant improvement in symptomatology.

Identification of two AGTR2 mutations in male patients with non-syndromic mental retardation

Mutations in the coding region of the angiotensin II type 2 receptor gene (AGTR2) were recently identified to cause X-linked recessive mental retardation. We report a mutation screening of the AGTR2 gene in 57 Finnish male patients with non-syndromic mental retardation. We identified two mutations, a 62G-->T transversion, which leads to a substitution of glycine for valine (G21V) and a 157A-->T transversion, which causes a substitution of isoleucine for phenylalanine (I53F). The patients with AGTR2 sequence variants had severe/profound mental retardation, epileptic seizures, restlessness, hyperactivity, and disturbed development of speech.

Rett syndrome: the complex nature of a monogenic disease.

Rett syndrome (RTT) is a severe neurodevelopmental disorder affecting almost exclusively girls. It is currently considered a monogenic X-linked dominant disorder due to mutations in MECP2 gene, encoding the methyl-CpG binding protein 2. A few RTT male cases, resulting from mosaicism for MECP2 mutations, have been reported. Male germline MECP2 mutations cause either severe encephalopathy with death at birth (usually in brothers of classical RTT females) or X-linked recessive mental retardation (XLMR). To date the wide phenotypic heterogeneity associated with MECP2 mutations in females (from classical RTT to healthy carriers) has been explained by differences in X chromosome inactivation. However, conflicting results have been obtained in different studies, with both random and highly skewed X-inactivation reported in healthy carrier females. Consequently it is possible that mechanisms other than X-inactivation play a role in the expressivity of MECP2 mutations. To explain the phenotypic heterogeneity associated with MECP2 mutations we propose a digenic model in which the presence of a "mutated" allele in a second gene, leading to a less functional protein, determines the clinical severity of the MECP2 mutation. The model is supported by the identification of the same mutation in XLMR and RTT cases. The carrier mothers of XLMR families are clinically asymptomatic and present balanced X chromosome inactivation. Therefore the same mutation arising in different genetic backgrounds can cause XLMR in males, remain silent in the carrier females and cause classic RTT in females. MECP2 mutations account for approximately 70-80% of classic RTT cases. MECP2 negative cases might result from mutations in noncoding regions of MECP2 gene. Alternatively, these cases might be due to mutations in other genes (locus heterogeneity). This hypothesis is supported by the identification of several chromosomal rearrangements in MECP2 negative patients with RTT and RTT-like phenotypes. MeCP2 is considered a general transcriptional repressor. However, conditional mouse mutants with selective loss of Mecp2 in the brain develop clinical manifestations similar to RTT, indicating that MECP2 is exclusively required for central nervous system function. The involvement of MeCP2 in methylation-specific transcriptional repression suggests that MECP2 related disorders result from dysregulated gene expression. Studies on gene expression have been performed in mouse and human brains. A relatively small number of gene expression changes were identified. It is possible that MeCP2 causes dysregulation of a very small subset of genes that are not detected with this method of analysis, or that very subtle changes in many genes cause the neuronal phenotype.

The phenotypic consequences of MECP2 mutations extend beyond Rett syndrome.

Although MECP2 was initially identified as the causative gene in classic Rett syndrome (RTT), the gene has now been implicated in several phenotypes that extend well beyond the clinically defined disorder. MECP2 mutations have been found in people with various disorders, including neonatal onset encephalopathy, X-linked recessive mental retardation (MRX), classic and atypical RTT, autism, and Angelman syndrome, as well as mildly affected females and normal carrier females. To make matters more complex, in approximately 20% of classic sporadic RTT cases and more than 50% of affected sister pairs, no mutation in MECP2 has been found. X-chromosome inactivation patterns can clearly affect the phenotypic expression in females, while the effect of the type and position of the mutation is more apparent in the broader phenotype than in RTT. Both males and females are at risk, although an excess of paternally derived mutations are found in most cases of classic RTT. Thus, because of the range of disparate phenotypes, the gene may account for a relatively large portion of mental retardation in the population.

Associations between MeCP2 mutations, X-chromosome inactivation, and phenotype.

Rett syndrome is a neurodevelopmental disorder of early postnatal brain growth in girls. Patients show a normal neonatal period with subsequent developmental regression and a loss of acquired skills (communication and motor skills), deceleration of head growth, and development of typical hand stereotypies. Recent studies have shown that mutations in the X-linked methyl CpG binding protein 2 gene (MeCP2) cause most typical cases of Rett syndrome. The MeCP2 gene encodes a protein that binds methylated cytosine residues of CpG dinucleotides and mediates, with histone deacetylases and transcriptional repressors, the transcription "silencing" of other genes. Girls with Rett syndrome exhibit mosaic expression for the MeCP2 defect at the cellular level, with most patients showing random X-inactivation and approximately equal numbers of cells expressing the normal MeCP2 gene and the mutated MeCP2 gene. In rare cases, females with a MeCP2 mutation escape phenotypic expression of the disorder because of nonrandom X-inactivation and the preferential inactivation of the mutated MeCP2 allele. Nonrandom patterns of X-inactivation may also contribute to the clinical variability often seen in girls with Rett syndrome. The spectrum of clinical phenotype caused by MeCP2 mutations is wide, including milder "preserved speech" variants, the severe congenital Rett variant, and a subset of X-linked recessive mental retardation in boys. Studies have shown that atypical and classical Rett syndrome can caused by the same MeCP2 mutations, indicating clinical phenotype is variable even among girls with the same MeCP2 mutation. The relationship between type of MeCP2 mutation, X-inactivation status, and clinical phenotype of Rett syndrome is complex and likely involves other environmental and polygenic modifiers.

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.

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...

A clinical follow-up of British patients with FG syndrome

The FG syndrome is an X-linked recessive mental retardation syndrome. Ten patients are reviewed with special emphasis on the natural history of the intellectual development, constipation, and the prognosis for growth and behaviour. Six out of 10 patients are still macrocephalic, and there is no evidence for a specific growth pattern with respect to height. The degree of mental retardation is is usually severe. The behaviour is characteristically friendly, sociable and over-talkative, with periodic aggression. Six patients have seizures. A characteristic progression seems to occur from congenital hypotonia with joint hyperlaxity at birth, to joint contractures with apparent spasticity and unsteady gait later in life. The constipation was a temporary problem in five cases. The cowlick and the fetal pads persist and are important, but not specific, for the diagnosis.

761 MULTIPLE SIBLING MENTAL RETARDATION AND THE IMPACT OF THE FRAGILE X CHROMOSOME

Mental retardation in two or more siblings with two normal parents represents an area about which little is known and is a difficult problem with regard to accurate diagnosis, elucidation of genetic mechanisms, and genetic counseling. Sixty-one families with two or more mentally retarded siblings with two normal parents were ascertained from a large institution for the mentally retarded and a clinical genetics service. Investigation included pedigree analyses, physical examination, biochemical/metabolic analyses and chromosome analyses. A specific diagnosis and/or genetic mechanism was found in only 25% (15/61) of the families. Of the sixty-one families, 22 had affected males only, of which two had definite X-linked recessive mental retardation. When chromosome analysis utilizing Medium 199 was done, an Xq27 fragile site was found in 36% (8/22) of the families with affected males only, including one of the known X-linked recessive families. Thus, the ability to detect the Xq27 fragile site resulted in an increase to 36% (22/61) of definite diagnoses for all families. Multiple sibling mental retardation represents a major clinical problem for which the detection of the Xq27 fragile site has an important impact.