X-linked mental retardation

X chromosome exome sequencing reveals a novel ALG13 mutation in a nonsyndromic intellectual disability family with multiple affected male siblings

X-linked intellectual disability (XLID) is a heterogeneous condition associated with mutations in >100 genes, accounting for over 10% of all cases of intellectual impairment. The majority of XLID cases show nonsyndromic forms (NSXLID), in which intellectual disability is the sole clinically consistent manifestation. Here we performed X chromosome exome (X-exome) sequencing to identify the causative mutation in an NSXLID family with four affected male siblings and five unaffected female siblings. The X-exome sequencing at 88× coverage in one affected male sibling revealed a novel missense mutation (p.Tyr1074Cys) in the asparagine-linked glycosylation 13 homolog (ALG13) gene. Segregation analysis by Sanger sequencing showed that the all affected siblings were hemizygous and the mother was heterozygous for the mutation. Recently, a de novo missense mutation in ALG13 has been reported in a patient with X-linked congenital disorders of glycosylation type I. Our study reports the first case of NSXLID caused by a mutation in ALG13 involved in protein N-glycosylation.

[New X linked mental retardation syndrome]

INTRODUCTION

Researching inherited mental retardation, from a diagnostic and aetiological point of view, is a great challenge. A particular type of mental retardation is the one linked to the X chromosome which is classified under syndromic and non-syndromic types, according to the presence or absence of a specific physical, neurological or metabolic pattern associated with mental retardation.

PATIENTS AND METHOD

Five generations of a family have been studied with eight males suffering from mental retardation. Six of these males were clinically tested using anthropometric indicators and genetic tests: high resolution karyotypes, fragile X research, linkage and MID1 and PQBP1 gene studies.

RESULTS

Along with mental retardation, the clinical study showed a pattern of microcephaly, micrognathia, osteoarticular and genital anomalies, short stature and other less frequent malformations. The linkage study mapped the possible causal gene of this mental retardation syndrome and multiple congenital abnormalities in the Xp11.23-q21.32 segment, with a LOD score of 2. As far as we know, a medical profile, similar to the one these patients have, linked to this X segment has not been described.

CONCLUSIONS

We suspect that this family has a "new syndrome" of mental retardation and multiple congenital anomalies linked to the X chromosome.

An antisense transcript spanning the CGG repeat region of FMR1 is upregulated in premutation carriers but silenced in full mutation individuals

Expansion of the polymorphic CGG repeats within the 5'-UTR of the FMR1 gene is associated with variable transcriptional regulation of FMR1. Here we report a novel gene, ASFMR1, overlapping the CGG repeat region of FMR1 and transcribed in the antisense orientation. The ASFMR1 transcript is spliced, polyadenylated and exported to the cytoplasm. Similar to FMR1, ASFMR1 is upregulated in individuals with premutation alleles and is not expressed from full mutation alleles. Moreover, it exhibits premutation-specific alternative splicing. Taken together, these observations suggest that in addition to FMR1, ASFMR1 may contribute to the variable phenotypes associated with the CGG repeat expansion.

The Italian XLMR bank: a clinical and molecular database

Mental retardation (MR) is a nonprogressive condition characterized by a significant impairment of intellectual capabilities with deficit of cognitive and adaptive functioning and onset before 18 years. Mental retardation occurs in about 2 to 3% of the general population and it is estimated that 25 to 35% of the cases may be due to genetic causes. Among these "genetic" MR, 25 to 30% are probably due to mutations in a gene on the X chromosome (X-linked mental retardation, XLMR). Given the genetic heterogeneity of XLMR, the availability of a considerable number of patients with accurate phenotypic classification is a crucial factor for research. The X-linked Mental Retardation Italian Network, which has been active since 2003, has collected detailed clinical information and biological samples from a vast number of MR patients. Collected samples and clinical information are inserted within the XLMR bank, a comprehensive molecular and clinical web-based database available at the address http://xlmr.unisi.it. The database is organized in three distinct parts. Part I and II contain several electronic schedules to register information on the family, the phenotypic description, the photographs, and a 20 sec movie of the patient. Part III allows the registration of molecular analyses performed on each case; samples and clinical data are usable via password-restricted access. Clinical and molecular centers interested in joining the network may request a password by simply contacting the Medical Genetics of the University of Siena. The XLMR bank is an innovative biological database that allows the collection of molecular and clinical data, combines descriptive and iconographic resources, and represents a fundamental tool for researchers in the field of mental retardation.

Non-syndromic X-linked mental retardation: From a molecular to a clinical point of view

This review focuses on the 19 identified genes involved in X-linked "non-syndromic" mental retardation (MR) and defines the signaling pathways in which they are involved, focusing on emerging common mechanisms. The majority of proteins are involved in three distinct pathways: (1) Rho GTPases pathway modulating neuronal differentiation and synaptic plasticity; (2) Rab GTPases pathway regulating synaptic vesicle cycling; (3) gene expression regulation. The function of four proteins (ACSL4, AT2, SLC6A8, and SAP102) could not be reconciled to a common pathway. From a clinical point of view, the review discusses whether some common dysmorphic features can be identified even in non-syndromic MR patients and whether it is correct to maintain the distinction between "non-syndromic" and "syndromic" MR.

X-linked mental retardation (XLMR): from clinical conditions to cloned genes

X-linked mental retardation (XLMR) is a heterogenous set of conditions responsible for a large proportion of inherited mental retardation. Approximately 200 XLMR conditions and 45 cloned genes are now listed in our catalogue on the Internet at http://xlmr.interfree.it/home.htm. Traditionally, XLMR conditions were subdivided into specific (MRXS) and nonspecific (MRX) forms, depending on their clinical presentation. Now that a growing number of candidate genes have become available for screening XLMR families and patients, this distinction is becoming less useful and similar conditions that had been previously listed as separate can now be grouped together because different mutations in the same gene have been identified. Furthermore, different mutations in the same XLMR gene may account for diseases of increasing severity, but can also cause different phenotypes. As the functions of proteins corresponding to these genes are characterized, biological networks involved in causing mental retardation and conversely in supporting normal intellectual functioning will be discovered. Molecular biologists and neurobiologists will need to cooperate in order to verify the effects of XLMR gene mutations in the context of neuronal circuitry. Eventually, DNA and protein microarray technologies will assist researchers and physicians in reaching a diagnosis even in small families or in individual patients with XLMR.

A third MRX family (MRX68) is the result of mutation in the long chain fatty acid-CoA ligase 4 (FACL4) gene: proposal of a rapid enzymatic assay for screening mentally retarded patients

BACKGROUND

The gene encoding fatty acid CoA ligase 4 (FACL4) is mutated in families with non-specific X linked mental retardation (MRX) and is responsible for cognitive impairment in the contiguous gene syndrome ATS-MR (Alport syndrome and mental retardation), mapped to Xq22.3. This finding makes this gene a good candidate for other mental retardation disorders mapping in this region.

METHODS

We have screened the FACL4 gene in eight families, two MRX and six syndromic X linked mental retardation (MRXS), mapping in a large interval encompassing Xq22.3.

RESULTS

We have found a missense mutation in MRX68. The mutation (c.1001C>T in the brain isoform) cosegregates with the disease and changes a highly conserved proline into a leucine (p.P375L) in the first luciferase domain, which markedly reduces the enzymatic activity. Furthermore, all heterozygous females showed completely skewed X inactivation in blood leucocytes, as happens in all reported females with other FACL4 point mutations or deletions.

CONCLUSIONS

Since the FACL4 gene is highly expressed in brain, where it encodes a brain specific isoform, and is located in hippocampal and cerebellar neurones, a role for this gene in cognitive processes can be expected. Here we report the third MRX family with a FACL4 mutation and describe the development of a rapid enzymatic assay on peripheral blood that we propose as a sensitive, robust, and efficient diagnostic tool in mentally retarded males.

In search of the MRX genes

Mental retardation (MR) is one of the most common human disorders. MR may be just one of the clinical signs of a complex syndrome or it may be associated with metabolic disorders or with disorders of brain development, but in many patients [nonspecific MR (NSMR)], it is the only consistent clinical manifestation. It is expected that NSMR is caused by alterations in molecular pathways important for cognitive functions. Insights into NSMR have recently come from the study of X-linked MR as eight genes were identified during the last few years. This development has represented a fundamental breakthrough in our understanding of NSMR and of cognitive functions and has opened new perspectives in the study of MR. The new genes identified are a heterogeneous group, but it is very intriguing that they are all directly or indirectly involved in signaling pathways and that the majority are proteins that regulate members of the Ras superfamily of small GTP binding proteins.

Pharmacological reactivation of inactive genes: the fragile X experience

The present review on the pharmacological reactivation of inactive genes focuses on our experience with the fragile X syndrome. The fragile X syndrome of mental retardation is the prototype of a series of inherited neurological disorders caused by abnormal expansion of repeated trinucleotide sequences embedded in various genes. In a number of these disorders, such as Huntington disease and several forms of spinocerebellar ataxias, the expanded CAG repeat is translated, resulting in a polyglutamine-containing protein that indirectly causes neurodegeneration. On the contrary, in the fragile X syndrome, the expanded CGG repeat is contained in the regulatory region of the FMR1 gene and causes transcriptional inactivation. The mutation spares the coding region of the FMR1 gene, which potentially would allow synthesis of a normal protein if transcription could be restored. This prompted us to try and reactivate the gene function with different pharmacological regimens. We discuss our successful results with DNA demethylating and histone hyperacetylating drugs and their implications for future treatments of the fragile X syndrome.

XLMR genes: update 2000

This is the sixth edition of the catalogue of XLMR genes, ie X-linked genes whose malfunctioning causes mental retardation. The cloning era is not yet concluded, actually much remains to be done to account for the 202 XLMR conditions listed in this update. Many of these may eventually prove to be due to mutations in the same gene but the present number of 33 cloned genes falls surely short of the actual total count. It is now clear that even small families or individual patients with cytogenetic rearrangements can be instrumental in pinning down the remaining genes. DNA chip technology will hopefully allow (re)screening large numbers of patients for mutations in candidate genes or testing the expression levels of many candidate genes in informative families. Slowly, our knowledge of the structure and functioning of the proteins encoded by these genes is beginning to cast some light on the biological pathways required for the normal development of intelligence. Correlations between the molecular defects and the phenotypic manifestations are also being established. In order to facilitate the exchange of existing information and to allow its timely update, we prepared the first edition of the XLMR database (available at http://homepages.go.com/~xlmr/home.htm) and invite all colleagues, expert in the field, to contribute with their experience.

Genetics of mental retardation

Aim of this review is to present the latest advances in the identification of the genetic determinants of intellectual deficiency. Mental retardation (MR) is often associated with other neurologic symptoms, metabolic disorders, or malformation syndromes. The purpose of the review is to subdivide the large field of MR into categories that may help professionals in making a diagnosis. Nonspecific MR can also segregate in families and the mapping and cloning of corresponding mutant genes will eventually advance our understanding of normal and abnormal brain functioning. Several genes responsible for nonspecific X-linked mental retardation have been identified in the last 12 to 24 months and are being intensively investigated. This will hopefully lead to new possibilities of either genetic or pharmacological therapy.