X-linked mental retardation with variable stature, head circumference, and testicular volume linked to Xq12-q21

Loss of ARHGEF6 Causes Hair Cell Stereocilia Deficits and Hearing Loss in Mice

ARHGEF6 belongs to the family of guanine nucleotide exchange factors (GEFs) for Rho GTPases, and it specifically activates Rho GTPases CDC42 and RAC1. Arhgef6 is the X-linked intellectual disability gene also known as XLID46, and clinical features of patients carrying Arhgef6 mutations include intellectual disability and, in some cases, sensorineural hearing loss. Rho GTPases act as molecular switches in many cellular processes. Their activities are regulated by binding or hydrolysis of GTP, which is facilitated by GEFs and GTPase-activating proteins, respectively. RAC1 and CDC42 have been shown to play important roles in hair cell (HC) stereocilia development. However, the role of ARHGEF6 in inner ear development and hearing function has not yet been investigated. Here, we found that ARHGEF6 is expressed in mouse cochlear HCs, including the HC stereocilia. We established Arhgef6 knockdown mice using the clustered regularly interspaced short palindromic repeat-associated Cas9 nuclease (CRISPR-Cas9) genome editing technique. We showed that ARHGEF6 was indispensable for the maintenance of outer hair cell (OHC) stereocilia, and loss of ARHGEF6 in mice caused HC stereocilia deficits that eventually led to progressive HC loss and hearing loss. However, the loss of ARHGEF6 did not affect the synapse density and did not affect the mechanoelectrical transduction currents in OHCs at postnatal day 3. At the molecular level, the levels of active CDC42 and RAC1 were dramatically decreased in the Arhgef6 knockdown mice, suggesting that ARHGEF6 regulates stereocilia maintenance through RAC1/CDC42.

Exome sequencing in children of women with skewed X-inactivation identifies atypical cases and complex phenotypes

BACKGROUND

More than 100 X-linked intellectual disability (X-LID) genes have been identified to be involved in 10-15% of intellectual disability (ID).

METHOD

To identify novel possible candidates, we selected 18 families with a male proband affected by isolated or syndromic ID. Pedigree and/or clinical presentation suggested an X-LID disorder. After exclusion of known genetic diseases, we identified seven cases whose mother showed a skewed X-inactivation (>80%) that underwent whole exome sequencing (WES, 50X average depth).

RESULTS

WES allowed to solve the genetic basis in four cases, two of which (Coffin-Lowry syndrome, RPS6K3 gene; ATRX syndrome, ATRX gene) had been missed by previous clinical/genetics tests. One further ATRX case showed a complex phenotype including pontocerebellar atrophy (PCA), possibly associated to an unidentified PCA gene mutation. In a case with suspected Lujan-Fryns syndrome, a c.649C>T (p.Pro217Ser) MECP2 missense change was identified, likely explaining the neurological impairment, but not the marfanoid features, which were possibly associated to the p.Thr1020Ala variant in fibrillin 1. Finally, a c.707T>G variant (p.Phe236Cys) in the DMD gene was identified in a patient retrospectively recognized to be affected by Becker muscular dystrophy (BMD, OMIM 300376).

CONCLUSION

Overall, our data show that WES may give hints to solve complex ID phenotypes with a likely X-linked transmission, and that a significant proportion of these orphan conditions might result from concomitant mutations affecting different clinically associated genes.

Aberrant Rho GTPases signaling and cognitive dysfunction: in vivo evidence for a compelling molecular relationship

Rho GTPases are key intracellular signaling molecules that coordinate dynamic changes in the actin cytoskeleton, thereby stimulating a variety of processes, including morphogenesis, migration, neuronal development, cell division and adhesion. Deviations from normal Rho GTPases activation state have been proposed to disrupt cognition and synaptic plasticity. This review focuses on the functional consequences of genetic ablation of upstream and downstream Rho GTPases molecules on cognitive function and neuronal morphology and connectivity. Available information on this issue is described and compared to that gained from mice carrying mutations in the most studied Rho GTPases and from pharmacological in vivo studies in which brain Rho GTPases signaling was modulated. Results from reviewed literature provide definitive evidence of a compelling link between Rho GTPases signaling and cognitive function, thus supporting the notion that Rho GTPases and their downstream effectors may represent important therapeutic targets for disorders associated with cognitive dysfunction.

[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.

A new X-linked mental retardation (XLMR) syndrome with late-onset primary testicular failure, short stature and microcephaly maps to Xq25–q26

X-linked mental retardation (XLMR) is a heterogeneous disorder with both syndromic and non-syndromic forms. Here we describe the clinical and molecular characterisation of a family with a syndromic form of XLMR with hypogonadism and short stature. We investigated a family in which four male members in two generations presented with hypergonadotrophic hypogonadism associated with development of small and abnormal testes. In two of the males, late-onset testicular ascent was noted. In addition, all affected males had short stature (<0.4th centile) and mild learning difficulties and three out of the four had microcephaly. Karyotypes were normal and endocrine investigations confirmed primary testicular failure. The phenotype segregated as an X-linked trait. Haplotype and genetic two-point linkage analysis with 22 microsatellites excluded the whole X chromosome except for a region on Xq25-Xq27 encompassing 13.7Mb with a maximum LOD score of 1.1 for marker DXS8038 at theta=0.05. One family previously described as having XLMR with hypogonadism and short stature maps to the same X chromosome region implicated in our family. However, the more severe mental retardation, muscle wasting and tremor described in this other family would suggest that our family is affected by a novel XLMR syndrome.

Increased Membrane/Nuclear Translocation and Phosphorylation of p90 KD Ribosomal S6 Kinase in the Brain of Hypoxic Preconditioned Mice

Our previous studies have demonstrated that hypoxic precondition (HPC) increased membrane translocation of protein kinase C isoforms and decreased phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) in the brain of mice. The goal of this study was to determine the involvement of p90 KD ribosomal S6 kinase (RSK) in cerebral HPC of mice. Using Western-blot analysis, we found that the levels of membrane/nuclear translocation, but not protein expression of RSK increased significantly in the frontal cortex and hippocampus of HPC mice. In addition, we found that the phosphorylation levels of RSK at the Ser227 site (a PDK1 phosphorylation site), but not at the Thr359/Ser363 sites (ERK1/2 phosphorylated sites) increased significantly in the brain of HPC mice. Similar results were confirmed by an immunostaining study of total RSK and phospho-Ser227 RSK. To further define the cellular populations to express phospho-Ser227 RSK, we found that the expression of phospho-Ser227 RSK co-localized with neurogranin, a neuron-specific marker, in cortex and hippocampus of HPC mice by using double-labeled immunofluorescent staining method. These results suggest that increased RSK membrane/nuclear translocation and PDK1 mediated neuron-specific phosphorylation of RSK at Ser227 might be involved in the development of cerebral HPC of mice.

Ribosomal protein gene knockdown causes developmental defects in zebrafish

The ribosomal proteins (RPs) form the majority of cellular proteins and are mandatory for cellular growth. RP genes have been linked, either directly or indirectly, to various diseases in humans. Mutations in RP genes are also associated with tissue-specific phenotypes, suggesting a possible role in organ development during early embryogenesis. However, it is not yet known how mutations in a particular RP gene result in specific cellular changes, or how RP genes might contribute to human diseases. The development of animal models with defects in RP genes will be essential for studying these questions. In this study, we knocked down 21 RP genes in zebrafish by using morpholino antisense oligos to inhibit their translation. Of these 21, knockdown of 19 RPs resulted in the development of morphants with obvious deformities. Although mutations in RP genes, like other housekeeping genes, would be expected to result in nonspecific developmental defects with widespread phenotypes, we found that knockdown of some RP genes resulted in phenotypes specific to each gene, with varying degrees of abnormality in the brain, body trunk, eyes, and ears at about 25 hours post fertilization. We focused further on the organogenesis of the brain. Each knocked-down gene that affected the morphogenesis of the brain produced a different pattern of abnormality. Among the 7 RP genes whose knockdown produced severe brain phenotypes, 3 human orthologs are located within chromosomal regions that have been linked to brain-associated diseases, suggesting a possible involvement of RP genes in brain or neurological diseases. The RP gene knockdown system developed in this study could be a powerful tool for studying the roles of ribosomes in human diseases.

A unique exonic splice enhancer mutation in a family with X-linked mental retardation and epilepsy points to a novel role of the renin receptor

The renin-angiotensin system (RAS) is essential for blood pressure control and water-electrolyte balance. Until the discovery of the renin receptor, renin was believed to be mainly a circulating enzyme with a unique function, the cleavage of angiotensinogen. We report a unique mutation in the renin receptor gene (ATP6AP2) present in patients with X-linked mental retardation and epilepsy (OMIM no. 300423), but absent in 1200 control X-chromosomes. A silent mutation (c.321C>T, p.D107D) residing in a putative exonic splicing enhancer site resulted in inefficient inclusion of exon 4 in 50% of renin receptor mRNA, as demonstrated by quantitative RT-PCR. Analysis of membrane associated-receptor molecular forms showed the presence of full-length and truncated proteins in the patient. Functional analysis demonstrated that the mutated receptor could bind renin and increase renin catalytic activity, similar to the wild-type receptor, but resulted in a modest and reproducible impairment of ERK1/2 activation. Thus, our findings confirm the importance of the RAS in cognitive processes and indicate a novel specific role for the renin receptor in cognitive functions and brain development.

Non-syndromic X-linked mental retardation associated with a missense mutation (P312L) in the FGD1 gene

Three brothers with non-syndromal X-linked mental retardation were found to have a novel missense mutation in FGD1, the gene associated with the Aarskog syndrome. Although the brothers have short stature and small feet, they lack distinct craniofacial, skeletal or genital findings suggestive of Aarskog syndrome. Their mother, the only obligate carrier available for testing, has the FGD1 mutation. The mutation, a C934T base change in exon 4, results in the proline at position 312 to be substituted with a leucine. This missense mutation is predicted to eliminate a beta-turn, creating an extra-long stretch of coiled sequence which may affect the orientations of an SH3 (Src homology 3) binding domain and the first structural conserved region. A new molecular defect associated with non-syndromal X-linked mental retardation affords an opportunity to seek specific diagnosis in males with previously unexplained developmental delays and this opens further predictive tests in families at risk.

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.