Expression of human PQBP-1 in Drosophila impairs long-term memory and induces abnormal courtship

Frame shift mutations of the polyglutamine binding protein-1 (PQBP1) gene lead to total or partial truncation of the C-terminal domain (CTD) and cause mental retardation in human patients. Interestingly, normal Drosophila homologue of PQBP-1 lacks CTD. As a model to analyze the molecular network of PQBP-1 affecting intelligence, we generated transgenic flies expressing human PQBP-1 with CTD. Pavlovian olfactory conditioning revealed that the transgenic flies showed disturbance of long-term memory. In addition, they showed abnormal courtship that male flies follow male flies. Abnormal functions of PQBP-1 or its binding partner might be linked to these symptoms.     

Xp22.3 interstitial deletion: A recognizable chromosomal abnormality encompassing VCX3A and STS genes in a patient with X-linked ichthyosis and mental retardation

X-linked ichthyosis is a genetic disorder affecting the skin and caused by a deficit in the steroid sulfatase enzyme (STS), often associated with a recurrent microdeletion at Xp22.31. Most of the STS deleted patients have X-linked ichthyosis as the only clinical feature and it is believed that patients with more complex disorders including mental retardation could be present as a result of contiguous gene deletion. In fact, VCX3A gene, a member of the VCX (variable charge, X chromosome) gene family, was previously proposed as the candidate gene for X-linked non-specific mental retardation in patients with X-linked ichthyosis.     

Segmental isotope-labeling of the intrinsically disordered protein PQBP1

Polyglutamine tract-binding protein 1 (PQBP1) is an intrinsically disordered protein abundantly expressed in the brain. Mutations in the PQBP1 gene are causative for X-linked mental retardation disorders. Here, we investigated the structure of the C-terminal segment within the context of full-length PQBP1. We produced a segmentally isotope-labeled PQBP1 composed of a non-labeled segment (residues 1–219; N-segment) and a ¹³C/¹⁵N-labeled segment (residues 220–265; C-segment). Our results demonstrate that the segmental isotope-labeling combined with NMR spectroscopy is useful for detecting a very weak intra-molecular interaction in an intrinsically disordered protein.     

Mutations in ZDHHC9, which encodes a palmitoyltransferase of NRAS and HRAS, cause X-linked mental retardation associated with a Marfanoid habitus

We have identified one frameshift mutation, one splice-site mutation, and two missense mutations in highly conserved residues in ZDHHC9 at Xq26.1 in 4 of 250 families with X-linked mental retardation (XLMR). In three of the families, the mental retardation phenotype is associated with a Marfanoid habitus, although none of the affected individuals meets the Ghent criteria for Marfan syndrome. ZDHHC9 is a palmitoyltransferase that catalyzes the posttranslational modification of NRAS and HRAS. The degree of palmitoylation determines the temporal and spatial location of these proteins in the plasma membrane and Golgi complex. The finding of mutations in ZDHHC9 suggests that alterations in the concentrations and cellular distribution of target proteins are sufficient to cause disease. This is the first XLMR gene to be reported that encodes a posttranslational modification enzyme, palmitoyltransferase. Furthermore, now that the first palmitoyltransferase that causes mental retardation has been identified, defects in other palmitoylation transferases become good candidates for causing other mental retardation syndromes.     

OFD1 Is Mutated in X-Linked Joubert Syndrome and Interacts with LCA5-Encoded Lebercilin

We ascertained a multi-generation Malaysian family with Joubert syndrome (JS). The presence of asymptomatic obligate carrier females suggested an X-linked recessive inheritance pattern. Affected males presented with mental retardation accompanied by postaxial polydactyly and retinitis pigmentosa. Brain MRIs showed the presence of a “molar tooth sign,” which classifies this syndrome as classic JS with retinal involvement. Linkage analysis showed linkage to Xpter-Xp22.2 and a maximum LOD score of 2.06 for marker DXS8022. Mutation analysis revealed a frameshift mutation, p.K948NfsX8, in exon 21 of OFD1. In an isolated male with JS, a second frameshift mutation, p.E923KfsX3, in the same exon was identified. OFD1 has previously been associated with oral-facial-digital type 1 (OFD1) syndrome, a male-lethal X-linked dominant condition, and with X-linked recessive Simpson-Golabi-Behmel syndrome type 2 (SGBS2). In a yeast two-hybrid screen of a retinal cDNA library, we identified OFD1 as an interacting partner of the LCA5-encoded ciliary protein lebercilin. We show that X-linked recessive mutations in OFD1 reduce, but do not eliminate, the interaction with lebercilin, whereas X-linked dominant OFD1 mutations completely abolish binding to lebercilin. In addition, recessive mutations in OFD1 did not affect the pericentriolar localization of the recombinant protein in hTERT-RPE1 cells, whereas this localization was lost for dominant mutations. These findings offer a molecular explanation for the phenotypic spectrum observed for OFD1 mutations; this spectrum now includes OFD1 syndrome, SGBS2, and JS.     

A Turkish family with Nance-Horan syndrome due to a novel mutation

Nance-Horan Syndrome (NHS) is a rare X-linked syndrome characterized by congenital cataract which leads to profound vision loss, characteristic dysmorphic features and specific dental anomalies. Microcornea, microphthalmia and mild or moderate mental retardation may accompany these features. Heterozygous females often manifest similarly but with less severe features than affected males. We describe two brothers who have the NHS phenotype and their carrier mother who had microcornea but not cataract. We identified a previously unreported frameshift mutation (c.558insA) in exon 1 of the NHS gene in these patients and their mother which is predicted to result in the incorporation of 11 aberrant amino acids prior to a stop codon (p.E186Efs11X). We also discussed genotype–phenotype correlation according to relevant literature.     

Cerebral Protein Synthesis in a Knockin Mouse Model of the Fragile X Premutation

The (CGG)n-repeat in the 5′-untranslated region of the fragile X mental retardation gene (FMR1) gene is polymorphic and may become unstable on transmission to the next generation. In fragile X syndrome, CGG repeat lengths exceed 200, resulting in silencing of FMR1 and absence of its protein product, fragile X mental retardation protein (FMRP). CGG repeat lengths between 55 and 200 occur in fragile X premutation (FXPM) carriers and have a high risk of expansion to a full mutation on maternal transmission. FXPM carriers have an increased risk for developing progressive neurodegenerative syndromes and neuropsychological symptoms. FMR1 mRNA levels are elevated in FXPM, and it is thought that clinical symptoms might be caused by a toxic gain of function due to elevated FMR1 mRNA. Paradoxically, FMRP levels decrease moderately with increasing CGG repeat length in FXPM. Lowered FMRP levels may also contribute to the appearance of clinical problems. We previously reported increases in regional rates of cerebral protein synthesis (rCPS) in the absence of FMRP in an Fmr1 knockout mouse model and in a FXPM knockin (KI) mouse model with 120 to 140 CGG repeats in which FMRP levels are profoundly reduced (80%–90%). To explore whether the concentration of FMRP contributes to the rCPS changes, we measured rCPS in another FXPM KI model with a similar CGG repeat length and a 50% reduction in FMRP. In all 24 brain regions examined, rCPS were unaffected. These results suggest that even with 50% reductions in FMRP, normal protein synthesis rates are maintained.     

Mutations in the gene encoding the Sigma 2 subunit of the adaptor protein 1 complex, AP1S2, cause X-linked mental retardation

In a systematic sequencing screen of the coding exons of the X chromosome in 250 families with X-linked mental retardation (XLMR), we identified two nonsense mutations and one consensus splice-site mutation in the AP1S2 gene on Xp22 in three families. Affected individuals in these families showed mild-to-profound mental retardation. Other features included hypotonia early in life and delay in walking. AP1S2 encodes an adaptin protein that constitutes part of the adaptor protein complex found at the cytoplasmic face of coated vesicles located at the Golgi complex. The complex mediates the recruitment of clathrin to the vesicle membrane. Aberrant endocytic processing through disruption of adaptor protein complexes is likely to result from the AP1S2 mutations identified in the three XLMR-affected families, and such defects may plausibly cause abnormal synaptic development and function. AP1S2 is the first reported XLMR gene that encodes a protein directly involved in the assembly of endocytic vesicles.     

Mutation c.359_363delGTATTinsATAC in the COL4A5 Causes alport syndrome in a Chinese family

The X-linked form of Alport syndrome is associated with mutations in the COL4A5 gene, which is located at Xq22.3 and encodes the α5 chain of type IV collagen. Here we clinically characterized a Chinese family with Alport Syndrome, but no ocular or hearing abnormalities have been observed in any patient in the family. Through Linkage analysis and direct DNA sequencing, a novel complex deletion/insertion mutation c.359_363delGTATTinsATAC in the COL4A5 gene was identified in the family. The mutation was found in all affected family members, but was not present in the unaffected family individuals or the 200 controls. The predicted mutant protein in the family is a truncated protein consisting of only 153 residues. Our report for the first time revealed that the frameshift mutation in the type IV collagen chain α5 causes only renal disease, without extrarenal lesion. Our study broadens genotypic and phenotypic spectrum of COL4A5 mutations associated with Alport syndrome.     

XLMR protein related to neurite extension (Xpn/KIAA2022) regulates cell–cell and cell–matrix adhesion and migration

X-linked mental retardation (XLMR) is a common cause of moderate to severe intellectual disability in males. XLMR protein related to neurite extension (Xpn, also known as KIAA2022) has been implicated as a gene responsible for XLMR in humans. Although Xpn is highly expressed in the developing brain and is involved in neurite outgrowth in PC12 cells and neurons, little is known about the functional role of Xpn. Here, we show that Xpn regulates cell–cell and cell–matrix adhesion and migration in PC12 cells. Xpn knockdown enhanced cell–cell and cell–matrix adhesion mediated by N-cadherin and β1-integrin, respectively. N-Cadherin and β1-integrin expression at the mRNA and protein levels was significantly increased in Xpn knockdown PC12 cells. Furthermore, overexpressed Xpn protein was strongly expressed in the nuclei of PC12 and 293T cells. Finally, depletion of Xpn perturbed cellular migration by enhancing N-cadherin and β1-integrin expression in a PC12 cell wound healing assay. We conclude that Xpn regulates cell–cell and cell–matrix adhesion and cellular migration by regulating the expression of adhesion molecules.     

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.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Making an impression on X-linked mental retardation

Oligophrenin-1 encodes a rhoGAP protein involved in X-linked mental retardationBilluart, P. (1998)Nature 392, 923–926PAK3 mutation in nonsyndromic X-linked mental retardationAllen, K.M. et al. (1998)Nat. Genet. 20, 25–30Mutations in GDI1 are responsible for X-linked non-specific mental retardationD'Adamo, P. et al. (1998)Nat. Genet. 19, 134–139Non-specific X-linked semidominant mental retardation by mutations in a Rab GDP-dissociation inhibitorBienvenu, T. et al. (1998)Hum. Mol. Genet. 7, 1311–1315     

Comparative aspects of polyglutamine binding domain in PQBP-1 among Vertebrata

We investigated the evolutionary conservation of polyglutamine binding protein-1 (PQBP-1) among Vertebrata. PQBP-1s were highly conserved and shared the same domain features including a WW domain, a polar amino acid rich domain (PRD), a nuclear localization signal (NLS), and a C-terminal domain (CTD) among Eutheria, but not always among Vertebrata. PQBP-1s of Vertebrata contained a variable region in the middle portion corresponding to the position of PRD. The full form of PRD including both 7aa and DR/ER repeats was specific to Eutheria. PRD of non-eutherian Amniota was minimal. Amphibia had no PRD. The DR/ER repeat was solo in fishes. Agnatha PRD was also rich in polar amino acids, but contained no repetitive sequence. We investigated 3 polyQ-containing proteins known to interact with PQBP-1: BRN-2, Huntingtin, and ATAXIN-1, and showed a diverse nature of protein-protein interaction in Vertebrata. There appears to be no interaction between PQBP-1 and BRN-2, Huntingtin, or ATAXIN-1 in Amphibia, while the interaction between PQBP-1 and BRN-2 is expected to be conserved among Mammalia, and the interaction between PQBP-1 and Huntingtin or ATAXIN-1 depends on the lineage in Eutheria.     

Angelman syndrome and severe infections in a patient with de novo 15q11.2–q13.1 deletion and maternally inherited 2q21.3 microdeletion

Angelman syndrome is a neurodevelopmental disorder characterized by mental retardation, severe speech disorder, facial dysmorphism, secondary microcephaly, ataxia, seizures, and abnormal behaviors such as easily provoked laughter. It is most frequently caused by a de novo maternal deletion of chromosome 15q11–q13 (about 70–90%), but can also be caused by paternal uniparental disomy of chromosome 15q11–q13 (3–7%), an imprinting defect (2–4%) or in mutations in the ubiquitin protein ligase E3A gene UBE3A mostly leading to frame shift mutation. In addition, for patients with overlapping clinical features (Angelman-like syndrome), mutations in methyl-CpG binding protein 2 gene MECP2 and cyclin-dependent kinase-like 5 gene CDKL5 as well as a microdeletion of 2q23.1 including the methyl-CpG binding domain protein 5 gene MBD5 have been described. Here, we describe a patient who carries a de novo 5 Mb-deletion of chromosome 15q11.2–q13.1 known to be associated with Angelman syndrome and a further, maternally inherited deletion 2q21.3 (~ 364 kb) of unknown significance. In addition to classic features of Angelman syndrome, she presented with severe infections in the first year of life, a symptom that has not been described in patients with Angelman syndrome. The 15q11.2–q13.1 deletion contains genes critical for Prader–Willi syndrome, the Angelman syndrome causing genes UBE3A and ATP10A/C, and several non-imprinted genes: GABRB3 and GABRA5 (both encoding subunits of GABA A receptor), GOLGA6L2, HERC2 and OCA2 (associated with oculocutaneous albinism II). The deletion 2q21.3 includes exons of the genes RAB3GAP1 (associated with Warburg Micro syndrome) and ZRANB3 (not disease-associated). Despite the normal phenotype of the mother, the relevance of the 2q21.3 microdeletion for the phenotype of the patient cannot be excluded, and further case reports will need to address this point.     

335.4 kb microduplication in chromosome band Xp11.2p11.3 associated with developmental delay, growth retardation, autistic disorder and dysmorphic features

About 10% of causative mutations for mental retardation in male patients involve X chromosome (X-linked mental retardation, XLMR). We describe a case of a 3-year-old boy presenting with developmental delay, autistic features and growth and speech delay. Array-CGH analysis detected a microduplication on the X chromosome (Xp11.2p11.3), spanning 335.4 kb and including 3 known genes (ZNF81, ZNF182 and SPACA5). Genome-wide association studies show that approximately 30% of mutations causing XLMR are located in Xp11.2p11.3, where few pathogenic genes have been identified to date (such as ZNF41, PQB1 and ZNF81). ZNF81 codifies a zinc finger protein and mutations (non-sense mutations, deletions and structural rearrangements) involving this gene have already been described in association with mental retardation. Larger duplications in the same region have also been observed in association with mental retardation, and, in one case, the over-expression of ZNF81 has also been verified by mRNA quantification. No duplications of the single gene have been identified. To our knowledge, the microduplication found in our patient is the smallest ever described in Xp11.2p11.3. This suggests that the over-expression of ZNF81 could have pathological effects.