Skewed X-inactivation in a family with mental retardation and PQBP1 gene mutation

PQBP1: The Key to Intellectual Disability, Neurodegenerative Diseases, and Innate Immunity

The idea that a common pathology underlies various neurodegenerative diseases and dementias has attracted considerable attention in the basic and medical sciences. Polyglutamine binding protein-1 (PQBP1) was identified in 1998 after a molecule was predicted to bind to polyglutamine tract amino acid sequences, which are associated with a family of neurodegenerative disorders called polyglutamine diseases. Hereditary gene mutations of PQBP1 cause intellectual disability, whereas acquired loss of function of PQBP1 contributes to dementia pathology. PQBP1 functions in innate immune cells as an intracellular receptor that recognizes pathogens and neurodegenerative proteins. It is an intrinsically disordered protein that generates intracellular foci, similar to other neurodegenerative disease proteins such as TDP43, FUS, and hnRNPs. The knowledge accumulated over more than 20 years has given rise to a new concept that shifts in the equilibrium between physiological and pathological processes have their basis in the dysregulation of common protein structure-linked molecular mechanisms.

The Impact of X-Chromosome Inactivation on Phenotypic Expression of X-Linked Neurodevelopmental Disorders

Nearly 20% of genes located on the X chromosome are associated with neurodevelopmental disorders (NDD) due to their expression and role in brain functioning. Given their location, several of these genes are either subject to or can escape X-chromosome inactivation (XCI). The degree to which genes are subject to XCI can influence the NDD phenotype between males and females. We provide a general review of X-linked NDD genes in the context of XCI and detailed discussion of the sex-based differences related to MECP2 and FMR1, two common X-linked causes of NDD that are subject to XCI. Understanding the effects of XCI on phenotypic expression of NDD genes may guide the development of stratification biomarkers in X-linked disorders.

Renpenning syndrome in a female

Renpenning syndrome (OMIM: 309500) is a rare X-linked disorder that causes intellectual disability, microcephaly, short stature, a variety of eye anomalies, and characteristic craniofacial features. This condition results from pathogenic variation of PQBP1, a polyglutamine-binding protein involved in transcription and pre-mRNA splicing. Renpenning syndrome has only been reported in affected males. Carrier females do not usually have clinical features, and in reported families with Renpenning syndrome, most female carriers exhibit favorable skewing of X-chromosome inactivation. We describe a female with syndromic features typical of Renpenning syndrome. She was identified by exome sequencing to have a de novo heterozygous c.459_462delAGAG mutation in PQBP1 (Xp11.23), affecting the AG hexamer in exon 4, which is the most common causative mutation in this syndrome. Streaky hypopigmentation of the skin was observed, supporting a hypothesized presence of an actively expressed, PQBP1 mutation-bearing X-chromosome in some cells. X-inactivation studies on peripheral blood cells demonstrated complete skewing in both the proband and her mother with preferential inactivation of the maternal X chromosome in the child. We demonstrated expression of the PQBP1 mutant transcript in leukocytes of the affected girl. Therefore, it is highly likely that the PQBP1 mutation arose from the paternal X chromosome.

In utero gene therapy rescues microcephaly caused by Pqbp1-hypofunction in neural stem progenitor cells

Human mutations in PQBP1, a molecule involved in transcription and splicing, result in a reduced but architecturally normal brain. Examination of a conditional Pqbp1-knockout (cKO) mouse with microcephaly failed to reveal either abnormal centrosomes or mitotic spindles, increased neurogenesis from the neural stem progenitor cell (NSPC) pool or increased cell death in vivo. Instead, we observed an increase in the length of the cell cycle, particularly for the M phase in NSPCs. Corresponding to the developmental expression of Pqbp1, the stem cell pool in vivo was decreased at E10 and remained at a low level during neurogenesis (E15) in Pqbp1-cKO mice. The expression profiles of NSPCs derived from the cKO mouse revealed significant changes in gene groups that control the M phase, including anaphase-promoting complex genes, via aberrant transcription and RNA splicing. Exogenous Apc4, a hub protein in the network of affected genes, recovered the cell cycle, proliferation, and cell phenotypes of NSPCs caused by Pqbp1-cKO. These data reveal a mechanism of brain size control based on the simple reduction of the NSPC pool by cell cycle time elongation. Finally, we demonstrated that in utero gene therapy for Pqbp1-cKO mice by intraperitoneal injection of the PQBP1-AAV vector at E10 successfully rescued microcephaly with preserved cortical structures and improved behavioral abnormalities in Pqbp1-cKO mice, opening a new strategy for treating this intractable developmental disorder.

The Renpenning syndrome spectrum: new clinical insights supported by 13 new PQBP1-mutated males

Since the first reports of polyglutamine-binding protein 1 (PQBP1) mutations in Renpenning syndrome and related disorders, the spectrum of PQBP1-linked clinical manifestations has been outlined from rare published case reports. The phenotypic description is often obtained from medical archives, and therefore, heterogeneous. Moreover, some aspects such as brain imaging or cognitive and behavioral functioning are rarely described. In this study, 13 PQBP1-mutated French patients were subjected to a standardized clinical, cognitive and behavioral assessment. Physical measurements of their relatives were also collected. We report on a recognizable clinical and radiological phenotype. All patients presented with microcephaly, leanness and mild short stature, relative to familial measurements. Three new clinical features are described: upper back progressive muscular atrophy, metacarpophalangeal ankylosis of the thumb and velar dysfunction. The specific facial dysmorphic features included at least four of the following signs: long triangular face, large ridged nose, half-depilated eyebrows, dysplastic or protruding ears and rough slightly sparse hair. An over-aged appearance was noticed in elderly patients. Cortical gyrification was normal based on available magnetic brain imaging of six patients. PQBP1-linked microcephaly (or Renpenning syndrome) is an X-linked mental retardation syndrome, which has clinically recognizable features.

A two base pair deletion in the PQBP1 gene is associated with microphthalmia, microcephaly, and mental retardation

X-linked mental retardation has been traditionally divided into syndromic (S-XLMR) and non-syndromic forms (NS-XLMR), although the borderlines between these phenotypes begin to vanish and mutations in a single gene, for example PQBP1, can cause S-XLMR as well as NS-XLMR. Here, we report two maternal cousins with an apparently X-linked phenotype of mental retardation (MR), microphthalmia, choroid coloboma, microcephaly, renal hypoplasia, and spastic paraplegia. By multipoint linkage analysis with markers spanning the entire X-chromosome we mapped the disease locus to a 28-Mb interval between Xp11.4 and Xq12, including the BCOR gene. A missense mutation in BCOR was described in a family with Lenz microphthalmia syndrome, a phenotype showing substantial overlapping features with that described in the two cousins. However, no mutation in the BCOR gene was found in both patients. Subsequent mutation analysis of PQBP1, located within the delineated linkage interval in Xp11.23, revealed a 2-bp deletion, c.461_462delAG, that cosegregated with the disease. Notably, the same mutation is associated with the Hamel cerebropalatocardiac syndrome, another form of S-XLMR. Haplotype analysis suggests a germline mosaicism of the 2-bp deletion in the maternal grandmother of both affected individuals. In summary, our findings demonstrate for the first time that mutations in PQBP1 are associated with an S-XLMR phenotype including microphthalmia, thereby further extending the clinical spectrum of phenotypes associated with PQBP1 mutations.

Exonic microdeletions in the X-linked PQBP1 gene in mentally retarded patients: a pathogenic mutation and in-frame deletions of uncertain effect

Mutations in PQBP1 were recently identified in families with syndromic and non-syndromic X-linked mental retardation (XLMR). Clinical features frequently associated with MR were microcephaly and/or short stature. The predominant mutations detected so far affect a stretch of six AG dinucleotides in the polar-amino-acid-rich domain (PRD), causing frameshifts in the fourth coding exon. We searched for PQBP1 exon 4 frameshifts in 57 mentally retarded males in whom initial referral description indicated at least one of the following criteria: microcephaly, short stature, spastic paraplegia or family history compatible with XLMR, and in 772 mentally retarded males not selected for specific clinical features or family history. We identified a novel frameshift mutation (23 bp deletion) in two half-brothers with specific clinical features, and performed prenatal diagnosis in this family. We also found two different 21 bp in-frame deletions (c.334-354del(21 bp) and c.393-413del(21 bp)) in four unrelated probands from various ethnic origins, each deleting one of five copies of an imperfect seven amino-acid repeat. Although such deletions have not been detected in 1180 X chromosomes from European controls, the c. 334-354del(21 bp) was subsequently found in two of 477 Xs from Indian controls. We conclude that pathogenic frameshift mutations in PQBP1 are rare in mentally retarded patients lacking specific associated signs and that the 21 bp in-frame deletions may be non-pathogenic, or alternatively could act subtly on PQBP1 function. This touches upon a common dilemma in XLMR, that is, how to distinguish between mutations and variants that may be non-pathogenic or represent risk factors for cognitive impairment.