Opitz G/BBB syndrome in Xp22: mutations in the MID1 gene cluster in the carboxy-terminal domain

Opitz syndrome: improving clinical interpretation of intronic variants in MID1 gene

BACKGROUND

Loss-of-function variants in MID1 are the most common cause of Opitz G/BBB syndrome (OS). The interpretation of intronic variants affecting the splicing is a rising issue in OS.

METHODS

Exon sequencing of a 2-year-old boy with OS showed that he was a carrier of the de novo c.1286-10G>T variant in MID1. In silico predictions and minigene assays explored the effect of the variant on splicing. The minigene approach was also applied to two previously identified MID1 c.864+1G>T and c.1285+1G>T variants.

RESULTS

Minigene assay demonstrated that the c.1286-10G>T variant generated the inclusion of eight nucleotides that predicted generation of a frameshift. The c.864+1G>T and c.1285+1G>T variants resulted in an in-frame deletion predicted to generate a shorter MID1 protein. In hemizygous males, this allowed reclassification of all the identified variants from "of unknown significance" to "likely pathogenic."

CONCLUSIONS

Minigene assay supports functional effects from MID1 intronic variants. This paves the way to the introduction of similar second-tier investigations in the molecular diagnostics workflow of OS.

IMPACT

Causative intronic variants in MID1 are rarely investigated in Opitz syndrome. MID1 is not expressed in blood and mRNA studies are hardly accessible in routine diagnostics. Minigene assay is an alternative for assessing the effect of intronic variants on splicing. This is the first study characterizing the molecular consequences of three MID1 variants for diagnostic purposes and demonstrating the efficacy of minigene assays in supporting their clinical interpretation. Review of the criteria according to the American College of Medical Genetics reassessed all variants as likely pathogenic.

SPECC1L Mutations Are Not Common in Sporadic Cases of Opitz G/BBB Syndrome

Opitz G/BBB syndrome (OS) is a rare genetic developmental condition characterized by congenital defects along the midline of the body. The main clinical signs are represented by hypertelorism, laryngo–tracheo–esophageal defects and hypospadias. The X-linked form of the disease is associated with mutations in the MID1 gene located in Xp22 whereas mutations in the SPECC1L gene in 22q11 have been linked to few cases of the autosomal dominant form of this disorder, as well as to other genetic syndromes. In this study, we have undertaken a mutation screening of the SPECC1L gene in samples of sporadic OS cases in which mutations in the MID1 gene were excluded. The heterozygous missense variants identified are already reported in variant databases raising the issue of their pathogenetic meaning. Recently, it was reported that some clinical manifestations peculiar to OS signs are not observed in patients carrying mutations in the SPECC1L gene, leading to the proposal of the designation of ‘SPECC1L syndrome’ to refer to this disorder. Our study confirms that patients with diagnosis of OS, mainly characterized by the presence of hypospadias and laryngo–tracheo–esophageal defects, do not carry pathogenic SPECC1L mutations. In addition, SPECC1L syndrome-associated mutations are clustered in two specific domains of the protein, whereas the missense variants detected in our work lies elsewhere and the impact of these variants in the function of this protein is difficult to ascertain with the current knowledge and will require further investigations. Nonetheless, our study provides further insight into the SPECC1L syndrome classification.

Differential accumulation of tau pathology between reciprocal F1 hybrids of rTg4510 mice

Tau, a family of microtubule-associated proteins, forms abnormal intracellular inclusions, so-called tau pathology, in a range of neurodegenerative diseases collectively known as tauopathies. The rTg4510 mouse model is a well-characterized bitransgenic F1 hybrid mouse model of tauopathy, which was obtained by crossing a Camk2α-tTA mouse line (on a C57BL/6 J background) with a tetO-MAPT*P301L mouse line (on a FVB/NJ background). The aim of this study was to investigate the effects of the genetic background and sex on the accumulation of tau pathology in reciprocal F1 hybrids of rTg4510 mice, i.e., rTg4510 on the (C57BL/6 J × FVB/NJ)F1 background (rTg4510_CxF) and on the (FVB/NJ × C57BL/6 J)F1 background (rTg4510_FxC). As compared with rTg4510_CxF mice, the rTg4510_FxC mice showed marked levels of tau pathology in the forebrain. Biochemical analyses indicated that the accumulation of abnormal tau species was accelerated in rTg4510_FxC mice. There were strong effects of the genetic background on the differential accumulation of tau pathology in rTg4510 mice, while sex had no apparent effect. Interestingly, midline-1 (Mid1) was identified as a candidate gene associated with this difference and exhibited significant up/downregulation according to the genetic background. Mid1 silencing with siRNA induced pathological phosphorylation of tau in HEK293T cells that stably expressed human tau with the P301L mutation, suggesting the role of Mid1 in pathological alterations of tau. Elucidation of the underlying mechanisms will provide novel insights into the accumulation of tau pathology and is expected to be especially informative to researchers for the continued development of therapeutic interventions for tauopathies.

Molecular characterization of Rhodeus uyekii tripartite motif protein 1 (TRIM1) involved in IFN-γ/LPS-induced NF-κB signaling

The tripartite motif-containing (TRIM) proteins are involved in a wide range of cellular processes, and the role of TRIM1 in immunity has been explored. However, fundamental studies on fish TRIM1 are lacking. In this study, we cloned and characterized TRIM1 cDNA from the Korean rose bitterling, Rhodeus uyekii (RuTRIM1). Two RuTRIM1 isoforms (RuTRIM1-X1 and RuTRIM1-X2) were identified. The coding sequence (CDS) of RuTRIM1-X1 comprised 2157 bp encoding a 718-aa protein, and the CDS of RuTRIM1-X2 comprised 1929 bp encoding a 642-aa protein. Both RuTRIM1 isoforms contained a RING finger domain, B-box 1, B-box 2, coiled-coil domain, COS box, FN3 motif, and PRY/SPRY domain. The deduced RuTRIM1-X1 and RuTRIM1-X2 proteins had high amino acid identity (76.27-98.89%) with orthologs from various other species, and a phylogenetic tree was constructed. RuTRIM1-X1 and RuTRIM1-X2 mRNA were expressed in all tissues examined, with the highest expression levels detected in the hepatopancreas. During early development, RuTRIM1-X1 and RuTRIM1-X2 mRNA levels changed differently from the gastrula period to the first feeding stage. An in vivo ubiquitination assay showed that RuTRIM1 exhibited RING-dependent E3 ubiquitin ligase activity, mainly by comparing RuTRIM1-X2 to RuTRIM1-X1. The subcellular localization of the two RuTRIM1 protein isoforms was characterized, revealing that they formed aggregates in cytoplasmic bodies in Raw264.7 cells. Interferon-γ/lipopolysaccharide-induced nuclear factor-κB signaling was negatively regulated by RuTRIM1-X1 and RuTRIM1-X2, and the negative effect was reversed in RING deletion mutants. To our knowledge, this is the first study to characterize fish TRIM1, which may play a role in the inflammatory response.

Modulation of F-actin dynamics by maternal Mid1ip1L controls germ plasm aggregation and furrow recruitment in the zebrafish embryo

During the early embryonic cell cycles, zebrafish germ plasm ribonucleoparticles (RNPs) gradually multimerize and become recruited to the forming furrows. RNPs multimerization occurs prior to and during furrow initiation, as forming aggregates move outward through their association with the tips of growing interphase astral microtubules. Germ plasm RNPs are also associated with short cortical F-actin. We show that, in embryos mutant for the cytoskeletal regulator mid1ip1l, germ plasm RNPs fail to become recruited to the furrow, accumulating instead at the periphery of the blastodisc. RNP aggregates are associated with zones of mid1ip1l-dependent cyclical local cortical F-actin network enrichments, as well as contractions at both the cortex and the contractile ring. F-actin inhibition in wild-type embryos mimics the RNP peripheral accumulation defect of mid1ip1l mutants. Our studies suggest that a common mechanism underlies distinct steps of germ plasm RNP segregation. At the cortex, this process attenuates microtubule-dependent outward RNP movement to retain RNPs in the blastodisc cortex and allow their recruitment to the furrows. F-actin network contraction likely also facilitates higher-order germ plasm RNP multimerization.

Two Novel Pathogenic MID1 Variants and Genotype-Phenotype Correlation Reanalysis in X-Linked Opitz G/BBB Syndrome

X-linked Opitz G/BBB syndrome (XLOS) is a multisystemic congenital condition, caused by mutations in the midline-1 gene (MID1), characterized by a large inter- and intrafamilial phenotypic variability and often associated with intellectual disability (ID). We report clinical, genetic, and molecular findings in 4 patients with typical XLOS dysmorphic features belonging to 2 unrelated families. Two novel pathogenic loss-of-function MID1 variants, a maternally inherited c.1656del and a de novo c.1215_1228dup, were identified. Subsequently, we performed a genotype-phenotype analysis using data from 91 male XLOS patients. To test the mutation impact on the phenotype; the type of mutation, the MID1-impaired domain and function were compared with the presence of each of the major clinical features (hypertelorism, clefts of the lip and/or palate, laryngo-tracheo-esophageal abnormalities, hypospadias and ID) and minor clinical features (brain, heart, and anal defects). No statistically significant correlation was found with these features. Further investigations, as well as exhaustive and unequivocal phenotyping, may be required to improve our knowledge of the biological mechanisms underlying this syndrome and to provide more adequate disease management.

Solution structure of the microtubule-targeting COS domain of MID1

The human MID1 protein is required for the proper development during embryogenesis. Mutations of MID1 are associated with X-linked Opitz G syndrome, characterized by midline anomalies. MID1 associates with the microtubules and functions as an ubiquitin E3 ligase, targeting protein phosphatase 2A for ubiquitin-mediated regulation. The mechanism of microtubule association is not known. Recently, a 60-amino acid region termed the C-terminal subgroup One Signature (COS) box/domain was identified at the C-terminal end of the coiled-coil (CC) domain that facilitates microtubule localization. Insertion of the MID1 COS domain at the C-terminal end of the CC domain of a nonmicrotubule-associated TRIM protein confers microtubule localization. Here, we report the solution structure of the COS domain of MID1. The domain adopts a helix-loop-helix structure in which the N- and C-terminal ends are in close proximity. Hydrophobic residues stabilizing the interaction of the two α-helices form a central hydrophobic core. The loop separating the α-helices is structured, with two of its hydrophobic residues making contact with the central core. On the outer surface, positively charged residues form a distinct basic patch near the termini that we postulate is important for microtubule binding. A model of the structure of the preceding coiled-coil and COS domains (CC-COS) show that the COS domain forms a helical bundle at the C-terminal end of the CC domain similar to the spectrin-like fold observed with some known microtubule-binding proteins. Interestingly, the CC-COS domains bind to microtubules, demonstrating for the first time that MID1 can directly associate with the microtubules.

DATABASE

Structural data are available in PDB database under the accession number 5IM8.

aura (mid1ip1l) regulates the cytoskeleton at the zebrafish egg-to-embryo transition

Embryos from females homozygous for a recessive maternal-effect mutation in the gene aura exhibit defects including reduced cortical integrity, defective cortical granule (CG) release upon egg activation, failure to complete cytokinesis, and abnormal cell wound healing. We show that the cytokinesis defects are associated with aberrant cytoskeletal reorganization during furrow maturation, including abnormal F-actin enrichment and microtubule reorganization. Cortical F-actin prior to furrow formation fails to exhibit a normal transition into F-actin-rich arcs, and drug inhibition is consistent with aura function promoting F-actin polymerization and/or stabilization. In mutants, components of exocytic and endocytic vesicles, such as Vamp2, Clathrin and Dynamin, are sequestered in unreleased CGs, indicating a need for CG recycling in the normal redistribution of these factors. However, the exocytic targeting factor Rab11 is recruited to the furrow plane normally at the tip of bundling microtubules, suggesting an alternative anchoring mechanism independent of membrane recycling. A positional cloning approach indicates that the mutation in aura is associated with a truncation of Mid1 interacting protein 1 like (Mid1ip1l), previously identified as an interactor of the X-linked Opitz G/BBB syndrome gene product Mid1. A Cas9/CRISPR-induced mutant allele in mid1ip1l fails to complement the originally isolated aura maternal-effect mutation, confirming gene assignment. Mid1ip1l protein localizes to cortical F-actin aggregates, consistent with a direct role in cytoskeletal regulation. Our studies indicate that maternally provided aura (mid1ip1l) acts during the reorganization of the cytoskeleton at the egg-to-embryo transition and highlight the importance of cytoskeletal dynamics and membrane recycling during this developmental period.

Mid1/Mid2 expression in craniofacial development and a literature review of X-linked opitz syndrome

Background

Opitz syndrome (OS) is a genetic disorder that affects mainly the development of midline structures, including the craniofacial region, embryonic heart, and urogenital system. The manifestations of X‐linked OS are believed to be results of a malfunctioned gene, MID1, whose product has been shown to have ubiquitin E3 ligase activity and regulate the turnover of microtubular protein phosphatase 2Ac. MID2, a homolog of MID1, shares high structural and functional similarities with MID1. Identification of a missense mutation in MID2 in an Indian family causing overlapping phenotypes with OS provided the first evidence that MID2 might be involved in similar pathogenesis.

Methods

The clinic features and the genetic findings of all reported X‐linked OS were collectively summarized in this research. Real‐time RT‐PCR and in situ hybridization were used in the expression studies of Mid1/Mid2 in mouse embryos.

Results

Up‐to‐date, 88 different mutations have been identified in MID1 and most mutations occurred on the conserved amino acids of MID1 and MID2. Expression studies using real‐time RT‐PCR implicated a tendency of a mutually repressive expression pattern between Mid1 and Mid2 in mouse embryos. Further investigations using in situ hybridization revealed strong expressions of Mid1 and Mid2 in the epithelium of approaching facial prominences and downregulated expressions after fusion in mouse embryos.

Conclusions

Our results support the hypothesis of functional redundancy of Mid1/Mid2 and their potential roles in regulating tissue remodelling in early development.

The Challenge of Prenatal Diagnostic Work-Up of Maternally Inherited X-Linked Opitz G/BBB: Case Report and Literature Review

Background. Prenatal diagnosis of Optiz G/BBB syndrome (OS) is challenging because the characteristic clinical features, such as facial and genitourinary anomalies, may be subtle at sonography and rather unspecific. Furthermore, molecular testing of the disease gene is not routinely performed, unless a specific diagnosis is suggested. Method. Both familial and ultrasound data were used to achieve the diagnosis of X-linked OS (XLOS), which was confirmed by molecular testing of MID1 gene (Xp22.3) at birth. Results. Sequencing of MID1 gene disclosed the nucleotide change c.1285 +1 G>T, previously associated with XLOS. Conclusions. This case illustrates current challenges of the prenatal diagnostic work-up of XLOS and exemplifies how clinical investigation, including family history, and accurate US foetal investigations can lead to the correct diagnosis.

A novel Netrin-1-sensitive mechanism promotes local SNARE-mediated exocytosis during axon branching

Localized plasma membrane expansion during axon branching mediated by Netrin-1 occurs via TRIM9-dependent regulation of SNARE-mediated vesicle fusion.

Developmental axon branching dramatically increases synaptic capacity and neuronal surface area. Netrin-1 promotes branching and synaptogenesis, but the mechanism by which Netrin-1 stimulates plasma membrane expansion is unknown. We demonstrate that SNARE-mediated exocytosis is a prerequisite for axon branching and identify the E3 ubiquitin ligase TRIM9 as a critical catalytic link between Netrin-1 and exocytic SNARE machinery in murine cortical neurons. TRIM9 ligase activity promotes SNARE-mediated vesicle fusion and axon branching in a Netrin-dependent manner. We identified a direct interaction between TRIM9 and the Netrin-1 receptor DCC as well as a Netrin-1–sensitive interaction between TRIM9 and the SNARE component SNAP25. The interaction with SNAP25 negatively regulates SNARE-mediated exocytosis and axon branching in the absence of Netrin-1. Deletion of TRIM9 elevated exocytosis in vitro and increased axon branching in vitro and in vivo. Our data provide a novel model for the spatial regulation of axon branching by Netrin-1, in which localized plasma membrane expansion occurs via TRIM9-dependent regulation of SNARE-mediated vesicle fusion.

A novel mutation in MID1 in a patient with X-linked Opitz G/BBB syndrome

Opitz G/BBB syndrome (OS) is a genetically heterogeneous disease. We report on an OS patient with a novel inherited mutation in MID1. Metaphase analysis showed a normal male karyotype. Array CGH revealed a maternally inherited duplication at Xp22.31 (6,467,203-7,992,261, hg18), the size was estimated to 1.5Mb. Sequence analysis of the MID1 coding region revealed a novel missense mutation in exon 8 (c.1561C>T/p. R521C) which resulted in an ammonia acid substitution (R521C) in the PRX domain of the MID1 protein. The mutation was inherited from unaffected grandmother and mildly affected mother. Prenatal diagnosis was performed for the third pregnancy after identification of the causative mutation in the family. The third fetus was found to be a female carrier. Postnatal follow-up at 2-month-old showed normal phenotype. In conclusion, we reported a familial OS patient with a novel mutation in exon 8 which provided another evidence for that mutation clustered in C-terminal domain of MID1. The newly identified mutation in our patient expands mutation spectrum in MID1 gene.

X-linked microtubule-associated protein, Mid1, regulates axon development

Opitz syndrome (OS) is a genetic neurological disorder. The gene responsible for the X-linked form of OS, Midline-1 (MID1), encodes an E3 ubiquitin ligase that regulates the degradation of the catalytic subunit of protein phosphatase 2A (PP2Ac). However, how Mid1 functions during neural development is largely unknown. In this study, we provide data from in vitro and in vivo experiments suggesting that silencing Mid1 in developing neurons promotes axon growth and branch formation, resulting in a disruption of callosal axon projections in the contralateral cortex. In addition, a similar phenotype of axonal development was observed in the Mid1 knockout mouse. This defect was largely due to the accumulation of PP2Ac in Mid1-depleted cells as further down-regulation of PP2Ac rescued the axonal phenotype. Together, these data demonstrate that Mid1-dependent PP2Ac turnover is important for normal axonal development and that dysregulation of this process may contribute to the underlying cause of OS.

Complex rearrangement of the exon 6 genomic region among Opitz G/BBB Syndrome MID1 alterations

Opitz G/BBB Syndrome (OS) is a multiple congenital anomaly disorder characterized by developmental defects of midline structures. The most relevant clinical signs are ocular hypertelorism, hypospadias, cleft lip and palate, laryngo-tracheo-esophageal abnormalities, imperforate anus, and cardiac defects. Developmental delay, intellectual disability and brain abnormalities are also present. The X-linked form of this disorder is caused by mutations in the MID1 gene coding for a member of the tripartite motif family of E3 ubiquitin ligases. Here, we describe 12 novel patients that carry MID1 mutations emphasizing that laryngo-tracheo-esophageal defects are very common in OS patients and, together with hypertelorism and hypospadias, are the most frequent findings among the full spectrum of OS clinical manifestations. Besides missense and nonsense mutations, small insertions and deletions scattered along the entire length of the gene, we found that a consistent number of MID1 alterations are represented by the deletion of single coding exons. Deep characterization of one of these deletions reveals, for the first time within the MID1 gene, a complex rearrangement composed of two deletions, an inversion and a small insertion that may suggest the involvement of concurrent non-homologous mechanisms in the generation of the observed structural variant.

Exon 2 duplication of the MID1 gene in a patient with a mild phenotype of Opitz G/BBB syndrome

The X-linked form of Opitz G/BBB syndrome is a congenital midline malformation syndrome caused by MID1 loss-of-function mutations, including point mutations and small-sized duplications, insertions, and deletions. Three patients with an Opitz G/BBB syndrome phenotype and relatively large duplications of part of the MID1 gene have been described up to date. Here we report a 2-months-old boy with a very mild phenotype including craniofacial dysmorphism, swallowing difficulties, and a normal psychomotor development. Molecular karyotyping revealed a 57-kb duplication involving exon 2 of the MID1 gene. The in-frame tandem duplication was confirmed by MID1 transcript analysis. This alteration results likely in a mutant MID1 protein which contains 32 duplicated amino acids in the first part of the coiled-coil domain. The mild phenotype of the patient with the microduplication suggests that MID1 mutations can be found in patients with hypertelorism with or without other clinical signs and MID1 alterations might be missed in individuals not fulfilling the minimal criteria for diagnosis of X-linked Opitz G/BBB syndrome. This report further emphasizes the genotype-first approach in medical genetics in general and patients with unspecific clinical features in particular.

A pronounced evolutionary shift of the pseudoautosomal region boundary in house mice

The pseudoautosomal region (PAR) is essential for the accurate pairing and segregation of the X and Y chromosomes during meiosis. Despite its functional significance, the PAR shows substantial evolutionary divergence in structure and sequence between mammalian species. An instructive example of PAR evolution is the house mouse Mus musculus domesticus (represented by the C57BL/6J strain), which has the smallest PAR among those that have been mapped. In C57BL/6J, the PAR boundary is located just ~700 kb from the distal end of the X chromosome, whereas the boundary is found at a more proximal position in Mus spretus, a species that diverged from house mice 2-4 million years ago. In this study we used a combination of genetic and physical mapping to document a pronounced shift in the PAR boundary in a second house mouse subspecies, Mus musculus castaneus (represented by the CAST/EiJ strain), ~430 kb proximal of the M. m. domesticus boundary. We demonstrate molecular evolutionary consequences of this shift, including a marked lineage-specific increase in sequence divergence within Mid1, a gene that resides entirely within the M. m. castaneus PAR but straddles the boundary in other subspecies. Our results extend observations of structural divergence in the PAR to closely related subspecies, pointing to major evolutionary changes in this functionally important genomic region over a short time period.

A MID1 gene mutation in a patient with Opitz G/BBB syndrome that altered the 3D structure of SPRY domain

Mutations in the MID1 gene result in X-linked Opitz G/BBB syndrome (OS), a disorder that affects development of midline structures and comprises hypertelorism, cleft lip/palate, hypospadias, and laryngo-tracheo-esophageal abnormalities, and, at times, neurological, anal, and cardiac defects. MID1 gene abnormalities include missense, nonsense, and splicing mutations, small insertions, small deletions, and complex rearrangements. Here, we present a patient with Opitz G/BBB syndrome and a unique MID1 gene point mutation c.1703T Collapse

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MESH Headings

• Abnormalities, Multiple/genetics
• Base Sequence
• Carrier Proteins/genetics
• Chromosome Disorders/genetics
• Esophagus/abnormalities
• Humans
• Hypertelorism/genetics
• Hypospadias/genetics
• Infant, Newborn
• Male
• Microtubule Proteins/genetics
• Nuclear Proteins/genetics
• Point Mutation
• Polymerase Chain Reaction/methods
• Protein Structure, Tertiary
• Sequence Alignment
• Sequence Analysis, DNA
• Transcription Factors/genetics
• Tripartite Motif Proteins
• Ubiquitin-Protein Ligases

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Affiliation(s)

Ching-Hsuan Hu Department of Pediatrics, Chung Shan Medical University Hospital, Taichung, Taiwan
Yu-Fan Liu
Ju-Shan Yu
Yan-Yan Ng
Suh-Jen Chen
Pen-Hua Su
Jia-Yuh Chen

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NMR studies of the C-terminus of alpha4 reveal possible mechanism of its interaction with MID1 and protein phosphatase 2A

Alpha4 is a regulatory subunit of the protein phosphatase family of enzymes and plays an essential role in regulating the catalytic subunit of PP2A (PP2Ac) within the rapamycin-sensitive signaling pathway. Alpha4 also interacts with MID1, a microtubule-associated ubiquitin E3 ligase that appears to regulate the function of PP2A. The C-terminal region of alpha4 plays a key role in the binding interaction of PP2Ac and MID1. Here we report on the solution structure of a 45-amino acid region derived from the C-terminus of alpha4 (alpha45) that binds tightly to MID1. In aqueous solution, alpha45 has properties of an intrinsically unstructured peptide although chemical shift index and dihedral angle estimation based on chemical shifts of backbone atoms indicate the presence of a transient α-helix. Alpha45 adopts a helix-turn-helix HEAT-like structure in 1% SDS micelles, which may mimic a negatively charged surface for which alpha45 could bind. Alpha45 binds tightly to the Bbox1 domain of MID1 in aqueous solution and adopts a structure consistent with the helix-turn-helix structure observed in 1% SDS. The structure of alpha45 reveals two distinct surfaces, one that can interact with a negatively charged surface, which is present on PP2A, and one that interacts with the Bbox1 domain of MID1.

Detection and characterization of the in vitro e3 ligase activity of the human MID1 protein

Human MID1 (midline-1) is a microtubule-associated protein that is postulated to target the catalytic subunit of protein phosphatase 2A for degradation. It binds alpha4 that then recruits the catalytic subunit of protein phosphatase 2A. As a member of the TRIM (tripartite motif) family, MID1 has three consecutive zinc-binding domains-RING (really interesting new gene), Bbox1, and Bbox2-that have similar ββα-folds. Here, we describe the in vitro characterization of these domains individually and in tandem. We observed that the RING domain exhibited greater ubiquitin (Ub) E3 ligase activity compared to the Bbox domains. The amount of autopolyubiquitinated products with RING-Bbox1 and RING-Bbox1-Bbox2 domains in tandem was significantly greater than those of the individual domains. However, no polyubiquitinated products were observed for the Bbox1-Bbox domains in tandem. Using mutants of Ub, we observed that these MID1 domain constructs facilitate Ub chain elongation via Lys63 of Ub. In addition, we observed that the high-molecular-weight protein products were primarily due to polyubiquitination at one site (Lys154) on the Bbox1 domain of the RING-Bbox1 and RING-Bbox1-Bbox2 constructs. We observed that MID1 E3 domains could interact with multiple E2-conjugating enzymes. Lastly, a 45-amino-acid peptide derived from the C-terminus of alpha4 that binds tightly to Bbox1 was observed to be monoubiquitinated in the assay and appears to down-regulate the amount of polyubiquitinated products formed. These studies shed light on MID1 E3 ligase activity and show how its three zinc-binding domains can contribute to MID1's overall function.

Genetic and environmental factors in the etiology of esophageal atresia and/or tracheoesophageal fistula: an overview of the current concepts

Esophageal atresia and/or tracheoesophageal fistula (EA/TEF) are severe congenital anomalies. Although recent years have brought significant improvement in clinical treatment, our understanding of the etiology of these defects is lagging. Many genes and genetic pathways have been implicated in the development of EA/TEF, but only a few genes have been shown to be involved in humans, in animals, or in both. Extrapolating data from animal models to humans is not always straightforward. Environmental factors may also carry a risk, but the mechanisms are yet to be elucidated. This review gives an overview of the current state of knowledge about both genetic and environmental risk factors in the etiology of EA/TEF.

Active transport of the ubiquitin ligase MID1 along the microtubules is regulated by protein phosphatase 2A

Mutations in the MID1 protein have been found in patients with Opitz BBB/G syndrome (OS), which is characterised by multiple malformations of the ventral midline. MID1 is a microtubule-associated protein that stabilizes microtubules and, in association with the regulatory subunit of protein phosphatase 2A (PP2A), alpha4, provides ubiquitin ligase activity for the ubiquitin-specific modification of PP2A. Using Fluorescence Recovery After Photobleaching (FRAP) technology, we show here that MID1 is actively and bi-directionally transported along the microtubules, and that this movement is directly linked to its MAP kinase and PP2A-mediated phosphorylation status. Intact transport depends on both kinesins and dyneins and is inhibited upon colcemide treatments. MID1 proteins carrying missense mutations in the alpha4 binding domain still bind the microtubules but cannot be actively transported. Likewise, knock-down of the alpha4 protein, inhibition of PP2A activity by okadaic acid and fostriecin or the simulation of permanent phosphorylation at Ser96 in MID1 stop the migration of MID1-GFP, while preserving its microtubule-association. In summary, our data uncover an unexpected and novel function for PP2A, its regulatory subunit alpha4 and PP2A/alpha4/mTOR signaling in the active transport of the MID1 ubiquitin ligase complex along the cytoskeleton. Furthermore, a failure in the microtubule directed transport of this protein complex would be an attractive mechanism underlying the pathogenesis of OS in patients with B-box1 mutations.

Clinical and molecular studies of patients with characteristics of Opitz G/BBB syndrome shows a novel MID1 mutation

Opitz G/BBB syndrome is characterized by midline abnormalities such as hypertelorism, cleft palate, and hypospadias. This syndrome is heterogeneous with an X-linked recessive form caused by mutations in the MID1 gene at band Xp22.3. However, mutations in MID1 have only been identified in 47% of familial cases of X-linked Opitz G/BBB syndrome, and 13% of sporadic cases. We performed a phenotype-genotype analysis of a group of nine new patients with clinical characteristics commonly seen in Opitz G/BBB syndrome, and of previously reported patients. We identified a novel mutation in exon 9 of the MID1 gene, c.1941insTGAGTCATCATCC, leading to a premature termination codon at amino acid 514 in a patient with hypertelorism, apparently low-set ears, a short philtrum, bilateral cleft of lip and palate and hypospadias. This mutation affects the PRY domain of the C-terminus of the MID1 protein.

MID1 mutations in patients with X-linked Opitz G/BBB syndrome

Mutations in the MID1 gene are responsible for the X-linked form of Opitz G/BBB syndrome (OS), a disorder that affects the development of midline structures. OS is characterized by hypertelorism, hypospadias, laryngo-tracheo-esophageal (LTE) abnormalities, and additional midline defects. Cardiac, anal, and neurological defects are also present. The expressivity of OS is highly variable, even within the same family. We reviewed all the MID1 mutations reported so far, in both familial and sporadic cases. The mutations are scattered along the entire length of the gene and consist of missense and nonsense mutations, insertions and deletions, either in-frame or causing frameshifts, and deletions of either single exons or the entire MID1 coding region. The variety of described mutations and the lack of a strict genotype-phenotype correlation confirm the previous suggestion of the OS phenotype being caused by a loss-of-function mechanism. However, although a specific mutation cannot entirely account for the observed phenotype, we observed preferential association between some types of mutation and specific clinical manifestations, e.g., brain anatomical defects and truncating mutations. This may suggest that the pathogenetic mechanism underlying the OS phenotype is more complex and may vary among the affected organs.

Diagnosis of a terminal deletion of 4p with duplication of Xp22.31 in a patient with findings of Opitz G/BBB syndrome and Wolf-Hirschhorn syndrome

Opitz G/BBB syndrome (OS) is a congenital midline malformation syndrome characterized by hypertelorism, hypospadias, cleft lip/palate, laryngotracheoesophageal abnormalities, imperforate anus, developmental delay and cardiac defects. The X-linked form is caused by mutations in the MID1 gene, while no gene has yet been identified for the autosomal dominant form. Here, we report on a 15-year-old boy who was referred for MID1 mutation analysis with findings typical of OS, including apparent hypertelorism, hypospadias, a history of feeding difficulties, dysphagia secondary to esophageal arteria lusoria, growth retardation and developmental delay. No MID1 mutation was found, but subsequent sub-megabase resolution array CGH unexpectedly documented a 2.34 Mb terminal 4p deletion, suggesting a diagnosis of WHS, and a duplication in Xp22.31. Wolf-Hirschhorn syndrome (WHS) is a contiguous gene deletion syndrome involving terminal chromosome 4p deletions, in particular 4p16.3. WHS is characterized by typical facial appearance ("Greek helmet facies"), mental retardation, congenital hypotonia, and growth retardation. While the severity of developmental delay in this patient supports the diagnosis of WHS rather than OS, this case illustrates the striking similarities of clinical findings in seemingly unrelated syndromes, suggesting common or interacting pathways at the molecular and pathogenetic level. This is the first report of arteria lusoria (esophageal vascular ring) in a patient with WHS.

X-linked Opitz G/BBB syndrome: identification of a novel mutation and prenatal diagnosis in a Korean family

X-linked Opitz G/BBB syndrome (XLOS; MIM 300000) is a rare multiple congenital anomaly disorder that is characterized by facial anomalies, laryngeal/tracheal/esophageal defects and genitourinary abnormalities. XLOS is caused by mutations in the MID1 gene which encodes a microtubule-associated RING-Bbox-Coiled-coil (RBCC) protein. We recently found a four-year Korean male patient who was suspected of having XLOS. Mutation analysis of the MID1 gene in the patient and his mother demonstrated that the patient had a novel insertion mutation (c.1798_1799-insC), and his mother was a heterozygous carrier of the mutation. After identification of the causative mutation in this family, prenatal diagnosis of two consecutive fetuses were successfully undertaken. This is the first report on a genetically confirmed case of XLOS in Korea.

A mutation of beta -actin that alters depolymerization dynamics is associated with autosomal dominant developmental malformations, deafness, and dystonia

Actin, one of the major filamentous cytoskeletal molecules, is involved in a variety of cellular functions. Whereas an association between muscle actin mutations and skeletal and cardiac myopathies has been well documented, reports of human disease arising from mutations of nonmuscle actin genes have been rare. We have identified a missense point mutation in the gene coding for beta -actin that results in an arginine-to-tryptophan substitution at position 183. The disease phenotype includes developmental midline malformations, sensory hearing loss, and a delayed-onset generalized dystonia syndrome in monozygotic twins. Cellular studies of a lymphoblastoid cell line obtained from an affected patient demonstrated morphological abnormalities of the actin cytoskeleton and altered actin depolymerization dynamics in response to latrunculin A, an actin monomer-sequestering drug. Resistance to latrunculin A was also observed in NIH 3T3 cells expressing the mutant actin. These findings suggest that mutations in nonmuscle actins may be associated with a broad spectrum of developmental malformations and/or neurological abnormalities such as dystonia.

Xnf7 contributes to spindle integrity through its microtubule-bundling activity

Regulation of microtubule dynamics and organization in mitosis by a number of microtubule-associated proteins (MAPs) is required for proper bipolar spindle assembly, yet the precise mechanisms by which many MAPs function are poorly understood. One interesting class of MAPs is known to localize to the nucleus during interphase yet fulfill important spindle functions during mitosis. We have identified Xenopus nuclear factor 7 (Xnf7), a developmental regulator of dorsal-ventral patterning, as a microtubule-binding protein that also associates with the nuclear import receptor importin alpha/beta. Xnf7 localized to interphase nuclei and metaphase spindles both in Xenopus egg extracts and cultured cells. Xnf7-depleted spindles were hypersensitive to microtubule-depolymerizing agents. Functional characterization of Xnf7 revealed that it binds directly to microtubules, exhibits RING-finger-dependent E3-ubiquitin-ligase activity, and has C-terminal-dependent microtubule-bundling activity. The minimal microtubule-bundling domain of Xnf7 was sufficient to rescue the spindle-hypersensitivity phenotype. Thus, we have identified Xnf7 as a nuclear MAP whose microtubule-bundling activity, but not E3-ligase activity, contributes to microtubule organization and spindle integrity. Characterization of the multiple activities of Xnf7 may have implications for understanding human diseases caused by mutations in related proteins.

A structure-function study of MID1 mutations associated with a mild Opitz phenotype

The X-linked form of Opitz syndrome (OS) affects midline structures and produces a characteristic, but heterogeneous, phenotype that may include severe mental retardation, hypertelorism, broad nasal bridge, widow's peak, cleft lip/cleft palate, congenital heart disease, laryngotracheal defects, and hypospadias. The MID1 gene was implicated in OS by linkage to Xp22. It encodes a 667 amino acid protein that contains a RING finger motif, two B-box zinc fingers, a coiled-coil, a fibronectin type III (FNIII) domain, and a B30.2 domain. Several mutations in MID1 are associated with severe OS. Here, we describe an intelligent male with a milder phenotype characterized by hypertelorism, broad nasal bridge, widow's peak, mild hypospadias, pectus excavatum, and a surgically corrected tracheo-esophageal fistula. He has an above average intelligence and no cleft lip/palate or heart disease. We identified a novel mutation in MID1 (P441L) which is in exon 8 and functionally associated with the FNIII domain. While OS phenotypes have been attributed to mutations in the C-terminal part of MID1, little is currently known about the structure-function relationships of MID1 mutations, and how they affect phenotype. We find from a literature review that missense mutations within the FNIII domain of MID1 are associated with a milder presentation of OS than missense mutations elsewhere in MID1. All truncating mutations (frameshift, insertions/deletions) lead to severe OS. We used homology analysis of the MID1 FNIII domain to investigate structure-function changes caused by our missense mutation. This and other missense mutations probably cause disruption of protein-protein interactions, either within MID1 or between MID1 and other proteins. We correlate these protein structure-function findings to the absence of CNS or palatal changes and conclude that the FNIII domain of the MID1 protein may be involved in midline differentiation after neural tube and palatal structures are completed.

Structural Organization and Zn2+-dependent Subdomain Interactions Involving Autoantigenic Epitopes in the Ring-B-box-Coiled-coil (RBCC) Region of Ro52

Ro52 is one of the major autoantigens targeted in the autoimmune disease Sjögren syndrome. By sequence similarity, Ro52 belongs to the RING-B-box-coiled-coil (RBCC) protein family. Disease-related antibodies bind Ro52 in a conformation-dependent way both in the coiled-coil region and in the Zn2+-binding Ring-B-box region. Primarily associated with Sjögren syndrome, Ro52 autoantibodies directed to a specific, partially structured epitope in the coiled-coil region may also induce a congenital heart block in the fetus of pregnant Ro52-positive mothers. To improve our understanding of the pathogenic effects of autoantibody binding to the Zn2+-binding region, a multianalytical mapping of its structural, biophysical, and antigenic properties is presented. Structure content and ligand binding of subregions, dissected by peptide synthesis and subcloning, were analyzed by fluorescence and circular dichroism spectroscopy. A novel matrix-assisted laser desorption ionization time-of-flight mass spectrometry strategy for time-resolved proteolysis experiments of large protein domains was developed to facilitate analysis and to help resolve the tertiary arrangement of the entire RBCC subregion. The linker region between the RING and B-box motifs is crucial for full folding, and Zn2+ affinity of the RING-B-box region is further protected in the entire RBCC region and appears to interact with the coiled-coil region. Murine monoclonal antibodies raised toward the RING-B-box region were primarily directed toward the linker, further supporting a highly functional role for the linker in a cellular environment. Taken together with our previous analysis of autoantigenic epitopes in the coiled-coil region, localization of autoantigenic epitopes in Ro52 appears closely related to molecular functionalities.

Microphthalmia with linear skin defects (MLS) syndrome: Clinical, cytogenetic, and molecular characterization of 11 cases

The microphthalmia with linear skin defects (MLS) syndrome (MIM 309801) is a severe and rare developmental disorder, which is inherited as an X-linked dominant trait with male lethality. In the vast majority of patients, this syndrome is associated with terminal deletion of the Xp22.3 region. Thirty-five cases have been described to date in the literature since the first description of the syndrome in the early 1990s. We now report on the clinical, cytogenetic, and molecular characterization of 11 patients, 7 of whom have not been described previously. Seven of these patients have chromosomal abnormalities of the short arm of the X-chromosome, which were characterized and defined by fluorescence in situ hybridization (FISH) analysis. Intriguingly, one of the patients displays an interstitial Xp22.3 deletion, which to the best of our knowledge is the first reported for this condition. Finally we report on the identification and molecular characterization of four cases with clinical features of MLS but apparently normal karyotypes, verified by FISH analysis using genomic clones spanning the MLS minimal critical region, and with genome-wide analysis using a 1 Mb resolution BAC microarray. These patients made it possible to undertake mutation screening of candidate genes and may prove critical for the identification of the gene responsible for this challenging and intriguing genetic disease.

Evidence of functional redundancy between MID proteins: implications for the presentation of Opitz syndrome

Opitz G/BBB syndrome (OS) is a congenital defect characterized by hypertelorism and hypospadias, but additional midline malformations are also common in OS patients. X-linked OS is caused by mutations in the ubiquitin ligase MID1. In chick, MID1 is involved in left-right determination: a mutually repressive relationship between Shh and cMid1 in Hensen's node plays a key role in establishing the avian left-right axis. We have utilized our existing knowledge of the molecular basis of avian L/R determination to investigate the possible existence of functional redundancy between MID1 and its close homologue MID2. The expression of cMid2 overlaps with that of cMid1 in the node, and we demonstrate that MID2 can both mimic MID1 function as a right side determinant and rescue the laterality defects caused by knocking down endogenous MID proteins in the node. Our results show that MID2 is able to compensate for an absence in MID1 during chick left-right determination and may explain why OS patients do not suffer laterality defects despite the association between midline and L/R development. The demonstration of functional redundancy between MID1 and MID2 in the node provides supports for the hypothesis that partial functional redundancy between MID proteins in other developing structures contributes to the wide variability of OS phenotype.

Mild phenotypes in a series of patients with Opitz GBBB syndrome with MID1 mutations

Opitz syndrome (OS; MIM 145410 and MIM 300000) is a congenital midline malformation syndrome characterized by hypertelorism, hypospadias, cleft lip/palate, laryngotracheoesophageal (LTE) abnormalities, imperforate anus, developmental delay, and cardiac defects. The X-linked form (XLOS) is caused by mutations in the MID1 gene, which encodes a microtubule-associated RBCC protein. In this study, phenotypic manifestations of patients with and without MID1 mutations were compared to determine genotype-phenotype correlations. We detected 10 novel mutations, 5 in familial cases, 2 in sporadic cases, and 3 in families for whom it was not clear if they were familial or sporadic. The genotype and phenotype was compared for these 10 families, clinically diagnosed OS patients found not to have MID1 mutations, and 4 families in whom we have previously reported MID1 mutations. This combined data set includes clinical and mutation data on 70 patients. The XLOS patients with MID1 mutations were less severely affected than patients with MID1 mutations reported in previous studies, particularly in functionally significant neurologic, LTE, anal, and cardiac abnormalities. Minor anomalies were more prevalent in patients with MID1 mutations compared to those without mutations in this study. Female MID1 mutation carriers had milder phenotypes compared to male MID1 mutation carriers, with the most common manifestation being hypertelorism in both sexes. Most of the anomalies found in the patients of the present study do not correlate with the MID1 mutation type, with the possible exception of LTE malformations. This study demonstrates the wide spectrum of severity and manifestations of OS. It also shows that XLOS patients with MID1 mutations may be less severely affected than indicated in prior reports.

A Single Amino Acid Change in the SPRY Domain of Human Trim5α Leads to HIV-1 Restriction

Retroviral restriction factors are cellular proteins that interfere with retrovirus replication at a postpenetration, preintegration stage in the viral life cycle. The first restriction activity described was the mouse Fv1 gene. Three different alleles of Fv1, capable of restricting different murine leukaemia viruses (MLV), have been characterized at the molecular level. Two further activities, Ref1, which acts on MLV, and Lv1, which acts on lentiviruses, have been identified in other mammalian species. Recently, it has become clear that Ref1 and Lv1 are encoded by the same gene, Trim5alpha, which inhibits retrovirus replication in a species-specific manner. A series of chimeras between the human and rhesus monkey Trim5 genes were created to map and identify these specificity determinants. The Trim5alpha SPRY domain was found to be responsible for targeting HIV-1 restriction. By contrast, N-MLV restriction appears dependent on both the coiled-coil domain and the SPRY domain. A single amino acid substitution (R332P) in the human Trim5alpha can confer the ability to restrict HIV-1, suggesting that small changes during evolution may have profound effects on our susceptibility to cross-species infection.

Mig12, a novel Opitz syndrome gene product partner, is expressed in the embryonic ventral midline and co-operates with Mid1 to bundle and stabilize microtubules

BACKGROUND

Opitz G/BBB syndrome is a genetic disorder characterized by developmental midline abnormalities, such as hypertelorism, cleft palate, and hypospadias. The gene responsible for the X-linked form of this disease, MID1, encodes a TRIM/RBCC protein that is anchored to the microtubules. The association of Mid1 with the cytoskeleton is regulated by dynamic phosphorylation, through the interaction with the alpha4 subunit of phosphatase 2A (PP2A). Mid1 acts as an E3 ubiquitin ligase, regulating PP2A degradation on microtubules.

RESULTS

In spite of these findings, the biological role exerted by the Opitz syndrome gene product is still unclear and the presence of other potential interacting moieties in the Mid1 structure prompted us to search for additional cellular partners. Through a yeast two-hybrid screening approach, we identified a novel gene, MIG12, whose protein product interacts with Mid1. We confirmed by immunoprecipitation that this interaction occurs in vivo and that it is mediated by the Mid1 coiled-coil domain. We found that Mig12 is mainly expressed in the neuroepithelial midline, urogenital apparatus, and digits during embryonic development. Transiently expressed Mig12 is found diffusely in both nucleus and cytoplasm, although it is enriched in the microtubule-organizing center region. Consistently with this, endogenous Mig12 protein is partially detected in the polymerized tubulin fraction after microtubule stabilization. When co-transfected with Mid1, Mig12 is massively recruited to thick filamentous structures composed of tubulin. These microtubule bundles are resistant to high doses of depolymerizing agents and are composed of acetylated tubulin, thus representing stabilized microtubule arrays.

CONCLUSIONS

Our findings suggest that Mig12 co-operates with Mid1 to stabilize microtubules. Mid1-Mig12 complexes might be implicated in cellular processes that require microtubule stabilization, such as cell division and migration. Impairment in Mig12/Mid1-mediated microtubule dynamic regulation, during the development of embryonic midline, may cause the pathological signs observed in Opitz syndrome patients.

X-linked Opitz syndrome: novel mutations in the MID1 gene and redefinition of the clinical spectrum

Opitz (or G/BBB) syndrome is a pleiotropic genetic disorder characterized by hypertelorism, hypospadias, and additional midline defects. This syndrome is heterogeneous with an X-linked (XLOS) and an autosomal dominant (ADOS) form. The gene implicated in the XLOS form, MID1, encodes a protein containing a RING-Bbox-Coiled-coil motif belonging to the tripartite motif (TRIM) family. To further clarify the molecular basis of XLOS, we have undertaken mutation analysis of the MID1 gene in patients with Opitz syndrome (OS). We found novel mutations in 11 of 63 male individuals referred to us as sporadic or familial X-linked OS cases. The mutations are scattered throughout the gene, although more are represented in the 3' region. By reviewing all the MID1-mutated OS patients so far described, we confirmed that hypertelorism and hypospadias are the most frequent manifestations, being present in almost every XLOS individual. However, it is clear that laryngo-tracheo-esophageal (LTE) defects are also common anomalies, being manifested by all MID1-mutated male patients. Congenital heart and anal abnormalities are less frequent than reported in literature. In addition, we can include limb defects in the OS clinical synopsis as we found a MID1-mutated patient showing syndactyly. The low frequency of mutations in MID1 and the high variability of the phenotype suggest the involvement of other genes in the OS phenotype.

The Opitz syndrome gene MID1 is essential for establishing asymmetric gene expression in Hensen's node

Patterning the avian left-right (L/R) body axis involves the establishment of asymmetric molecular signals on the left and right sides of Hensen's node. We have examined the role of the chick Midline 1 gene, cMid1, in generating asymmetric gene expression in the node. cMid1 is initially expressed bilaterally, but its expression is then confined to the right side of the node. We show that this restriction of cMid1 expression is a result of repression by Shh on the left side of the node. Misexpression of cMid1 on the left side of the node results in bilateral Bmp4 expression and a loss of Shh expression. Correspondingly, downstream left pathway genes are repressed while right pathway genes are ectopically activated. Conversely, knocking down endogenous right-sided cMid1 results in a loss of Bmp4 expression and bilateral Shh expression. This results in an absence of right pathway genes and the ectopic activation of the left pathway on the right. Here, we present a revised model for the establishment of asymmetric gene expression in Hensen's node based on the epistatic interactions observed between Shh, cMid1, and Bmp4.

The MID1/PP2A complex: a key to the pathogenesis of Opitz BBB/G syndrome

Opitz BBB/G syndrome is a monogenic disorder that is characterized by malformations of the ventral midline. Investigations into the underlying genetic defects and the pathobiochemistry of this syndrome have already shed light on the mechanisms of both the physiological and the pathological development of the ventral midline, a complicated multistep process. Moreover, these studies have revealed the ubiquitin-dependent regulation of microtubule-associated phosphatase 2A, a central mechanism in many cellular processes. In this review, we summarize recent findings and speculate upon their implications for both medical and general research.

Duplication of the MID1 first exon in a patient with Opitz G/BBB syndrome

Opitz G/BBB syndrome is a malformation syndrome of the ventral midline mainly characterized by hypertelorism, swallowing difficulties, hypospadias and developmental delay. SSCP analysis and genomic sequencing of the MID1 open reading frame have identified mutations in 80% of the families with X-linked inheritance. However, in many patients the underlying genetic defect remains undetected by these techniques. Using RNA diagnostics we have now identified a duplication of the MID1 first exon in a patient with X-linked Opitz G/BBB syndrome. This duplication introduces a premature termination codon. In addition, we could significantly lower the threshold for mutation detection on the DNA level by combining SSCP analysis with DHPLC technology.

The Opitz syndrome gene Mid1 is transcribed from a human endogenous retroviral promoter

Human endogenous retroviruses (HERVs) and other long terminal repeat (LTR)-containing elements comprise a significant portion (8%) of the human genome and are likely vestiges of retroviral infections during primate evolution. Many of the HERVs present in human DNA have retained functional promoter, enhancer, and polyadenylation signals, and these regulatory sequences have the potential to modify the expression of nearby genes. To identify retroviral elements that contribute to the transcription of human genes, we screened sequence databases for chimeric (viral-cellular) transcripts. These searches revealed a fusion transcript containing the LTR of an HERV-E element linked to the Opitz syndrome gene Mid1. We confirmed the authenticity of the chimeric transcript by 5' rapid amplification of cDNA ends (RACE) and established that the Mid1 mRNA isoform was transcribed from a retroviral LTR. The identification of a retroviral first exon suggested the existence of alternative promoters for Mid1 because nonretroviral (native) 5' untranslated regions (UTRs) had been reported previously for this gene. Although Mid1 transcripts could be detected in all tissues tested, quantitative real-time reverse transcription-polymerase chain reaction indicated that the retroviral promoter contributes significantly to the level of Mid1 transcripts in placenta and embryonic kidney, where chimeric mRNAs were found to represent 25% and 22% of overall Mid1 mRNAs, respectively. Transient transfection studies supported a role for the LTR as a strong tissue-specific promoter in placental and embryonic kidney cell lines and suggested a function for the LTR as an enhancer. These findings provide further evidence that some endogenous retroviruses have evolved a biological function by contributing transcriptional regulatory elements to cellular genes.

MID1 and MID2 homo- and heterodimerise to tether the rapamycin-sensitive PP2A regulatory subunit, alpha 4, to microtubules: implications for the clinical variability of X-linked Opitz GBBB syndrome and other developmental disorders

BACKGROUND

Patients with Opitz GBBB syndrome present with a variable array of developmental defects including craniofacial, cardiac, and genital anomalies. Mutations in the X-linked MID1 gene, which encodes a microtubule-binding protein, have been found in approximately 50% of Opitz GBBB syndrome patients consistent with the genetically heterogeneous nature of the disorder. A protein highly related to MID1, called MID2, has also been described that similarly associates with microtubules.

RESULTS

To identify protein partners of MID1 and MID2 we undertook two separate yeast two-hybrid screens. Using this system we identified Alpha 4, a regulatory subunit of PP2-type phosphatases and a key component of the rapamycin-sensitive signaling pathway, as a strong interactor of both proteins. Analysis of domain-specific deletions has shown that the B-boxes of both MID1 and MID2 mediate the interaction with Alpha 4, the first demonstration in an RBCC protein of a specific role for the B-box region. In addition, we show that the MID1/2 coiled-coil motifs mediate both homo- and hetero-dimerisation, and that dimerisation is a prerequisite for association of the MID-Alpha 4 complex with microtubules.

CONCLUSIONS

Our findings not only implicate Alpha 4 in the pathogenesis of Opitz GBBB syndrome but also support our earlier hypothesis that MID2 is a modifier of the X-linked phenotype. Of further note is the observation that Alpha 4 maps to Xq13 within the region showing linkage to FG (Opitz-Kaveggia) syndrome. Overlap in the clinical features of FG and Opitz GBBB syndromes warrants investigation of Alpha 4 as a candidate for causing FG syndrome.

FXY2/MID2, a gene related to the X-linked Opitz syndrome gene FXY/MID1, maps to Xq22 and encodes a FNIII domain-containing protein that associates with microtubules

Opitz G/BBB syndrome (OS) is a genetically heterogeneous disorder with an X-linked locus and an autosomal locus linked to 22q11.2. OS affects multiple organ systems with often variable severity even between siblings. The clinical features, which include hypertelorism, cleft lip and palate, defects of cardiac septation, hypospadias, and anorectal anomalies, indicate an underlying disturbance of the developing ventral midline of the embryo. The gene responsible for X-linked OS, FXY/MID1, is located on the short arm of the human X chromosome within Xp22.3 and encodes a protein with both an RBCC (RING finger, B-box, coiled coil) and a B30.2 domain. The Fxy gene in mice is also located on the X chromosome but spans the pseudoautosomal boundary in this species. Here we describe a gene closely related to FXY/MID1, called FXY2, which also maps to the X chromosome within Xq22. The mouse Fxy2 gene is located on the distal part of the mouse X chromosome within a region syntenic to Xq22. Analysis of genes flanking both FXY/MID1 and FXY2 (as well as their counterparts in mouse) suggests that these regions may have arisen as a result of an intrachromosomal duplication on an ancestral X chromosome. We have also identified in both FXY2 and FXY/MID1 proteins a conserved fibronectin type III domain located between the RBCC and B30.2 domains that has implications for understanding protein function. The FXY/MID1 protein has previously been shown to colocalize with microtubules, and here we show that the FXY2 protein similarly associates with microtubules in a manner that is dependent on the carboxy-terminal B30.2 domain.

Molecular cloning of rat efp: expression and regulation in primary osteoblasts

We have previously identified an estrogen-responsive gene, efp (estrogen-responsive finger protein), by genomic binding-site cloning method. Here, we isolated a rat homologue of efp cDNA that encodes an open reading frame of 644 amino acids sharing high homology with human efp (69% identity at the protein level) and mouse efp (80% identity at the protein level). The efp protein has a RING finger, a variant type of zinc finger motif, B1 box and B2 box, each having a pair of zinc fingers, and coiled-coil domain, belonging to the RING finger-B box-Coiled Coil (RBCC) family. Several members of RBCC family including efp have characteristic C-terminal domain, forming a subfamily. Next, we detected efp mRNA in primary osteoblasts, one of estrogen target cells, derived from the calvariae of rat fetus. An anti-efp antibody revealed the efp protein is expressed and regulated by estrogen in the primary osteoblasts. Interestingly, the efp protein in primary osteoblasts is down-regulated by 1alpha,25-dihydroxyvitamin D(3) treatment that promotes the differentiation of the cells, whereas it is up-regulated by TGF-beta1 treatment that inhibits the differentiation of the cells. These findings suggest the possible involvement of the efp in the differentiation of osteoblastic cells.

MID2, a homologue of the Opitz syndrome gene MID1: similarities in subcellular localization and differences in expression during development

The B-box family is an expanding new family of genes encoding proteins involved in diverse cellular functions such as developmental patterning and oncogenesis. A member of this protein family, MID1, is the gene responsible for the X-linked form of Opitz G/BBB syndrome, a developmental disorder characterized by defects of the midline structures. We now report the identification of MID2, a new transcript closely related to MID1. MID2 maps to Xq22 in human and to the syntenic region on the mouse X chromosome. The two X-linked genes share the same domains, the same exon-intron organization, a high degree of similarity at the protein level and the same subcellular localization, both being confined to the cytoplasm in association to micro-tubular structures. The expression pattern studied by RNA in situ hybridization in mouse revealed that Mid2 is expressed early in development and the highest level of expression is detected in the heart, unlike Mid1 for which no expression was detected in the developing heart. Together, these data suggest that midin and MID2 have a similar biochemical function but a different physiological role during development.