An intron mutation in the human alpha 1(I) collagen gene alters the efficiency of pre-mRNA splicing and is associated with osteogenesis imperfecta type II

A novel deletion mutation in CCM1 gene (krit1) is detected in a Chinese family with cerebral cavernous malformations

Cerebral Cavernous Malformations (CCM) are vascular malformations that are mostly located in the central nervous system (CNS) and occasionally within the skin and retina, which are classified into three types (CCM1, CCM2 and CCM3) by being located at different loci on chromosomes. At present, CCM1 (7q21), CCM2 (7p13-p15) and CCM3 (3q25.2-q27) are respectively linked to krit1 (Krev interaction trapped gene 1), MGC4607 and PDCD10 (programmed cell death 10). In this work, we identified a novel "GTA" deletion mutation at the acceptor splicing site of intron9/exon10 on krit1. The mutation results in an abnormally spliced protein by creating a premature termination code at the 23rd amino acid downstream from the sequence alteration. Our results are consistent with previous research on krit1 mutations and confirm the conclusion that KRIT1 haploinsufficiency may be the underlying mechanism of CCM1.

Novel mouse model of autosomal semidominant adult hypophosphatasia has a splice site mutation in the tissue nonspecific alkaline phosphatase gene Akp2

Deactivating mutations in the TNSALP gene cause HPP. Akp2(-/-) mice model severe infantile HPP, but there is no model for the relatively mild adult form. Here we report on mice with an induced mutation in Akp2 that affects splicing. The phenotype of homozygotes mirror aspects of the adult form of HPP.

INTRODUCTION

Hypophosphatasia (HPP) is a clinically varied skeletal disorder resulting from deficiency of tissue nonspecific alkaline phosphatase (TNSALP). Mice lacking Akp2 model infantile HPP characterized by skeletal hypomineralization, impaired growth, seizures, and perinatal mortality. No animal model exists to study the less severe forms of the disease that typically present in later life.

MATERIALS AND METHODS

N-ethyl-N-nitrosourea (ENU) mutagenesis was used to generate mouse models of human disease. A mouse with low plasma alkaline phosphatase (ALP) activity was identified by our clinical chemistry screen. Its offspring were used for inheritance studies and subjected to biochemical, histological, and radiological phenotyping. DNA was extracted for mapping and osteoblasts harvested for functional studies.

RESULTS

We showed semidominant inheritance of the low ALP phenotype and mapped the underlying point mutation to Akp2. Affected offspring bear the splice site mutation 862 + 5G>A-a hypomorphic allele named Akp2(Hpp). The same mutation has been reported in a patient. Akp2(Hpp/+) mice have approximately 50% of normal plasma ALP but display no other biochemical or skeletal abnormalities. Unlike Akp2(-/-) mice, Akp2(Hpp/Hpp) mice have normal initial skeletal development and growth, a normal lifespan and do not have seizures. TNSALP is low but detectable in Akp2(Hpp/Hpp) plasma. Osteoblasts display approximately 10% of normal ALP activity and reduced intracellular inorganic phosphate levels, yet are capable of normal mineralization in vitro. TNSALP substrates are significantly elevated in urine (inorganic pyrophosphate and phosphoethanolamine) and plasma (pyridoxal 5'-phosphate), whereas plasma inorganic pyrophosphate levels are normal. Akp2(Hpp/Hpp) mice develop late-onset skeletal disease, notably defective endochondral ossification and bone mineralization that leads to arthropathies of knees and shoulders.

CONCLUSIONS

Akp2(Hpp/Hpp) mice mirror a number of clinical features of the human adult form of HPP. These mice provide for the first time an animal model of late onset HPP that will be valuable in future mechanistic studies and for the evaluation of therapies such as those aimed at HPP.

A sequence motif in the simian virus 40 (SV40) early core promoter affects alternative splicing of transcribed mRNA

To identify new sequence elements in the promoter that affect splicing patterns of pre-mRNAs, we analyzed effects of different promoters on alternative splicing of model reporter genes. We compared the E1a alternative splicing pattern in transcripts expressed from the full-length cytomegalovirus, SV40 early, or a hybrid cytomegalovirus/SV40 early promoter and found that the hybrid promoter improved selection of the suboptimal E1a 5'SS-1. Expressing RNA from the hybrid promoter also enhanced selection of suboptimal splice sites in other alternatively spliced reporter genes, demonstrating the generality of this effect. Unlike previously defined promoter elements shown to affect alternative splicing, which were located in the enhancer/upstream activating sequences, the motif identified in this work is positioned within the core promoter; it is comprised of eight T-residues directly upstream of the SV40 early TATA box. This motif was previously implicated in DNA bending and negative regulation of transcription. Together, these results suggest that the identity of transcription complex assembled in the core promoter-dependent fashion can affect splice site selection during pre-mRNA splicing, perhaps by influencing the processivity of transcription elongation.

The molecular basis of familial hypercholesterolaemia in Turkish patients

Familial hypercholesterolaemia (FH) is an autosomal dominant disorder of lipoprotein metabolism. In the majority of patients FH is caused by mutations in the gene for the low-density lipoprotein receptor (LDLR), and to date more than 700 mutations have been reported worldwide. In this study, 36 paediatric patients with a clinical diagnosis of FH (20 homozygous and 16 heterozygotes) were screened for mutations in the LDLR gene. Each exon, with intron-exon junctions, was screened by capillary fluorescent SSCP (F-SSCP) and heteroduplex analysis. Samples showing different band patterns were sequenced. Ten novel (including three frame shift small deletions or insertions) and seven known mutations were detected. A total of 37 out of the predicted 56 FH-causing alleles were identified (66.1%). No patients with the R3500Q mutation in the APOB gene were found. W556R was the most common mutation, explaining 21.4% of the predicted defective LDLR alleles. The novel sequence changes were deemed to be pathogenic if they altered a conserved amino acid (L143P, D147E, Q233H-C234G, C347G) or occurred in or close to a splice site (IVS 16+5) and were absent in DNA from 50 healthy Turkish subjects. These data confirm the genetic heterogeneity of FH in Turkey, and demonstrate the usefulness of F-SSCP for mutation detection.

Intracellular trafficking and degradation of unassociated proalpha2 chains of collagen type I

Procollagen I is a trimer consisting of two proalpha1(I) chains and one proalpha 2(I) chain. In certain cases of mild osteogenesis imperfecta, abnormal proalpha1(I) chains are degraded very soon after synthesis. As a consequence, the cells produce excess proalpha2(I) chains, which cannot form trimers and are not secreted. The objective of this work was to determine the intracellular fate of unassociated proalpha2(I) chains. Mov13 mouse fibroblasts, which do not synthesize proalpha1(I) mRNA, but do produce proalpha2(I) mRNA, were incubated with radioactive amino acids using pulse-chase protocols, and proteins were analyzed by gel electrophoresis, autoradiography, and Western blotting. Mov13 cells produced proalpha2(I) chains that were degraded intracellularly within 30 min. Degradation was inhibited when cells were treated with brefeldin-A, which blocks transit from endoplasmic reticulum to Golgi. Fixed cells exposed to various immunofluorescence markers and imaged by confocal laser scanning microscopy showed that proalpha2(I) chains colocalized with Golgi and lysosome markers. Degradation was inhibited and chains were secreted when cells were treated with wortmannin, which blocks trafficking to lysosomes. These results demonstrate that unassociated proalpha2(I) chains leave the endoplasmic reticulum, transit the Golgi, and enter lysosomes where they are degraded.

A recurrent RNA-splicing mutation in the SEDL gene causes X-linked spondyloepiphyseal dysplasia tarda

Spondyloepiphyseal dysplasia tarda (SEDL) is a genetically heterogeneous disorder characterized by mild-to-moderate short stature and early-onset osteoarthritis. Both autosomal and X-linked forms have been described. Elsewhere, we have reported the identification of the gene for the X-linked recessive form, which maps to Xp22.2. We now report characterization of an exon-skipping mutation (IVS3+5G-->A at the intron 3 splice-donor site) in two unrelated families with SEDL. Using reverse transcriptase (RT)-PCR, we demonstrated that the mutation resulted in elimination of the first 31 codons of the open reading frame. The mutation was not detected in 120 control X chromosomes. Articular cartilage from an adult who had SEDL and carried this mutation contained chondrocytes with abundant Golgi complexes and dilated rough endoplasmic reticulum (ER). RT-PCR experiments using mouse/human cell hybrids revealed that the SEDL gene escapes X inactivation. Homologues of the SEDL gene include a transcribed retropseudogene on chromosome 19, as well as expressed genes in mouse, rat, Drosophila melanogaster Caenorhabditis elegans, and Saccharomyces cerevisiae. The latter homologue, p20, has a putative role in vesicular transport from ER to Golgi complex. These data suggest that SEDL mutations may perturb an intracellular pathway that is important for cartilage homeostasis.

Reduced expression of dentin sialophosphoprotein is associated with dysplastic dentin in mice overexpressing transforming growth factor-beta 1 in teeth

Transforming growth factor (TGF)-beta1 is expressed in developing tooth from the initiation stage through adulthood. Odontoblast-specific expression of TGF-beta1 in the tooth continues throughout life; however, the precise biological functions of this growth factor in the odontoblasts are not clearly understood. Herein, we describe the generation of transgenic mice that overexpress active TGF-beta1 predominantly in the odontoblasts. Teeth of these mice show a significant reduction in the tooth mineralization, defective dentin formation, and a relatively high branching of dentinal tubules. Dentin extracellular matrix components such as type I and III collagens are increased and deposited abnormally in the dental pulp, similar to the hereditary human tooth disorders such as dentin dysplasia and dentinogenesis imperfecta. Calcium, one of the crucial inorganic components of mineralization, is also apparently increased in the transgenic mouse teeth. Most importantly, the expression of dentin sialophosphoprotein (dspp), a candidate gene implicated in dentinogenesis imperfecta II (MIM 125420), is significantly down-regulated in the transgenic teeth. Our results provide in vivo evidence suggesting that TGF-beta1 mediated expression of dspp is crucial for dentin mineralization. These findings also provide for the first time a direct experimental evidence indicating that decreased dspp gene expression along with the other cellular changes in odontoblasts may result in human hereditary dental disorders like dentinogenesis imperfecta II (MIM 125420) and dentin dysplasia (MIM 125400 and 125420).

Novel point mutation in the splice donor site of exon-intron junction 6 of the androgen receptor gene in a patient with partial androgen insensitivity syndrome

Androgen receptor (AR) gene mutations have been shown to cause androgen insensitivity syndrome with altered sexual differentiation in XY individuals, ranging from a partial insensitivity with male phenotype and azoospermia to a complete insensitivity with female phenotype and the absence of pubic and axillary sexual hair after puberty. In this study we present an 11-yr-old XY girl, with clinical manifestations peculiar for impaired androgen biological action, including female phenotype, blind-ending vagina, small degree of posterior labial fusion, and absence of uterus, fallopian tubes, and ovaries. At the time of the diagnosis the patient had a FSH/LH ratio according to the puberal stage, undetectable 17beta-estradiol, and high levels of testosterone (80.1 ng/mL). After bilateral gonadectomy, performed at the age of 11 yr, histological examination showed small embryonic seminiferous tubules containing prevalently Sertoli cells and occasional spermatogonia together with abundant fibrous tissue. Molecular study of the patient showed a guanine to thymine transversion in position +5 of the donor splice site in the junction between exon 6 and intron 6 of the AR gene. The result of RT-PCR amplification of the AR messenger ribonucleic acid from cultured genital skin fibroblasts of the patient suggests that splicing is defective, and intron 6 is retained in most of the receptor messenger ribonucleic acid molecules. We show by immunoblotting that most of the expressed protein lacks part of the C-terminal hormone-binding domain, and a small amount of normal receptor is observed. This is probably responsible for the reduced binding capacity in genital skin fibroblasts of the patient. The molecular basis of the alteration in this case is a novel, uncommon mutation, leading to a phenotype indicative of a partial androgen insensitivity syndrome, Quigley's grade 5.

Information analysis of human splice site mutations

Splice site nucleotide substitutions can be analyzed by comparing the individual information contents (Ri, bits) of the normal and variant splice junction sequences [Rogan and Schneider, 1995]. In the present study, we related splicing abnormalities to changes in Ri values of 111 previously reported splice site substitutions in 41 different genes. Mutant donor and acceptor sites have significantly less information than their normal counterparts. With one possible exception, primary mutant sites with <2.4 bits were not spliced. Sites with Ri values > or = 2.4 bits but less than the corresponding natural site usually decreased, but did not abolish splicing. Substitutions that produced small changes in Ri probably do not impair splicing and are often polymorphisms. The Ri values of activated cryptic sites were generally comparable to or greater than those of the corresponding natural splice sites. Information analysis revealed preexisting cryptic splice junctions that are used instead of the mutated natural site. Other cryptic sites were created or strengthened by sequence changes that simultaneously altered the natural site. Comparison between normal and mutant splice site Ri values distinguishes substitutions that impair splicing from those which do not, distinguishes null alleles from those that are partially functional, and detects activated cryptic splice sites.

Identification of novel PAX6 mutations in two families with bilateral aniridia. Mutations in brief no. 167. Online

We report two novel PAX6 mutations in aniridia patients of two Swiss pedigrees (We, Sc) which give rise to different phenotypes. An SSCP analysis of the PAX6 14 exons reveals electrophoretic mobility shifts exclusively in exons 5 and 12 of aniridia patients. As determined by bidirectional sequencing and restriction digest analysis, these shifts are caused by mono-allelic base transitions in exon 5 (c.547C-->T; R44X; We) and intron 12 (IVS12+5G-->A; Sc). Each mutation co-segregates with the trait in the affected family with complete penetrance. The Sc mutation in the splicing donor site of intron 12 may result in either intron inclusion or exon skipping, both giving rise to a truncated PAX6 protein which may retain a residual transactivating activity. In contrast, the We genetic alteration is a loss-of-function mutation leading to a more severe phenotype than that observed in the Sc pedigree.

Normolipidemia and hypercholesterolemia in persons heterozygous for the same 1592 + 5G --> A splice site mutation in the low-density lipoprotein receptor gene

In the present study, we have characterized a unique splice donor G to A substitution in the moderately conserved + 5 position in intron 10 of the low-density lipoprotein (LDL) receptor gene. In two Danish families, carriers of the 1592 + 5G --> A mutation display a clinical phenotype ranging from healthy normocholesterolemic persons to classical heterozygous familial hypercholesterolemia (FH) patients. Reverse transcription-polymerase chain reaction (RT-PCR) of RNA from Epstein Barr virus (EBV)-transformed lymphoblasts obtained from members of both families demonstrated abnormal splicing generating two aberrant mRNAs due to either alternative splicing and skipping of exon 10 or activation of a cryptic splice site in intron 10 inserting 66 intronic base pairs. These abnormally spliced mRNAs were predicted to encode two abnormal receptor proteins containing an in-frame deletion of 75 amino acids and an insertion of 22 novel amino acids, respectively. Results obtained by immunofluorescence staining, flow cytometry, and confocal microscopy of transfected Chang and COS-7 cells expressing normal and mutant LDL receptors were compatible with nearly complete retention of the mutant proteins in the endoplasmic reticulum. Quantitative measurements of LDL receptor mRNAs from EBV-transformed lymphoblasts, however, did not reveal any significant differences in variant mRNA contents between mutation carriers in the families that could be related to degree of hypercholesterolemia.

Identification of novel pro-alpha2(IX) collagen gene mutations in two families with distinctive oligo-epiphyseal forms of multiple epiphyseal dysplasia

Multiple epiphyseal dysplasia (MED) is a genetically heterogeneous disorder with marked clinical and radiographic variability. Traditionally, the mild "Ribbing" and severe "Fairbank" types have been used to define a broad phenotypic spectrum. Mutations in the gene encoding cartilage oligomeric-matrix protein have been shown to result in several types of MED, whereas mutations in the gene encoding the alpha2 chain of type IX collagen (COL9A2) have so far been found only in two families with the Fairbank type of MED. Type IX collagen is a heterotrimer of pro-alpha chains derived from three distinct genes-COL9A1, COL9A2, and COL9A3. In this article, we describe two families with distinctive oligo-epiphyseal forms of MED, which are heterozygous for different mutations in the COL9A2 exon 3/intron 3 splice-donor site. Both of these mutations result in the skipping of exon 3 from COL9A2 mRNA, but the position of the mutation in the splice-donor site determines the stability of the mRNA produced from the mutant COL9A2 allele.

Alternative splicing of LTBP-3

LTBPs bind the 100-kDa latent TGF-beta complex and thereby regulate TGF-beta assembly, tissue localization, and function. However, the 100-kDa complex is not always associated with LTBP, and, conversely, evidence suggests that LTBP has a distinct role in the extracellular matrix. As yet, there are no data to explain how the binding interaction between LTBP and the 100-kDa complex is regulated. This report provides the first direct evidence of alternative splicing of an LTBP gene. Two alternative splice sites in the mouse LTBP-3 gene have been identified based on in vivo and in vitro studies. Alternative splicing at one site in particular was found to disrupt a structural motif involved in the binding interaction with the 100-kDa latent TGF-beta complex. Therefore, alternative splicing may represent a molecular mechanism by which the uncomplexed form of LTBP-3 is produced, and, as a corollary, by which the 100-kDa latent TGF-beta 1 complex is produced.

Human genes for dental anomalies

The development of the tooth at gene level is beginning to be understood. This paper reviews current knowledge and the advances in research on human genes whose defect leads to dental anomalies. Amelogenesis imperfecta (AI) is a diverse group of hereditary disorders characterized by a variety of developmental enamel defects including hypoplasia and hypomineralization, some of which have been revealed to be associated with defective amelogenin genes. The human amelogenin genes on X and Y chromosomes have been cloned and investigated extensively. Although autosomally inherited forms of AI are more common than the X-linked forms, most studies on the genes causing AI have been performed on the genes of X-linked forms. Recently, the gene for the human tuftelin protein (an enamelin) has been cloned as a candidate gene for the autosomal forms of AI with another gene on chromosome 4 involved in some families. Dentinogenesis imperfecta (DI) may be associated with osteogenesis imperfecta (OI), which is an autosomal dominant bone disease. Most patients with OI have mutations in either the COLIA1 or COLIA2 genes, which encode the alpha 1(I) or alpha 2(I) subunits of type I collagen, the major organic component of bone and dentin. Gene defects causing isolated DI have not been identified. Recently, it was demonstrated that a missense mutation of MSXI, a human homeobox gene, causes autosomal dominant agenesis of second premolars and third molars. Data indicating an important function for MSXI, the mouse counterpart of the human MSXI gene, in mouse tooth development have been accumulating since 1991. Knockout mice lacking this gene exhibited multiple craniofacial anomalies including complete tooth agenesis. X-linked anhidrotic ectodermal dysplasia (EDA), characterized by abnormal hair, teeth, and sweat glands, was demonstrated to be caused by a mutation in a novel transmembrane protein gene that is expressed in epithelial cells and in other adult and fetal tissues. The predicted EDA protein may belong to a novel class of proteins with a role in epithelial-mesenchymal signaling. Several mutations have been reported in genes causing hypophosphatasia, which is characterized by defective mineralization of the skeletal and dental structures.

Mutation producing alternative splicing of exon 26 in the COL1A2 gene causes type IV osteogenesis imperfecta with intrafamilial clinical variability

We have characterized a familial form of osteogenesis imperfecta (OI). Following the identification by ultrasound of short limbs and multiple fractures in a fetus at 25 weeks of gestation, the family was referred with a provisional diagnosis of severe OI. We detected subtle clinical and radiological signs of OI in the father and in the paternal grandmother of the proposita, who had never received a diagnosis of OI. Linkage analysis indicated COL1A2 as the disease locus. Heteroduplex analysis of reverse transcription-polymerase chain reaction (RT-PCR) amplification products of pro alpha2(I) mRNA from an affected member and subsequent sequencing of the candidate region demonstrated the presence of normal transcripts and a minority of transcripts lacking exon 26 (54 bp) of COL1A2. Sequencing of PCR-amplified genomic DNA identified an A --> G transition in the moderately conserved +3 position of the IVS 26 donor splice site. The mutant pre-mRNA molecules were alternatively spliced, yielding both full-length and deleted transcripts that represented less than 30% of the total pro alpha2(I) mRNA. The biochemical data on type I collagen synthesized by dermal fibroblasts showed intracellular retention of the mutant protein; failure to detect the shortened alpha2(I) chains either in the medium or in the cell layer may be the consequence of their instability at physiological temperature. These observations justified the mild resulting phenotype.

Splice site mutation causing deletion of exon 21 sequences from the pro alpha 2(I) chain of type I collagen in a patient with severe dentinogenesis imperfecta but very mild osteogenesis imperfecta

An eight-year-old boy was referred for dental assessment of dentinogenesis imperfecta, a full clinical examination also revealed joint hypermobility and some features of mild osteogenesis imperfecta although he had suffered few fractures. Analysis of the collagens produced by both gingival and skin fibroblast cultures showed the synthesis and intracellular retention of an abnormal alpha 2(I) chain that migrated faster than normal on SDS-PAGE. Cyanogen bromide peptide mapping of this intracellular protein indicated a probable deletion in the N-terminal peptide alpha 2CB4. The denaturation temperature of the mutant protein was only 36 degrees C, some 6 degrees C below normal. At 37 degrees C secretion of abnormal protein was not detectable but a lower temperature (30 degrees C) some was secreted into the medium. RT-PCR amplification of mRNA coding for alpha 2CB4 revealed a heterozygous deletion of the 108 bp exon 21 of COL1A2. Sequencing of PCR amplified genomic DNA identified a G --> A transition in the moderately conserved + 5 position of the IVS 21 5' consensus splice site causing the skipping of exon 21. Hybridization with allele-specific oligonucleotides showed no other family member had this base change. Since the cDNA deletion was associated with the (-) allele of a Pvu II polymorphism in exon 25 of COL1A2 we could demonstrate that the mutant pre-mRNA was alternatively spliced yielding both full length and deleted transcripts. Family genotype analysis indicated the mutation had originated in the paternal alpha 2(I) gene.

Skipping of exon 12 as a consequence of a point mutation (1898 + 5G-->T) in the cystic fibrosis transmembrane conductance regulator gene found in a consanguineous Chinese family

A point mutation (1898 + 5G-->T) located five base pairs downstream from the donor splice site in intron 12 of the CFTR gene has been identified in a consanguineous CF patient of Chinese origin. To determine if this nucleotide substitution could affect mRNA splicing, PCR analysis was performed with RNA isolated from the lymphoblastoid cell line of the mother of the deceased patient. While exon 12-minus transcript was detected in this sample, it was also found in individuals without 1898 + 5G-->T, albeit in a smaller proportion. Using a sequence polymorphism associated with each of the two alleles in the mother, however, we showed that mutant transcript was almost exclusively produced by the 1898 + 5G-->T allele. Skipping of exon 12 would result in the deletion of 29 amino acids from the first nucleotide binding domain of CFTR, rendering the protein non-functional. The possibility of a low level (< or = 2.5%) of normal transcript from the mutant allele cannot be excluded and it may explain the pancreatic sufficient phenotype of the patient. The 1898 + 5G-->T mutation was found in two other CF patients of Chinese origin, but it was not detected in 192 CF chromosomes of Caucasian origin and 30 other chromosomes from Chinese individuals without a family history of CF.

Aberrant splicing of the type III procollagen mRNA leads to intracellular degradation of the protein in a patient with Ehlers-Danlos type IV

Ehlers-Danlos syndrome type IV (EDS IV) is an autosomal dominant disorder characterized by fragile skin, blood vessels, and internal organs and associated with decreased production, secretion, or thermal stability of type III procollagen. Mutations in the gene for type III procollagen have been identified in patients exhibiting decreased secretion or thermal stability of the protein, but no defect has been elucidated to explain the decreased production of type III procollagen in some patients with EDS IV. We report on a patient with a moderate case of EDS IV who produced decreased amounts of type III procollagen despite normal levels of translatable type III procollagen mRNA. S1 nuclease analysis of the type III procollagen mRNA indicated a defect in the region encoding exon 27. Sequence analysis of cDNA clones and genomic fragments generated by polymerase chain reaction amplification revealed that sequences encoded by exon 27 were absent from 3 out of 5 cDNA clones and that a G at the +5 position of the splice donor site in intron 27 was changed to an A in one allele of the patient's type III procollagen gene. Using a cDNA-genomic DNA hybrid probe in S1 nuclease analysis, fragments consistent with mRNA species containing and lacking exon 27 were detected in a 1:1 ratio. Pulse label and chase experiments in the presence or absence of brefeldin A indicated that most of the type III procollagen molecules synthesized by the patient's fibroblasts were not secreted into the medium but were degraded in the endoplasmic reticulum-Golgi compartment by a nonlysosomal mechanism.

An exon-skipping mutation in the btk gene of a patient with X-linked agammaglobulinemia and isolated growth hormone deficiency

X-linked agammaglobulinemia (XLA) is an inherited immunodeficiency disease associated with a block in differentiation from pre-B to B cells. The XLA gene encodes a 659 amino acids cytoplasmic protein tyrosine kinase named btk (Bruton's tyrosine kinase). The few btk gene alterations so far reported in XLA patients are heterogenous and distributed in all domains of the btk protein. They appear to be responsible for a range of B cell immunodeficiency disorders of variable severity. Rare families in which XLA is inherited together with isolated growth hormone deficiency (IGHD) have been reported. Genetic analysis has shown that this disease association maps to the same region of the X chromosome as XLA, but whether the two phenotypes are caused by a common or different developmental or biochemical mechanism is unknown. We have analyzed the btk gene of a patient with XLA and IGHD. RT-PCR analysis of btk transcripts, sequencing data obtained from cDNA and genomic DNA and in vitro splicing assays showed that an intronic point mutation (1882 + 5G-->A) is responsible for skipping of an exon located in the tyrosine kinase domain. This exon-skipping event results in a frameshift leading to a premature stop codon 14 amino acids downstream, and in the loss of the last 61 residues of the carboxy-terminal end of the protein. Although we studied a sporadic case, the results suggest that an alteration of the btk gene might cause this unusual phenotype.

Chromosomal localization of the protein tyrosine phosphatase G1 gene and characterization of the aberrant transcripts in human colon cancer cells

We have recently described the isolation of the human PTPG1 gene which encodes a member of intracellular protein tyrosine phosphatases that may be candidates for tumor suppressor genes. In order to investigate the abnormality of the PTPG1 transcript in various human cancer cell lines, we have analyzed the consensus catalytic region of PTPG1 cDNA, using the reverse transcription polymerase chain reaction. In a colorectal carcinoma cell line, DLD-1, we found three aberrant transcripts. Sequencing analysis revealed that one had a missense point mutation and the remainders contained 77 bp and 173 bp deletions, respectively. These alterations might directly affect their phosphatase activities. Our findings provide the first evidence for the aberrant transcripts of the protein tyrosine phosphatase in human cancer cells, and suggest that the aberration of PTPG1 gene might be involved in the tumorigenesis. Moreover, the human PTPG1 gene is localized on chromosome 7q11.23, a region with frequent abnormalities implicated in some human cancers.

Collagen genes: mutations affecting collagen structure and expression

It is to be expected that more collagen genes will be identified and that additional heritable connective tissue diseases will be shown to arise from collagen mutations. Further progress will be fostered by the coordinated study of naturally occurring and induced heritable connective tissues diseases. In some instances, human mutations will be studied in more detail using transgenic mice, while in others, transgenic studies will be used to determine the type of human phenotype that is likely to result from mutations of a given collagen gene. Further studies of transcriptional regulation of the collagen genes will provide the prospect for therapeutic control of expression of specific collagen genes in patients with genetically determined collagen disorders as well as in a wide range of common human diseases in which abnormal formation of the connective tissues is a feature.

Further evidence that the failure to cleave the aminopropeptide of type I procollagen is the cause of Ehlers-Danlos syndrome type VII

Dermal fibroblasts from a Chinese Ehlers-Danlos syndrome type VII patient synthesized approximately equal amounts of normal pro-alpha 2(I) chains of type I procollagen and abnormal ones with electrophoretic mobility of pN alpha 2(I) chains, in which the amino-propeptide (N-propeptide) was retained. Reverse-transcriptase PCR analysis of the proband's RNA showed outsplicing of the 54 base exon 6 in half of the pro-alpha 2(I) mRNAs. Exon 6 encodes 18 amino acids of the N-telopeptide which contains the procollagen N-proteinase cleavage site and a cross-link precursor lysine. Loss of these sequences would result in failure to cleave the amino-propeptide of pro-alpha 2(I) and the accumulation of pN-alpha 2(I) chains. Nucleotide sequencing analyses of the proband's COL1A2 gene showed the presence of a T to C transition at position +2 of intron 6 in one allele and the proband is heterozygous for the defect. This mutation which destroyed the consensus GT dinucleotide at the 5' splice donor site of the intron is responsible for the loss of exon 6 by exon skipping. Electron microscopic analysis of the patient's dermis showed the presence of abnormal collagen I fibrils of irregular diameter and circularity. This mutation in COL1A2 in an EDS VII patient is the first reported case in the Chinese population and is identical to one reported for another EDS-VII (Libyan) patient. The occurrence of an identical mutation in two probands of different ethnic origin is direct evidence that the mutant genotype is the cause of the EDS VII phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

A homozygous insertion-deletion in the type VII collagen gene (COL7A1) in Hallopeau-Siemens dystrophic epidermolysis bullosa

The Hallopeau-Siemens type of recessive dystrophic epidermolysis bullosa (HS-RDEB) is a life-threatening autosomal disease characterized by loss of dermal-epidermal adherence with abnormal anchoring fibrils (AF). We recently linked HS-RDEB to the type VII collagen gene (COL7A1) which encodes the major component of AF. We describe a patient who is homozygous for an insertion-deletion in the FN-4A domain of the COL7A1 gene. This defect causes a frameshift mutation which leads to a premature stop codon in the FN-5A domain, resulting in a marked diminution in mutated mRNA levels, with no detectable type VII collagen polypeptide in the patient. Our data suggest strongly that this null allele prevents normal anchoring fibril formation in homozygotes and is the underlying cause of HS-RDEB in this patient.

A T+6 to C+6 mutation in the donor splice site of COL3A1 IVS7 causes exon skipping and results in Ehlers-Danlos syndrome type IV

Ehlers-Danlos syndrome type IV is usually caused by mutations in COL3A1, the gene coding for type III collagen. In a woman with a milder form of this disease, analysis of type III collagen synthesised by her cultured skin fibroblasts showed an apparently shorter form of the protein. Amplification of overlapping cDNAs, encoding the triple helical region of the molecule, showed a deletion near the 5' end of the gene. Sequencing showed that exon 7 was missing from the cDNA sequence. Analysis of genomic DNA showed that this was the result of a T+6 to C+6 mutation in the donor splice site of intron 7. The proband's parents and 35 normal controls were homozygous for T+6 at this position, indicating that the C+6 mutation was causative.

Temperature sensitivity of aberrant RNA splicing with a mutation in the G+5 position of intron 37 of the gene for type III procollagen from a patient with Ehlers-Danlos syndrome type IV

A single-base mutation in intron 37 of the gene for type III procollagen (COL3A1) was found in a proband with the type IV variant of Ehlers-Danlos syndrome. Probe-protection experiments with S1 nuclease and RNA from fibroblasts incubated at 37 degrees C demonstrated that about 35% of the total mRNA or about 70% of the mRNA from mutated allele was spliced by exon skipping. The effects of the mutation were temperature-sensitive in that the amount of RNA from the mutated allele that was spliced by exon skipping was 87.1 +/- 7.7% at 31 degrees C, 70.1 +/- 6.5% at 37 degrees C, and 85.4 +/- 11.1% at 42 degrees C. The effects of temperature on aberrant RNA splicing were, therefore, the reverse of those reported for four previous mutants in collagen genes. The increase in abnormal RNA splicing when the temperature was raised from 31 degrees to 37 degrees C seen with previously reported mutants suggested that RNA-RNA hybridization of U1snRNA to the 5'-splice site in the substrate may be limiting in the processing of transcripts from the mutated alleles, since RNA-RNA hybridizations become less favorable at higher temperatures. The decrease in abnormal RNA splicing seen here when the temperature was raised from 31 degrees to 37 degrees C suggested that protein-RNA or protein-protein binding steps become rate limiting with the G+5 mutation in intron 37 of the COL3A1 gene.

Paternal mosaicism for a COL1A1 dominant mutation (alpha 1 Ser-415) causes recurrent osteogenesis imperfecta

We describe a dominant point mutation in the COL1A1 gene causing extremely severe osteogenesis imperfecta (OI type II/III) which was detected in the dermal fibroblasts of a proband, diagnosed by ultrasonography at 24 weeks of gestation. Type I collagen secretion was reduced and pro alpha 1(I) chains were overmodified. The mutation was localised in one COL1A1 allele by chemical cleavage of mismatched bases in normal cDNA/proband's mRNA heteroduplexes, and identified by cloning and sequencing. A G-to-A transition which causes the substitution of Gly-415 with serine in the alpha 1(I) triple helical domain was found. The same mutation was detected in the father's spermatozoa and lymphocytes. Mosaicism in the father's germline explains the occurrence in the family of two additional OI pregnancies, which were documented by X-ray and ultrasound investigations.

An error in dystrophin mRNA processing in golden retriever muscular dystrophy, an animal homologue of Duchenne muscular dystrophy

Golden retriever muscular dystrophy (GRMD) is a spontaneous, X-linked, progressively fatal disease of dogs and is also a homologue of Duchenne muscular dystrophy (DMD). Two-thirds of DMD patients carry detectable deletions in their dystrophin gene. The defect underlying the remaining one-third of DMD patients is undetermined. Analysis of the canine dystrophin gene in normal and GRMD dogs has failed to demonstrate any detectable loss of exons. Here, we have demonstrated a RNA processing error in GRMD that results from a single base change in the 3' consensus splice site of intron 6. The seventh exon is then skipped, which predicts a termination of the dystrophin reading frame within its N-terminal domain in exon 8. This is the first example of dystrophin deficiency caused by a splice-site mutation.

Invariant exon skipping in the human alpha-galactosidase A pre-mRNA: Ag+1 to t substitution in a 5'-splice site causing Fabry disease

Fabry disease, an inborn error of glycosphingolipid catabolism, results from lesions in the X-linked gene encoding the human lysosomal hydrolase, alpha-galactosidase A (alpha-D-galactoside galactohydrolase; EC 3.2.1.22). To detect alpha-galactosidase A RNA processing or stability defects causing Fabry disease, Northern hybridization analyses were performed with poly(A)+ RNA isolated from cultured lymphoblasts from unrelated Fabry hemizygotes. Using a riboprobe complimentary to the normal 1.45-kb alpha-galactosidase A mRNA, a single 1.25-kb transcript was identified in three classically affected brothers from a Japanese Fabry family. Densitometric analysis revealed that the 1.25-kb transcripts were present at 50 to 60% of normal amounts. RNase A analysis identified a deletion of about 200 bp that appeared to include the entire 198 bp of exon 6. Amplification and direct sequencing of a genomic region containing exon 6 from an affected hemizygote revealed a g+1 to t transversion in the invariant gt consensus 5'-splice site of intron 6, which resulted in the deletion of the entire exon 6 sequence. This novel splicing lesion causing Fabry disease is the first g+1 to t transversion of a mammalian 5'-splice site that consistently eliminates the preceding exon.

Mutations of chromosome 5q21 genes in FAP and colorectal cancer patients

Previous studies suggested that one or more genes on chromosome 5q21 are responsible for the inheritance of familial adenomatous polyposis (FAP) and Gardner's syndrome (GS), and contribute to tumor development in patients with noninherited forms of colorectal cancer. Two genes on 5q21 that are tightly linked to FAP (MCC and APC) were found to be somatically altered in tumors from sporadic colorectal cancer patients. One of the genes (APC) was also found to be altered by point mutation in the germ line of FAP and GS patients. These data suggest that more than one gene on chromosome 5q21 may contribute to colorectal neoplasia, and that mutations of the APC gene can cause both FAP and GS. The identification of these genes should aid in understanding the pathogenesis of colorectal neoplasia and in the diagnosis and counseling of patients with inherited predispositions to colorectal cancer.

Osteogenesis imperfecta: translation of mutation to phenotype

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Control of alternative splicing by the differential binding of U1 small nuclear ribonucleoprotein particle

Cellular factors controlling alternative splicing of precursor messenger RNA are largely unknown, even though this process plays a central role in specifying the diversity of proteins in the eukaryotic cell. For the identification of such factors, a segment of the rat preprotachykinin gene was used in which differential expression of neuropeptides gamma and K is dependent on alternative splicing of the fourth exon (E4). Sequence variants of the three-exon segment, (E3-E4-E5) were created, resulting in a sensitive assay for factors mediating the splicing switch between E4-skipping and E4-inclusion. A dinucleotide mutation in the 5' splice site of E4 that increase base-pairing of this site to U1 small nuclear RNA resulted in uniform selection of E4, whereas a control mutation that destroyed base-pairing resulted in uniform E4-skipping. Affinity selection of spliceosomes formed on these functionally distinct substrates revealed that the extreme difference in splicing was mediated by differential binding of the U1 small nuclear ribonucleoprotein particle (snRNP) to the 5' splice site of E4. These data show that, apart from its established role in selecting 5' splice sites, U1 snRNP plays a fundamental role in 3' exon selection and provides insight into possible mechanisms of alternative splicing.

Brittle bones--fragile molecules: disorders of collagen gene structure and expression

Mutations in the genes that encode the chains of type I collagen, the major structural protein in most tissues, usually produce brittle bones. The consequences of even apparently minor mutations--single base substitutions--can range from lethal to mild, and the phenotypic consequences reflect the nature and position of the mutation. The manner in which phenotypes are produced depends on the effect of the mutation on the structural integrity of the molecule and on whether or how the abnormal molecules can be incorporated into an extracellular matrix.