Pathogenic FBN1 mutations in 146 adults not meeting clinical diagnostic criteria for Marfan syndrome: further delineation of type 1 fibrillinopathies and focus on patients with an isolated major criterion.

Mutations in the FBN1 gene cause Marfan syndrome (MFS) and have been associated with a wide range of milder overlapping phenotypes. A proportion of patients carrying a FBN1 mutation does not meet diagnostic criteria for MFS, and are diagnosed with "other type I fibrillinopathy." In order to better describe this entity, we analyzed a subgroup of 146 out of 689 adult propositi with incomplete "clinical" international criteria (Ghent nosology) from a large collaborative international study including 1,009 propositi with a pathogenic FBN1 mutation. We focused on patients with only one major clinical criterion, [including isolated ectopia lentis (EL; 12 patients), isolated ascending aortic dilatation (17 patients), and isolated major skeletal manifestations (1 patient)] or with no major criterion but only minor criteria in 1 or more organ systems (16 patients). At least one component of the Ghent nosology, insufficient alone to make a minor criterion, was found in the majority of patients with isolated ascending aortic dilatation and isolated EL. In patients with isolated EL, missense mutations involving a cysteine were predominant, mutations in exons 24-32 were underrepresented, and no mutations leading to a premature truncation were found. Studies of recurrent mutations and affected family members of propositi with only one major clinical criterion argue for a clinical continuum between such phenotypes and classical MFS. Using strict definitions, we conclude that patients with FBN1 mutation and only one major clinical criterion or with only minor clinical criteria of one or more organ system do exist but represent only 5% of the adult cohort.

Severe congenital encephalopathy caused by MECP2 null mutations in males: central hypoxia and reduced neuronal dendritic structure.

Non-mosaic males with a 46,XY karyotype and a MECP2 null mutation display a phenotype of severe neonatal-onset encephalopathy that is distinctly different from Rett syndrome (RTT). To increase awareness of this rare disorder, we are reporting novel findings in a sporadic case, compare them to 16 previously reported cases and establish salient criteria for clinical diagnosis. The proband suffered from general hypotonia and hypoxia caused by hypoventilation and irregular breathing. He developed abnormal movements, seizures and electroencephalogram abnormalities. He failed to thrive and to reach any motor milestones and died at 15 months from central respiratory failure without a diagnosis. In a muscle biopsy, type II fibers were reduced in diameter, indicating central hypoxia. At autopsy, the brain was small with disproportionate reduction of the frontal and temporal lobes. Synaptophysin staining of synaptic vesicles was greatly reduced in cerebellar and spinal cord sections. Analysis of Golgi-stained pyramidal neurons from cortical layers III and V of the frontal and temporal lobes revealed drastically diminished dendritic trees. Post-mortem MECP2 mutation analysis on DNA and RNA from fibroblasts revealed a novel de novo 9-nucleotide deletion including the intron 3/exon 4 splice junction. The two nucleotides flanking the deletion form a new splice site, and the aberrantly spliced transcript lacks seven nucleotides (r.378_384delTCCCCAG), causing a frameshift and premature termination codon (p.I126fsX11). Males with congenital encephalopathy, not females with RTT, represent the true human counterpart for the commonly studied Mecp2-/y mouse model and provide unique insight into the mechanisms of MeCP2 deficiency.

Contribution of molecular analyses in diagnosing Marfan syndrome and type I fibrillinopathies: an international study of 1009 probands.

BACKGROUND: The diagnosis of Marfan syndrome (MFS) is usually initially based on clinical criteria according to the number of major and minor systems affected following international nosology. The number of FBN1 mutation carriers, at risk of aortic complications who would not be properly diagnosed based only on clinical grounds, is of growing importance owing to the increased availability of molecular screening. The aim of the study was to identify patients who should be considered for FBN1 mutation screening. METHODS: Our international series included 1009 probands with a known FBN1 mutation. Patients were classified as either fulfilling or not fulfilling "clinical" criteria. In patients with unfulfilled "clinical" criteria, we evaluated the percentage of additional patients who became positive for international criteria when the FBN1 mutation was considered. The aortic risk was evaluated and compared in patients fulfilling or not fulfilling the "clinical" international criteria. RESULTS: Diagnosis of MFS was possible on clinical grounds in 79% of the adults, whereas 90% fulfilled the international criteria when including the FBN1 mutation. Corresponding figures for children were 56% and 85%, respectively. Aortic dilatation occurred later in adults with unfulfilled "clinical criteria" when compared to the Marfan syndrome group (44% vs 73% at 40 years, p<0.001), but the lifelong risk for ascending aortic dissection or surgery was not significantly different in both groups. CONCLUSIONS: Because of its implications for aortic follow-up, FBN1 molecular analysis is recommended in newly suspected MFS when two systems are involved with at least one major system affected. This is of utmost importance in patients without aortic dilatation and in children.

Effect of mutation type and location on clinical outcome in 1,013 probands with Marfan syndrome or related phenotypes and FBN1 mutations: an international study.

Mutations in the fibrillin-1 (FBN1) gene cause Marfan syndrome (MFS) and have been associated with a wide range of overlapping phenotypes. Clinical care is complicated by variable age at onset and the wide range of severity of aortic features. The factors that modulate phenotypical severity, both among and within families, remain to be determined. The availability of international FBN1 mutation Universal Mutation Database (UMD-FBN1) has allowed us to perform the largest collaborative study ever reported, to investigate the correlation between the FBN1 genotype and the nature and severity of the clinical phenotype. A range of qualitative and quantitative clinical parameters (skeletal, cardiovascular, ophthalmologic, skin, pulmonary, and dural) was compared for different classes of mutation (types and locations) in 1,013 probands with a pathogenic FBN1 mutation. A higher probability of ectopia lentis was found for patients with a missense mutation substituting or producing a cysteine, when compared with other missense mutations. Patients with an FBN1 premature termination codon had a more severe skeletal and skin phenotype than did patients with an inframe mutation. Mutations in exons 24-32 were associated with a more severe and complete phenotype, including younger age at diagnosis of type I fibrillinopathy and higher probability of developing ectopia lentis, ascending aortic dilatation, aortic surgery, mitral valve abnormalities, scoliosis, and shorter survival; the majority of these results were replicated even when cases of neonatal MFS were excluded. These correlations, found between different mutation types and clinical manifestations, might be explained by different underlying genetic mechanisms (dominant negative versus haploinsufficiency) and by consideration of the two main physiological functions of fibrillin-1 (structural versus mediator of TGF beta signalling). Exon 24-32 mutations define a high-risk group for cardiac manifestations associated with severe prognosis at all ages.

Multi-exon deletions of the FBN1 gene in Marfan syndrome.

BACKGROUND: Mutations in the fibrillin -1 gene (FBN1) cause Marfan syndrome (MFS), an autosomal dominant multi-system connective tissue disorder. The 200 different mutations reported in the 235 kb, 65 exon-containing gene include only one family with a genomic multi-exon deletion. METHODS: We used long-range RT-PCR for mutation detection and long-range genomic PCR and DNA sequencing for identification of deletion breakpoints, allele-specific transcript analyses to determine stability of the mutant RNA, and pulse-chase studies to quantitate fibrillin synthesis and extracellular matrix deposition in cultured fibroblasts. Southern blots of genomic DNA were probed with three overlapping fragments covering the FBN1 coding exons RESULTS: Two novel multi-exon FBN1 deletions were discovered. Identical nucleotide pentamers were found at or near the intronic breakpoints. In a Case with classic MFS, an in-frame deletion of exons 42 and 43 removed the C-terminal 24 amino acids of the 5th LTBP (8-cysteine) domain and the adjacent 25th calcium-binding EGF-like (6-cysteine) domain. The mutant mRNA was stable, but fibrillin synthesis and matrix deposition were significantly reduced. A Case with severe childhood-onset MFS has a de novo deletion of exons 44-46 that removed three EGF-like domains. Fibrillin protein synthesis was normal, but matrix deposition was strikingly reduced. No genomic rearrangements were detected by Southern analysis of 18 unrelated MFS samples negative for FBN1 mutation screening. CONCLUSIONS: Two novel deletion cases expand knowledge of mutational mechanisms and genotype/phenotype correlations of fibrillinopathies. Deletions or mutations affecting an LTBP domain may result in unstable mutant protein cleavage products that interfere with microfibril assembly.

Evolutionary relationships among Rel domains indicate functional diversification by recombination.

The recent sequencing of several complete genomes has made it possible to track the evolution of large gene families by their genomic structure. Following the large-scale association of exons encoding domains with well defined functions in invertebrates could be useful in predicting the function of complex multidomain proteins in mammals produced by accretion of domains. With this objective, we have determined the genomic structure of the 14 genes in invertebrates and vertebrates that contain rel domains. The sequence encoding the rel domain is defined by intronic boundaries and has been recombined with at least three structurally and functionally distinct genomic sequences to generate coding sequences for: (i) the rel/Dorsal/NFkappaB proteins that are retained in the cytoplasm by IkB-like proteins; (ii) the NFATc proteins that sense calcium signals and undergo cytoplasmic-to-nuclear translocation in response to dephosphorylation by calcineurin; and (iii) the TonEBP tonicity-responsive proteins. Remarkably, a single exon in each NFATc family member encodes the entire Ca(2+)/calcineurin sensing region, including nuclear import/export, calcineurin-binding, and substrate regions. The Rel/Dorsal proteins and the TonEBP proteins are present in Drosophila but not Caenorhabditis elegans. On the other hand, the calcium-responsive NFATc proteins are present only in vertebrates, suggesting that the NFATc family is dedicated to functions specific to vertebrates such as a recombinational immune response, cardiovascular development, and vertebrate-specific aspects of the development and function of the nervous system.

Association of acetylated histones with paternally expressed genes in the Prader--Willi deletion region.

Imprinted genes within the Prader-Willi/Angelman syndrome region of human chromosome 15q11-q13 are regulated by a mechanism involving allele-specific DNA methylation. Since transcriptional regulation by DNA methylation involves histone deacetylation, we explored whether differences in histone acetylation exist between the two parental alleles of SNRPN and other paternally expressed genes in the region by using a chromatin immunoprecipitation assay with antibodies against acetylated histones H3 and H4. SNRPN exon 1, which is methylated on the silent maternal allele, was associated with acetylated histones on the expressed paternal allele only. SNRPN intron 7, which is methylated on the paternal allele, was not associated with acetylated histones on either allele. The paternally expressed genes NDN, IPW, PWCR1 and MAGEL2 were not associated with acetylated histones on either allele. Treatment of the lymphoblastoid cells with trichostatin A, a histone deacetylase inhibitor, did not result in any changes to SNRPN expression or association of acetylated histones with exon 1. Treatment with 5-aza-deoxycytidine (5-aza-dC), which inhibits DNA methylation, resulted in activation of SNRPN expression from the maternal allele, but was not accompanied by acetylation of histones. Our finding of allele-specific association of acetylated histones with the SNRPN exon 1 region, which encompasses the imprinting center, suggests that histone acetylation at this site may be important for regulation of SNRPN and of other paternally expressed genes in the region. On the silent allele, 5-aza-dC treatment altered SNRPN expression, but not association with acetylated histones, suggesting that histone acetylation is a secondary event in the process of gene reactivation by CpG demethylation.

Expression of human Wiskott-Aldrich syndrome protein in patients' cells leads to partial correction of a phenotypic abnormality of cell surface glycoproteins.

The Wiskott-Aldrich syndrome (WAS) is an uncommon X-linked recessive disease characterized by thrombocytopenia, eczema and immunodeficiency. The biochemical defect of this disorder primarily affects cells derived from bone marrow. To understand better the molecular mechanisms underlying this disease and to evaluate the possibility of correcting the genetic defects in hematopoietic cells, a Moloney murine leukemia virus (MoMLV)- based retroviral vector carrying a functional Wiskott-Aldrich syndrome protein (WASp) cDNA driven by an SV40 promoter (LNS-WASp) was constructed. A packaging cell line containing this vector produced a stable level of WAS protein and maintained a high titer of viral output. Epstein-Barr virus (EBV)-transformed B lymphoblastoid cell lines (B-LCL) from WAS patients, which lack expression of the WAS protein, were transduced by the LNS-WASp retroviral vector and showed expression of WASp by Western blot. Analysis of the O-glycan pattern on cell surface glycoproteins from WAS patients' B-LCL showed an altered glycosylation pattern, due to increased activity of beta-1, 6-N-acetylglucosaminyltransferase (C2GnT). Transduction by the retroviral vector carrying the functional WASp cDNA partially restored the abnormal glycosylation pattern, and was accompanied by a decreasing C2GnT activity. These findings imply a functional linkage between the WAS protein and the expression of the glycosyltransferase involved in the O-glycosylation, and also suggest a potential gene therapy via transferring a functional WASp cDNA into hematopoietic cells for Wiskott-Aldrich syndrome. Gene Therapy (2000) 7, 314-320.

Cysteine substitutions in epidermal growth factor-like domains of fibrillin-1: distinct effects on biochemical and clinical phenotypes.

Fibrillin-1 (FBN1) contains 47 epidermal growth factor (EGF)-like domains characterized by six conserved cysteine residues. Cysteine substitutions that disrupt one of the three disulfide bonds are frequent causes of Marfan syndrome (MFS). We identified 19 new substitutions involving cysteine residues in each of the six positions of EGF-like domains. Allele-specific mRNA assays revealed equal abundance of mutant and normal FBN1 transcripts in all 10 individuals studied. Quantitative pulse-chase analysis of fibrillin protein was performed on 25 mutant fibroblast strains with substitutions of 22 different cysteine residues in 18 different EGF-like domains spanning the entire gene. Normal synthesis and stability of mutant fibrillin molecules was seen in 20/25 individuals, 11 of whom showed delayed intracellular processing and/or secretion. In the remaining five cases, the mutant protein was apparently unstable. In four of these five cases, the second or third disulfide bond of EGF-like domains immediately preceding an 8-cysteine or hybrid domain was affected. All but two mutations caused severe reduction of matrix deposition, which was attributed to a dominant-negative effect of mutant molecules. For genotype/phenotype comparisons, clinical data on 25 probands and 19 mutation-positive family members were analyzed. Ocular manifestations were among the most consistent features (ectopia lentis in 86%, myopia in 80%). Nine mutations encoded by exons 26-32 resulted in early-onset classic MFS and, in one case, neonatal-lethal MFS. Mutations outside this region were associated with variable clinical phenotypes, including individuals with fibrillinopathies not meeting diagnostic criteria for MFS.

The human neuregulin-2 (NRG2) gene: cloning, mapping and evaluation as a candidate for the autosomal recessive form of Charcot-Marie-Tooth disease linked to 5q.

Neuregulin-2 (NRG2) is a novel member of the neuregulin family of growth and differentiation factors. Through interaction with the ErbB family of receptors, neuregulin-2 induces the growth and differentiation of epithelial, neuronal, glial and other types of cells. In this study, we have cloned the human neuregulin-2 gene, and determined its genomic structure and alternative splicing patterns. By using radiation hybrid mapping panels, the human NRG2 gene was mapped to the D5S658-D5S402 region within 5q23-q33, close to an autosomal recessive form of demyelinating Charcot-Marie-Tooth (CMT) disease. The NRG2 gene was found to be on two yeast artificial chromosomes overlapping the candidate interval and was, thus, considered a good positional candidate for this form of CMT. When the entire neuregulin-2 coding sequence and splice junctions were explored, however, no mutation was identified in one CMT family linked to 5q23-q33. In addition, three intronic single nucleotide polymorphisms were identified in the NRG2 gene. Genotyping in two families localized the NRG2 gene outside of the revised candidate interval between D5S402-D5S210 and excluded NRG2 as the gene responsible for this form of CMT disease.

Characterization and expression pattern of the frizzled gene Fzd9, the mouse homolog of FZD9 which is deleted in Williams-Beuren syndrome.

The frizzled gene family is conserved from insects to mammals and codes for putative Wnt receptors that share a cysteine-rich extracellular domain and seven transmembrane domains. We previously identified a novel frizzled gene, FZD3, now renamed FZD9, in the Williams-Beuren syndrome (WBS) deletion region at chromosomal band 7q11.23 and showed that its product can interact with the Drosophila wingless protein. Here, we report the characterization of the mouse homolog Fzd9. The Fzd9 gene produces a 2.4-kb transcript encoding a 592-amino-acid protein with 95% identity to the human FZD9. Fzd9 was mapped to the conserved syntenic region on distal mouse chromosome 5. By RNA in situ hybridization studies of whole-mount embryos and sections we delineated the temporal and spatial expression patterns in the neural tube, trunk skeletal muscle precursors (myotomes), limb skeletal anlagen, craniofacial regions, and nephric ducts. In adult mouse tissue, the Fzd9 transcript is abundantly present in heart, brain, testis, and skeletal muscle. In testis, Fzd9 is expressed in all spermatogenic cell types. Immunohistochemical studies of cells transfected with a Fzd9 expression construct confirm that Fzd9 is a membrane protein. These results suggest potential Wnt ligands of Fzd9, a role of Fzd9 in skeletal muscle specification, and contributions of FZD9 to the WBS phenotype.

In vivo nuclease hypersensitivity studies reveal multiple sites of parental origin-dependent differential chromatin conformation in the 150 kb SNRPN transcription unit.

Human chromosome region 15q11-q13 contains a cluster of oppositely imprinted genes. Loss of the paternal or the maternal alleles by deletion of the region or by uniparental disomy 15 results in Prader-Willi syndrome (PWS) or Angelman syndrome (AS), respectively. Hence, the two phenotypically distinct neurodevelopmental disorders are caused by the lack of products of imprinted genes. Subsets of PWS and AS patients exhibit 'imprinting mutations', such as small microdeletions within the 5' region of the small nuclear ribonucleoprotein polypeptide N ( SNRPN ) transcription unit which affect the transcriptional activity and methylation status of distant imprinted genes throughout 15q11-q13 in cis. To elucidate the mechanism of these long-range effects, we have analyzed the chromatin structure of the 150 kb SNRPN transcription unit for DNase I- and Msp I-hypersensitive sites. By using an in vivo approach on lymphoblastoid cell lines from PWS and AS individuals, we discovered that the SNRPN exon 1 is flanked by prominent hypersensitive sites on the paternal allele, but is completely inaccessible to nucleases on the maternal allele. In contrast, we identified several regions of increased nuclease hypersensitivity on the maternal allele, one of which coincides with the AS minimal microdeletion region and another lies in intron 1 immediately downstream of the paternal-specific hypersensitive sites. At several sites, parental origin-specific nuclease hypersensitivity was found to be correlated with hypermethylation on the allele contributed by the other parent. The differential parental origin-dependent chromatin conformations might govern access of regulatory protein complexes and/or RNAs which could mediate interaction of the region with other genes.

Five SWI/SNF-related, matrix-associated, actin-dependent regulator of chromatin (SMARC) genes are dispersed in the human genome.

The SWI/SNF-related, matrix-associated, actin-dependent regulators of chromatin (SMARC), also called BRG1-associated factors, are components of human SWI/SNF-like chromatin-remodeling protein complexes. We mapped five human SMARC genes toregions on four different human chromosomes, SMARCC1 to 3p23-p21, SMARCC2 to 12q13-q14, SMARCD1 to 12q13-q14, SMARCD2 to 17q23-q24, and SMARCD3 to 7q35-q36. SMARCC1, SMARCC2, and SMARCD1 are assigned to chromosomal regions that are frequently involved in somatic rearrangements in human cancers. SMARCD1 was mapped to the critical region of Allgrove syndrome; however, no mutation was identified in one Allgrove syndrome family studied.

p48 Activates a UV-damaged-DNA binding factor and is defective in xeroderma pigmentosum group E cells that lack binding activity.

A subset of xeroderma pigmentosum (XP) group E cells lack a factor that binds to DNA damaged by UV radiation. This factor can be purified to homogeneity as p125, a 125-kDa polypeptide. However, when cDNA encoding p125 is translated in vitro, only a small fraction binds to UV-damaged DNA, suggesting that a second factor is required for the activation of p125. We discovered that most hamster cell lines expressed inactive p125, which was activated in somatic cell hybrids containing human chromosome region 11p11.2-11cen. This region excluded p125 but included p48, which encodes a 48-kDa polypeptide known to copurify with p125 under some conditions. Expression of human p48 activated p125 binding in hamster cells and increased p125 binding in human cells. No such effects were observed from expression of p48 containing single amino acid substitutions from XP group E cells that lacked binding activity, demonstrating that the p48 gene is defective in those cells. Activation of p125 occurred by a "hit-and-run" mechanism, since the presence of p48 was not required for subsequent binding. Nevertheless, p48 was capable of forming a complex with p125 either bound to UV-damaged DNA or in free solution. It is notable that hamster cells fail to efficiently repair cyclobutane pyrimidine dimers in nontranscribed DNA and fail to express p48, which contains a WD motif with homology to proteins that reorganize chromatin. We propose that p48 plays a role in repairing lesions that would otherwise remain inaccessible in nontranscribed chromatin.

Imprinted genes in the Prader-Willi deletion.

Parent-of-origin-specific deletions of proximal chromosome 15q cause either the Prader-Willi syndrome (paternal deletion) or the Angelman syndrome (maternal deletion), two distinct neurodevelopmental disorders. In contrast to the Angelman syndrome, which can also be caused by mutations in a single gene (UBE3A, encoding a ubiquitin ligase), the Prader-Willi syndrome is caused by deletions in about two-thirds of cases and by maternal uniparental disomy in the remaining third. The consequence of both mechanisms, in addition to rare microdeletions or so-called 'imprinting mutations', is lack of the products of multiple genes in the region that are normally expressed only from the paternal chromosome. One gene that is consistently silent in the Prader-Willi syndrome is SNRPN, which encodes the small nuclear ribonucleoprotein particle-associated polypeptide N that forms part of the spliceosomes in the brain. A systematic search for other imprinted genes in the Prader-Willi syndrome region revealed a paternally expressed transcript (IPW, for imprinted in the Prader-Willi region) and a similarly imprinted mouse homologue (Ipw) in the conserved syntenic region on mouse chromosome 7. Ipw is highly expressed in the brain and alternatively spliced to generate different transcripts. Since there is no open reading frame that is conserved in the human and mouse IPW genes, they are postulated to function as untranslated RNAs, possibly regulating transcription in cis in the region.

Toso, a cell surface, specific regulator of Fas-induced apoptosis in T cells.

Fas is a surface receptor that can transmit signals for apoptosis. Using retroviral cDNA library-based functional cloning we identified a gene, toso, that blocks Fas-mediated apoptosis. Toso expression was confined to lymphoid cells and was enhanced after cell-specific activation processes in T cells. Toso appeared limited to inhibition of apoptosis mediated by members of the TNF receptor family and was capable of inhibiting T cell self-killing induced by TCR activation processes that up-regulate Fas ligand. We mapped the effect of Toso to inhibition of caspase-8 processing, the most upstream caspase activity in Fas-mediated signaling, potentially through activation of cFLIP. Toso therefore serves as a novel regulator of Fas-mediated apoptosis and may act as a regulator of cell fate in T cells and other hematopoietic lineages.

A mouse model for Prader-Willi syndrome imprinting-centre mutations.

Imprinting in the 15q11-q13 region involves an 'imprinting centre' (IC), mapping in part to the promoter and first exon of SNRPN. Deletion of this IC abolishes local paternally derived gene expression and results in Prader-Willi syndrome (PWS). We have created two deletion mutations in mice to understand PWS and the mechanism of this IC. Mice harbouring an intragenic deletion in Snrpn are phenotypically normal, suggesting that mutations of SNRPN are not sufficient to induce PWS. Mice with a larger deletion involving both Snrpn and the putative PWS-IC lack expression of the imprinted genes Zfp127 (mouse homologue of ZNF127), Ndn and Ipw, and manifest several phenotypes common to PWS infants. These data demonstrate that both the position of the IC and its role in the coordinate expression of genes is conserved between mouse and human, and indicate that the mouse is a suitable model system in which to investigate the molecular mechanisms of imprinting in this region of the genome.

Ascending aortic aneurysm with or without features of Marfan syndrome and other fibrillinopathies: new insights.

More than 70 unique fibrillin-1 mutations have been identified in individuals with a variety of phenotypic changes. These range from severe neonatal lethal forms of Marfan syndrome to adult onset manifestations, mitral valve prolapse syndromes to isolated features such as ectopia lentis, Marfanoid body habitus and ascending aortic aneurysm and/or dissection. Fibrillin-1 mutations result in structurally and functionally defective fibrillin-1 molecules and microfibrils. Recent molecular genetic and fibrillin-1 biosynthesis studies suggest that individuals with fibrillin-1 abnormalities can be further subdivided into groups that are associated with distinct differences in severity and prognosis. In recognition of the expanding scope of related connective tissue disorders, we propose the terms microfibrillar disorder for disorders affecting fibrillin-containing microfibrils, and the more narrow concept of fibrillinopathy for clinical entities associated with abnormalities of fibrillin-1 or fibrillin-2. This latter category includes the previously defined disorders Marfan syndrome, congenital contractual arachnodactyly, and forms of ascending aortic aneurysm and/or dissection.

Molecular cloning, mapping to human chromosome 1 q21-q23, and cell binding characteristics of Spalpha, a new member of the scavenger receptor cysteine-rich (SRCR) family of proteins.

CD5 and CD6, two type I cell surface antigens predominantly expressed by T cells and a subset of B cells, have been shown to function as accessory molecules capable of modulating T cell activation. Here we report the cloning of a cDNA encoding Spalpha, a secreted protein that is highly homologous to CD5 and CD6. Spalpha has the same domain organization as the extracellular region of CD5 and CD6 and is composed of three SRCR (scavenger receptor cysteine rich) domains. Chromosomal mapping by fluorescence in situ hybridization and radiation hybrid panel analysis indicated that the gene encoding Spalpha is located on the long arm of human chromosome 1 at q21-q23 within contig WC1.17. RNA transcripts encoding Spalpha were found in human bone marrow, spleen, lymph node, thymus, and fetal liver but not in non-lymphoid tissues. Cell binding studies with an Spalpha immunoglobulin (Spalpha-mIg) fusion protein indicated that Spalpha is capable of binding to peripheral monocytes but not to T or B cells. Spalpha-mIg was also found to bind to the monocyte precursor cell lines K-562 and weakly to THP-1 but not to U937. Spalpha-mIg also bound to the B cell line Raji and weakly to the T cell line HUT-78. These findings indicate that Spalpha, a novel secreted protein produced in lymphoid tissues, may regulate monocyte activation, function, and/or survival.