Human endogenous retroviruses: from infectious elements to human genes

Mammalian genomes contain a heavy load (42% in humans) of retroelements, which are mobile sequences requiring reverse transcription for their replicative transposition. A significant proportion of these elements is of retroviral origin, with thousands of sequences resembling the integrated form of infectious retroviruses, with two LTRs bordering internal regions homologous to the gag, prt, pol, and env genes. These elements, named endogenous retroviruses (ERVs), are most probably the proviral remnants of ancestral germ-line infections by active retroviruses, which have thereafter been transmitted in a Mendelian manner. The complete sequencing of the human genome now allows a comprehensive survey of human ERVs (HERVs), which can be grouped according to sequence homologies into approximately 80 distinct families, each containing a few to several hundred elements. As reviewed here, strong similarities between HERVs and present-day retroviruses can be inferred from phylogenetic analyses on the reverse transcriptase (RT) domain of the pol gene or the transmembrane subunit (TM) of the env gene, which disclose interspersion of both classes of elements and suggest a common history and shared ancestors. Similarities are also observed at the functional levels, since despite the fact that most HERVs have accumulated mutations, deletions, and/or truncations, several elements still possess some of the functions of retroviruses, with evidence for viral-like particle formation, and occurrence of envelope proteins allowing cell-cell fusion and even conferring infectivity to pseudotypes. Along this line, a genomewide screening for human retroviral genes with coding capacity has revealed 16 fully coding envelope genes. These genes are transcribed in several healthy tissues including the placenta, three of them at a very high level. Besides their impact in modelling the genome, HERVs thus appear to contain still active genes, which most probably have been subverted by the host for its benefit and should be considered as bona fide human genes. Some of their characteristic features and possible physiological roles, as well as potential pathological effects inherited from their retroviral ancestors are also reviewed.

Characterization of the three HERV-H proviruses with an open envelope reading frame encompassing the immunosuppressive domain and evolutionary history in primates

The HERV-H family is one of the largest human endogenous retrovirus families, with approximately 1000 elements. Using a direct coupled in vitro transcription/translation approach (PTT for protein truncation test) and an extended series of primers on human genomic DNA, on monochromosomal hybrids and on a BAC library, we could demonstrate that there are only three envelopes with a large open reading frame encompassing the immunosuppressive (ISU) domain, corresponding to 62-, 60-, and 59-kDa potential translational products. The associated proviruses, HERV-H/env62, HERV-H/env60, and HERV-H/env59 were sequenced together with their flanking DNA and mapped by FISH, and their entry times within the primate lineage were determined. Analysis of the LTR sequences revealed numerous recombinational and/or homogenization events in the course of evolution, with divergences between 5' and 3' LTRs higher than expected for a simple time-dependent genetic drift. PTT analyses further revealed that the three large envelopes in humans are prematurely stopped in the majority of primates, and sequencing of the largest envelope gene, from HERV-H/env62, in five human individuals revealed two polymorphic sites. The results are consistent with the absence of a strong selective pressure for the conservation of a functional envelope gene of possible benefit for the host, but do not exclude somatic effects possibly associated with the immunosuppressive domain carried by these genes.

ERV-3 envelope expression and congenital heart block: what does a physiological knockout teach us

Congenital heart block is a serious condition with significant mortality due in most cases to the transplacental transfer of autoantibodies from an otherwise asymptomatic mother. Although SSA/Ro and SSB/La autoantibodies have been implicated, attention has focused recently on autoantibodies to envelope proteins of endogenous retrovirus-3 (ERV-3). We have recently identified in 1% of the caucasian population a natural knock out of ERV-3 due to a premature stop mutation generating a severely truncated form of the protein [corrected]. If a pregnant female homozygous for the truncated form of the ERV-3 carries a foetus expressing the entire protein, the mother might be expected to acquire high titre immunity, while the foetus homozygous for the truncated form would not be expected to immunise its mother. In order to test whether this naturally occurring model could shed light on the pathogenesis of CHB, we determined the status of the ERV-3 stop polymorphism in 12 mothers of CHB infants [corrected]. The fact that none was homozygous for the stop mutation tends to rule out a role for the stop polymorphism of the mothers in the generation of the CHB disease, but does not exclude that other polymorphisms might be involved [corrected].

The long terminal repeats of the HERV-H human endogenous retrovirus contain binding sites for transcriptional regulation by the Myb protein

Binding sites for transcriptional regulation by the Myb protein have been identified in the long terminal repeats (LTRs) of the H-type human endogenous retrovirus-like elements (HERV-H). Transfection assays using reporter plasmids containing the luciferase gene under the control of a HERV-H LTR disclosed a sevenfold increase in promoter activity in human teratocarcinoma cells when cotransfected with an expression vector for the Myb protein. Binding sites for Myb were unambiguously identified within the LTR by both DNase I footprinting experiments and mobility shift assays using a bacterially expressed purified Myb recombinant protein. Possible roles of these Myb-responsive elements are discussed.

Clinical variability in patients with Apert's syndrome

OBJECT

Apert's syndrome is characterized by faciocraniosynostosis and severe bony and cutaneous syndactyly of all four limbs. The molecular basis for this syndrome appears remarkably specific: two adjacent amino acid substitutions (either S252W or P253R) occurring in the linking region between the second and third immunoglobulin domains of the fibroblast growth factor receptor (FGFR)2 gene. The goal of this study was to examine the phenotype/genotype correlations in patients with Apert's syndrome.

METHODS

In the present study, 36 patients with Apert's syndrome were screened for genetic mutations. Mutations were detected in all cases. In one of the patients there was a rare mutation consisting of a double-base pair substitution in the same codon (S252F). A phenotypical survey of our cases was performed and showed the clinical variability of this syndrome. In two patients there was no clinical or radiological evidence of craniosynostosis. In two other patients with atypical forms of syndactyly and cranial abnormalities, the detection of a specific mutation was helpful in making the diagnosis.

CONCLUSIONS

The P253R mutation appears to be associated with the more severe forms, with regard to the forms of syndactyly and to mental outcome. The fact that mutations found in patients with Apert' s syndrome are usually confined to a specific region of the FGFR2 exon IIIa may be useful in making the diagnosis and allowing genetic counseling in difficult cases.

Physiological knockout of the envelope gene of the single-copy ERV-3 human endogenous retrovirus in a fraction of the Caucasian population

ERV-3 is an evolutionarily conserved single-copy human endogenous retrovirus with a coding envelope gene potentially involved in important placental functions. We have investigated the sequence variability of this gene among 150 unrelated Caucasian individuals and found eight polymorphic sites. One of them corresponds to the introduction of a stop codon resulting in the production of a severely truncated ERV-3 envelope protein lacking both the fusion peptide and the immunosuppressive domain of the protein. The stop codon is observed in a homozygous state in approximately 1% of Caucasian individuals without evidence for counterselection, thus precluding the involvement of any essential function of the gene in placental implantation and development. This natural knockout provides a mean to investigate other potential roles for this otherwise highly conserved gene.

Increased calvaria cell differentiation and bone matrix formation induced by fibroblast growth factor receptor 2 mutations in Apert syndrome

Apert syndrome, associated with fibroblast growth factor receptor (FGFR) 2 mutations, is characterized by premature fusion of cranial sutures. We analyzed proliferation and differentiation of calvaria cells derived from Apert infants and fetuses with FGFR-2 mutations. Histological analysis revealed premature ossification, increased extent of subperiosteal bone formation, and alkaline phosphatase- positive preosteoblastic cells in Apert fetal calvaria compared with age-matched controls. Preosteoblastic calvaria cells isolated from Apert infants and fetuses showed normal cell growth in basal conditions or in response to exogenous FGF-2. In contrast, the number of alkaline phosphatase- positive calvaria cells was fourfold higher than normal in mutant fetal calvaria cells with the most frequent Apert FGFR-2 mutation (Ser252Trp), suggesting increased maturation rate of cells in the osteoblastic lineage. Biochemical and Northern blot analyses also showed that the expression of alkaline phosphatase and type 1 collagen were 2-10-fold greater than normal in mutant fetal calvaria cells. The in vitro production of mineralized matrix formed by immortalized mutant fetal calvaria cells cultured in aggregates was also increased markedly compared with control immortalized fetal calvaria cells. The results show that Apert FGFR-2 mutations lead to an increase in the number of precursor cells that enter the osteogenic pathway, leading ultimately to increased subperiosteal bone matrix formation and premature calvaria ossification during fetal development, which establishes a connection between the altered genotype and cellular phenotype in Apert syndromic craniosynostosis.

Possible genetic heterogeneity in the Saethre-Chotzen syndrome

Saethre-Chotzen syndrome is an autosomal dominant acrocephalosyndactyly syndrome whose gene has been assigned to chromosome 7p. Cytogenetic and linkage analyses have enabled the interval encompassing the disease gene to be delimited to a short region of chromosome 7p15.3-p21.2. Based on the genetic analysis of three unreported families, we confirm the location of the disease gene(s) in the interval defined by loci D7S664 and D7S493 (Zmax = 4.78 at [symbol: see text] = 0 at the D7S488 locus) but fail to decide whether one or more disease-causing genes map in this genetic interval.

No evidence of genetic heterogeneity in Crouzon craniofacial dysostosis

Crouzon craniofacial dysostosis (CFD) is an autosomal dominant form of craniosynostosis characterized by an abnormal skull shape, with hypertelorism, prominent eyes and midfacial retrusion. Recently, a gene for CFD has been mapped to chromosome 10q25-q26 and mutations in exon B of the fibroblast growth factor receptor 2 (FGFR2) gene have been identified. Here, we report the mapping of a CFD gene to chromosome 10q by close linkage to probe AFMa197wb1 at locus D10 S1483 in six unrelated families of French ancestry (Zmax = 4.69 at theta = 0) and provide additional evidence of genetic homogeneity of this condition. In addition, we report a novel mutation in exon B of the FGFR2 gene (Cys 342 Trp) in familial CFD and describe recurrent mutations at codon 342 as a particularly frequent event in CFD. Since mutations in the extracellular domain of the FGFR2 gene are observed in a few clinically distinct craniosynostosis syndromes (CFD, Jackson-Weiss, Apert and Pfeiffer), the present study gives support to the variable clinical expression of FGFR2 mutations in humans.

Isolation of new oncogenic forms of the murine c-fms gene

The c-fms gene encodes the receptor for the macrophage colony-stimulating factor, which plays a key role in the proliferation and differentiation of cells of the myelomonocytic lineage. In order to study the effects of overexpression of the macrophage colony-stimulating factor receptor in hematopoietic cells, a Harvey sarcoma virus-derived retroviral vector containing the murine c-fms cDNA was pseudotyped with Friend murine leukemia virus and inoculated into newborn DBA/2 mice. This viral complex induced monoclonal or oligoclonal leukemias with a shorter latency than that for Friend murine leukemia virus alone. Unexpectedly, 60% of the integrated fms proviruses had deletions at the 5' end of the c-fms gene. Sequence analysis of seven mutant proviruses indicated that the deletions always included the c-fms ligand binding domain and either occurred within the c-fms sequences, leaving the fms open reading frame unchanged, or joined VL30 sequences located at the 5' end of the parental retroviral vector to internal c-fms sequences, resulting in truncated fms proteins devoid of the canonical signal peptide. In contrast to all tyrosine kinase receptors transduced in retroviruses, no helper gag- or env-derived sequences were fused to the rearranged fms sequences. Viral supernatants isolated from hematopoietic tumors with viruses with deletions were able to transform NIH 3T3 cells as efficiently as parental fms virus, indicating that deletions resulted in constitutive activation of the c-fms gene. These oncogenic variants differ from those transduced in the Suzan McDonough strain of feline sarcoma viruses (L. Donner, L. A. Fedele, C. F. Garon, S. J. Anderson, and C. J. Sherr, J. Virol. 41:489-500, 1982). The high rate of c-fms rearrangement and its relevance in the occurrence of hematopoietic tumors are discussed.

Expression of functional beta-platelet-derived growth factor receptors on hematopoietic cell lines

The beta-type receptor of platelet-derived growth factor (beta PDGFR) is a class III transmembrane receptor with tyrosine kinase activity. The beta PDGFR gene is located on mouse chromosome 18 close to the c-fms gene which codes for the colony stimulating factor-1 receptor (CSF-1R). We previously reported that in a high percentage of myeloblastic leukemias induced by the Friend helper murine leukemia virus (F-MuLV), proviruses were integrated in the first intron of the c-fms gene leading to an enhanced expression of c-fms mRNA. Since activation by proviral insertion can act at long distance, we studied beta PDGF receptor gene expression in murine myeloblastic leukemias. This gene was found to be frequently expressed but the level of beta PDGF receptor mRNA was weak and not related to proviral activation. High affinity binding sites were expressed on myeloblastic cells and ligand binding induced cell proliferation. To determine whether beta PDGFR expression is a common feature in hematopoietic cells, we tested cell lines belonging to other hematopoietic lineages. We found that multipotent stem and mast cell lines also expressed the beta PDGF receptor gene. This suggests that PDGF, known as a mitogen for connective tissue cells, could also play a role in normal hematopoiesis.