Cloning an expressed gene shared by the human sex chromosomes

High-valency Anti-CD99 Antibodies Toward the Treatment of T Cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive form of leukemia that currently requires intensive chemotherapy. While childhood T-ALL is associated with high cure rates, adult T-ALL is not, and both are associated with significant short- and long-term morbidities. Thus, less toxic and effective strategies to treat T-ALL are needed. CD99 is overexpressed on T-ALL blasts at diagnosis and at relapse. Although targeting CD99 with cytotoxic antibodies has been proposed, the molecular features required for their activity are undefined. We identified human antibodies that selectively bound to the extracellular domain of human CD99, and the most potent clone, 10A1, shared an epitope with a previously described cytotoxic IgM antibody. We engineered clone 10A1 in bivalent, trivalent, tetravalent, and dodecavalent formats. Increasing the antibody valency beyond two had no effects on binding to T-ALL cells. In contrast, a valency of ≥3 was required for cytotoxicity, suggesting a mechanism of action in which an antibody clusters ≥3 CD99 molecules to induce cytotoxicity. We developed a human IgG-based tetravalent version of 10A1 that exhibited cytotoxic activity to T-ALL cells but not to healthy peripheral blood cells. The crystal structure of the 10A1 Fab in complex with a CD99 fragment revealed that the antibody primarily recognizes a proline-rich motif (PRM) of CD99 in a manner reminiscent of SH3-PRM interactions. This work further validates CD99 as a promising therapeutic target in T-ALL and defines a pathway toward the development of a selective therapy against T-ALL.

The molecular genetic background leading to the formation of the human erythroid-specific Xga/CD99 blood groups

The Xg^a and CD99 antigens of the human Xg blood group system show a unique and sex-specific phenotypic relationship. The phenotypic relationship is believed to result from transcriptional coregulation of the XG and CD99 genes, which span the pseudoautosomal boundary of the X and Y chromosomes. However, the molecular genetic background responsible for these blood groups has remained undetermined. During the present investigation, we initially conducted a pilot study aimed at individuals with different Xg^a/CD99 phenotypes; this used targeted next-generation sequencing of the genomic areas relevant to XG and CD99 This was followed by a large-scale association study that demonstrated a definite association between a single nucleotide polymorphism (SNP) rs311103 and the Xg^a/CD99 blood groups. The G and C genotypes of SNP rs311103 were associated with the Xg(a+)/CD99H and Xg(a-)/CD99L phenotypes, respectively. The rs311103 genomic region with the G genotype was found to have stronger transcription-enhancing activity by reporter assay, and this occurred specifically with erythroid-lineage cells. Such activity was absent when the same region with the C genotype was investigated. In silico analysis of the polymorphic rs311103 genomic regions revealed that a binding motif for members of the GATA transcription factor family was present in the rs311103[G] region. Follow-up investigations showed that the erythroid GATA1 factor is able to bind specifically to the rs311103[G] region and markedly stimulates the transcriptional activity of the rs311103[G] segment. The present findings identify the genetic basis of the erythroid-specific Xg^a/CD99 blood group phenotypes and reveal the molecular background of their formation.

Developments beyond blood group serology in the genomics era

Blood group serology and single nucleotide polymorphism-based genotyping platforms are accurate but do not provide a comprehensive cover for all 36 blood group systems and do not cover the antigen diversity observed among population groups. This review examines the extent to which genomics is shaping blood group serology. Resources for genomics include the Human Reference Genome Sequence assembly; curated blood group tables listing variants; public databases providing information on genetic variants from world-wide studies; and massively parallel sequencing technologies. Blood group genomic studies span the spectrum, from bioinformatic data mining of huge data sets containing whole genome and whole exome information to laboratory investigations utilising targeted sequencing approaches. Blood group predictions based on genome sequencing and genomic studies are proving accurate, and have shown utility in both research and reference settings. Overall, studies confirm the potential for blood group genomics to reshape donor and patient transfusion management strategies to provide more compatible blood transfusions.

CD317/tetherin is an organiser of membrane microdomains

The integral membrane protein tetherin has been associated with an eclectic mix of cellular processes, including restricting the release of a range of enveloped viruses from infected cells. The unusual topology of tetherin (it possesses both a conventional transmembrane domain and a glycosylphosphatidylinositol anchor), its localisation to membrane microdomains (lipid rafts) and the fact that its cytosolic domain can be linked (indirectly) to the actin cytoskeleton, led us to speculate that tetherin might form a 'tethered picket fence' and thereby play a role in the organisation of lipid rafts. We now show that knocking down expression of tetherin leads to changes in the distribution of lipid raft-localised proteins and changes in the organisation of lipids in the plasma membrane. These changes can be reversed by re-expression of wild-type tetherin, but not by any of a range of tetherin-based constructs, indicating that no individual feature of the tetherin sequence is dispensable in the context of its lipid raft organising function.

CD99/MIC2 Constitutes a Differentiation Antigen of a Human Osteoblast Cell Line

BACKGROUND

The histological origin of the Ewing's family of tumors (EFT) is still not clear. Since these small cell bone tumors may originate from osteogenic stem cells, the presence of the CD99/MIC2 antigen, known to be overexpressed in EFT, was studied in a human osteoblast cell line in response to differentiation inducers.

METHODS

The HBA-71 monoclonal antibody directed to the CD99/MIC2 antigen was used to stain a human osteoblast cell line as well as the two EFT cell lines KAL and EW-2 after pretreatment of the cells with the differentiation inducers calcitriol and the histone deacetylase (HDAC) inhibitors sodium butyrate (NaB), sodium phenylacetate (NaPA) as well as N, N'-hexamethylen-bis-acetamide (HMBA). Alkaline phosphatase (ALP) levels were determined as cellular differentiation marker.

RESULTS

Significant expression of the CD99/MIC2 antigen, yielding a molecular weight of 32 kD in Western blotting, was found in the human osteoblast cell line. Pretreatment of the osteoblasts with calcitriol and HMBA increased ALP content and suppressed the CD99/MIC2 antigen. Calcitriol had no major effect on CD99/MIC2 expression of both EFT cell lines, but HMBA enhanced ALP activity in KAL cells and downregulated CD99/MIC2. EW-2 cells exhibited reduced levels of both CD99/MIC2 and ALP.

CONCLUSIONS

This study supports the role of CD99/MIC2 as differentiation antigen of osteoblasts and a Ewing's sarcoma cell line with neuroectodermal phenotype. Response to calcitriol is absent or low in the two EFT cell lines tested.

Genes and phenotypes of the human Y chromosome

By 1959 it was recognized that the gene (or genes) responsible for initiating the human male phenotype were carried on the Y chromosome. But in subsequent years, few phenotypes were associated with the Y chromosome. Recently, using molecular techniques combined with classical genetics, the Y chromosome has been the focus of intensive and productive investigation. Some of the findings are unexpected and have extended our understanding of the functions of the human Y chromosome. The notion that the Y chromosome is largely devoid of genes is changing. At the present, over 20 Y chromosome genes or pseudogenes have been identified or cloned, a number that is rapidly increasing. A high proportion of Y chromosome sequences have been found to be related to X chromosome sequences: the assembly of a complete physical map of the Y chromosome euchromatic region (believed to carry all of the genes) has shown 25% of the region studied to have homology to the X chromosome.3 Several X-homologous genes are located in the X and Y chromosome pairing regions, an area predicted to have shared homology. Surprisingly, some of the Y-encoded genes that lie outside of the X and Y pairing region share high sequence similarity, and in at least one case, functional identity, with genes on the X chromosome.

Genetic basis of blood group diversity

In the last 18 years the genes that encode all but one of the 29 blood group systems present on red blood cells (RBCs) have been identified. This body of knowledge has permitted the application of molecular techniques to characterize the common blood group antigens and to elucidate the background for some of the variant phenotypes. Just as the RBC was used as a model for the biochemical characterization of cell membranes, so the genes encoding blood groups provide a readily accessible model for the study of gene expression and diversity. The application of genotyping techniques to identify fetuses at risk of haemolytic disease of the newborn is now the standard of care, and the expansion of nucleic acid testing platforms to include both disease testing and blood typing in the blood centre is on the horizon. This review summarizes the molecular basis of blood groups and illustrates the mechanisms that generate diversity through specific examples.

A study of the coregulation and tissue specificity of XGand MIC2 gene expression in eukaryotic cells

CD99, the product of the MIC2 gene, exhibits an erythroid-specific quantitative polymorphism coregulated with the polymorphism of the XG blood group gene. As a preliminary study of this phenomenon, human XG and CD99 recombinant proteins were expressed in murine RAG cells and analyzed by flow cytometry. Both proteins were expressed independently and at a similar level in single and double transfectants. Immunoprecipitation and Western blot analysis, using the murine monoclonal antibodies NBL-1 and 12E7, revealed species of 26 kd (XG) and 32 kd (CD99), respectively. A putative 28-kd intracellular precursor of CD99 was also detected, as was a 26-kd species after neuraminidase treatment of CD99-expressing cells. No evidence of association or complex formation between XG and CD99 proteins could be proven, either on transfected RAG cells or on human erythrocytes. These results were confirmed using somatic hybrids between single transfectants. These findings suggest that the phenotypic relationship between XG and CD99 is mostly regulated at the transcriptional level, but they do not formally exclude some posttranscriptional effect. Studies on the tissue specificity of XG expression showed that surface expression of the XG protein could not be restored in somatic hybrids between B-lymphoblastoid cell lines from Xg(a+) persons and fibroblasts (RAG) or erythroid (MEL) cells. RT-PCR analysis of the transcripts revealed the existence of an XG mRNA in each cell line, suggesting that the tissue-specific regulation of cell surface XG expression occurs either at a quantitative transcriptional level or is a posttranscriptional event. By Northern blot analysis,XG transcripts were detected in erythroid tissues and several nonerythroid tissues.

What is important on the red blood cell surface

Blood group antigens have provided tools for investigation of the red cell surface and been very useful as genetic markers in family, population and forensic studies. Precise definition of phenotypes is very important. Application of MAIEA (monoclonal antibody-specific immobilisation of erythrocyte antigen), a recently reported technique, to identify antigens and to assign red cell antigens to a particular membrane component is described: location of Knops system antigens on CR1 is confirmed and provisional assignment of Cromer system antigens to the different short consensus regions of decay accelerating factor (DAF) is described. Variability of red cell antigen expression is considered. The possibility is discussed that factors other than alterations in Rh genes may be responsible for some Rh variant phenotypes. Some C variants, two of which are associated with low incidence antigens, are described. The relationship of Xga with the quantitative polymorphism of 12E7 antigen is reconsidered in light of some recent immunochemical studies.

PBDX is the XG blood group gene

We have identified the Xga antigen, encoded by the XG blood group gene, by employing rabbit polyclonal and mouse monoclonal antibodies raised against a peptide derived from the N-terminal domain of a candidate gene, referred to earlier as PBDX. In indirect haemagglutination assays, these anti-peptide antibodies react with Xg(a+) but not Xg(a-) erythrocytes. In antibody-specific immobilization of antigen (ASIA) and immunoblot assays, the anti-peptide antibodies react with the same molecule as does human anti-Xga. Therefore, by its identity with PBDX, Xga is identified as a cell-surface protein that is 48% homologous to CD99 (previously designated the 12E7 antigen), the product of MIC2 which is tightly linked to XG. PBDX is renamed here XG.

Pseudoautosomal region in schizophrenia: sex concordance of the affected sibpairs and the association study with DNA markers

To test a hypothesis that the pseudoautosomal region of the sex chromosomes contributes to the pathogenesis of schizophrenia, we carried out the following studies: First, the sex concordant rates of 77 schizophrenic sibpairs were examined. Secondly, 46 schizophrenic patients and 150 healthy controls were tested for association with DXYS17, DXYS20, DXYS28, and MIC2 in the pseudoautosomal region. Sex concordant rates in sibpairs with schizophrenia were not higher than would be expected by chance. No significant associations were found between four DNA markers we tested and schizophrenia. These results did not support the hypothesis; however, linkage disequilibrium can only be detected if the marker and trait loci are located close enough. Linkage analyses in multiplex families need to be carried out before ruling out this region as a location for a gene for schizophrenia.

Inactivation of an X-linked transgene in murine extraembryonic and adult tissues

Transgenes located on the X chromosome have been used to study the mechanisms involved in X-chromosome inactivation. Analysis of the transgenic mouse strain M-TKneo1 carrying a neomycin resistance gene inserted in the X chromosome showed that, in adult somatic tissues, this transgene is subject to X-inactivation and to de novo methylation as other endogenous X-linked genes. During mouse embryogenesis, X-linked genes show a preferential paternal inactivation in extraembryonic tissues, whereas these genes are subject to random inactivation in embryonic tissues. It has been suggested that, in the mouse, the extraembryonic tissues carry a parental imprint at the time of inactivation. The study of the neo transgene expression in extraembryonic endoderm has shown not only that neo is inactivated but also that, at the RNA level, paternal inactivation of the transgene seems essentially complete. The differences between our results and previously obtained results with a mouse alpha-fetoprotein transgene, which was only inactivated in neonatal tissues but not in extraembryonic tissues, are discussed.

Heterozygous expression of X-linked chondrodysplasia punctata. Complex chromosome aberration including deletion of MIC2 and STS

Two females showing partial expression of X-linked chondrodysplasia punctata were identified in a family. Bone dysplasia was caused by an aberrant X chromosome that had an inverse duplication of the segment Xp21.2-Xp22.2 and a deletion of Xp22.3-Xpter. To characterise the aberrant X chromosome, dosage blots were performed on genomic DNA from a carrier using a number of X-linked probes. Anonymous sequences from Xp21.2-Xp22.2 to which probes D2, 99.61, C7, pERT87-15, and 754 bind were duplicated on the aberrant X chromosome. The proposita was heterozygous for all these markers. Dosage blots also showed that the loci for steroid sulfatase and the cell surface antigen 12E7 (MIC2) were deleted as expected from the cytogenetic results. Mouse human cell hybrids were constructed that retained the normal X in the active state. Analysis of these hybrid clones for the markers from Xp21.2-Xp22.2 revealed that all the alleles of the informative markers, present in a single dosage in the genomic DNA, were carried on the normal X chromosome of the proposita. The duplicated X chromosome therefore had two identical alleles, indicating that the aberration resulted from an intrachromosomal rearrangement.

Regulator genes affecting red cell antigens

Several mechanisms may be involved to explain the action of genes that regulate the expression of red cell antigens. When carbohydrate antigens are involved, lack of an enzyme in the biochemical pathway prevents formation of the precursor for the next and following steps of that path, or, alternatively, addition of an extra sugar to the immuno-dominant sugar may produce a new structure in which the expression of the expected antigen is masked. Thinking of genetic rather than biochemical interference, a regulator gene may "switch-off" the action of a structural gene, and this mechanism could involve the upset of repressor and/or derepressor genes. The mechanisms for the regulator genes described in this article are unknown. The effect of XGR is limited to red cells: the expression of 12E7 antigen on other tissues and cells, other than red cells, is invariable. The reported effects of XOr and XQ are for red cells, but it is unlikely that other cells and tissues have been studied intensively; propositi with these regulator genes are much rarer than people informative for XGR and In(Lu). The effects of In(Lu) are not limited to red cells but have been shown to regulate the expression of p80 on some white cells. Most of the abnormalities in Rhnull cells appear to be associated with the lack of the Rh antigens and lack of Rh proteins. The hypothesis of a functional complex involving Rh, lack of which affects incorporation of apparently unrelated proteins into the red cell membrane, is an attractive idea. Studies of the similar phenotype, Rhmod, suggest that some Rh specificities can be present in cells that appear to be as abnormal, serologically and morphologically, as Rhnull cells. Perhaps some polypeptides are functionally more important than others and perhaps all polypeptides required for the functional efficiency of the Rh complex have not yet been identified. Lack of Lutheran antigens is not always accompanied by modification of other red cell antigens. As suggested by Telen and green, if In(Lu) acts via a single mechanism, then that mechanism differs from that of XS2. Certainly the mechanisms of In(Lu) and XS2 differ in their action on the expression of CD44 or p80 antigens. The red cell surface is well charted territory, familiar to serologists, immunologists, biochemists, and geneticists. It still provides an excellent model for study of cell surface antigens and for the regulator genes described above that modify expression of some red cell antigens.(ABSTRACT TRUNCATED AT 400 WORDS)

Sex vesicle "entrapment": translocation or nonhomologous recombination of misaligned Yp and Xp as alternative mechanisms for abnormal inheritance of the sex-determining region

Abnormal inheritance of the sex determining region, normally located on Yp, results in about 1 in 20,000 phenotypic males with a 46,XX genotype. Studies to date indicate that many 46,XX males apparently arise due to a balanced, yet abnormal, nonhomologous interchange of Xp and Yp termini. However, 2 of the 5 XX males we report here have 3 copies of the pseudoautosomal locus, MIC2. Thus, they appear to have inherited the sex determining region as a result of Yp sequences being added onto the X pseudoautosomal region. Such an unequal, extremely nonhomologous interchange could alternatively be considered to arise from an unbalanced translocation of Yp to Xp. Our results suggest that very unequal interchange or translocation of Yp sequences onto the X pseudoautosomal region is not as rare a mechanism for XX males as originally thought. We also suggest that sex vesicle "entrapment" favors the association of a Yp fragment to the X pseudoautosomal region over a translocation to either Xq or an autosome.

Chromosomes of older humans are more prone to aminopterine-induced breakage

We have adopted a simplified version of the "cell hybrid cotransfer method" to test the hypothesis that human lymphocytes derived from elderly individuals have a higher chromosome instability. Peripheral blood lymphocytes from "old" male individuals and "young" controls were fused with a Chinese hamster cell line (CHO-YH21), yielding 10 HAT-resistant rodent-human clones from the old propositi and 22 from the young controls (HAT = hypoxanthine/aminopterin/thymidine). Both series of hybrid clones were analyzed with respect to the retention of the enzyme glucose-6-phosphate dehydrogenase and the surface antigen MIC2 identified by monoclonal antibody 12E7, two human X chromosome-linked markers located at opposite ends of the X chromosome. Cell hybrid clones with an X chromosome from a young control retained both markers in about 70% of the cells. In contrast, cell hybrid clones with an X chromosome from an old donor retained the MIC2 marker in only 30% of their cells. Slot-blot hybridization studies have established that the observed loss of the MIC2 marker is due to loss of the coding gene, not to suppression of its expression. Similar hybridization studies with molecular probes specific for other regions of the X chromosome suggest preferential chromosomal breakage sites. T lymphocytes from old donors were also found to have an LD50 for aminopterine significantly lower than the concentration of this drug in the HAT medium used to grow the hybrids, suggesting that the higher level of gene loss observed in the X chromosomes from old donors may be directly related to their increased sensitivity to the clastogenic effect of aminopterine. We speculate that the higher rate of chromosomal breakage and of marker loss observed along the "old-age" X chromosomes could be the result of "molecular scars" accumulated with aging at sites of constitutive chromosomal fragility.

MIC2: a human pseudoautosomal gene

MIC2 and XGR are the only known pseudoautosomal genes in man. MIC2 encodes the 12E7 antigen, a human cell-surface molecule of unknown function. XGR regulates, in cis, the expression of the XG and MIC2 genes. DNA probes derived from the MIC2 locus have been used in the construction of a meiotic map of the pseudoautosomal region and a long range restriction map into the X- and Y-specific chromosome domains. MIC2 is the most proximal marker in the pseudoautosomal region and recombination between the sex chromsomes only rarely includes the MIC2 locus. Our long-range restriction maps and chromosome walking experiments have localized the pseudoautosomal boundary within 40 kilobases adjacent to the 3' end of the MIC2 gene. The same maps have been used to predict the chromosomal location of TDF.

A monoclonal antibody, R1, and a polyclonal serum, S10, recognize the same molecules: a novel use of DNA transfectants

The monoclonal antibody R1 defines the product of the MIC5 locus which has been localized to the long arm of the human X chromosome in the same region as the fragile site FRAXA. DNA transfectants, selected by R1 on the fluorescence-activated cell sorter (FACS), have been used to demonstrate that a previously described polyclonal antiserum, S10 (Buck & Bodmer, 1976), recognizes the same cell surface molecule as R1. Immunoprecipitation from metabolically and surface-labelled cells has shown that this molecule is a Mr 200 K phosphoprotein which is synthesized as a Mr 180 K precursor and subsequently modified to a Mr 195 K extracellular form. A Mr 150 K molecule is coprecipitated with the Mr 200 K phosphoprotein, but is only detected in lysates of surface-labelled cells, raising the possibility that the 200 K molecule is a cell surface receptor and the 150 K molecule a ligand.

Absence of methylation of a CpG-rich region at the 5' end of the MIC2 gene on the active X, the inactive X, and the Y chromosome

We have identified and characterized a Hpa II tiny fragment (HTF) island associated with the promoter region of the pseudoautosomal gene MIC2. The MIC2 HTF island is unmethylated on both the active and inactive X chromosome and is similarly unmethylated on the Y chromosome. Unlike the majority of genes borne on the X chromosome, MIC2 fails to undergo X chromosome inactivation. HTF islands associated with X chromosome-linked genes that are inactivated are highly methylated on the inactive or transcriptionally silent homologue. The failure of MIC2 to undergo X chromosome inactivation correlates with the lack of methylation of the HTF island at the 5' end of the gene. These results provide further evidence that DNA methylation plays an important role in the phenomenon of X chromosome inactivation.

Analysis of methylation of a human X located gene which escapes X inactivation

The gene MIC2 is located in the pseudoautosomal region at the ends of the short arms of the X and Y chromosomes. In females MIC2 escapes X inactivation. We have analyzed the methylation pattern of MIC2 on the active X, the inactive X chromosomes, and the Y chromosome. The 5' end of the gene contains a GC rich region which is unmethylated on the active X, the inactive X and on the Y. The body of the gene is characterized by variable methylation.

Immunochemical characterization of the human blood cell membrane glycoprotein recognized by the monoclonal antibody 12E7

The 12E7 murine monoclonal antibody recognizes a protease-sensitive component of human red cells, platelets and lymphocytes which could not be detected on granulocytes. Scatchard analyses indicated that the 125I-labelled antibody binds to 1000, 4000 and 27,000 antigen sites on each red cell, platelet and lymphocyte respectively, with a binding constant ranging from 4 x 10(7) to 9 x 10(7) M-1. The membrane components recognized by the monoclonal antibody were characterized by immunostaining on nitrocellulose sheets. A 28 kDa sialoglycoprotein was visualized following electrophoretic transfer of the red cell and lymphocyte membrane proteins separated by SDS/polyacrylamide-gel electrophoresis. Another component of 25 kDa was also clearly identified in the lymphocyte and platelet lysates, but was barely detectable in the red cell membrane preparations. Enzyme treatment of intact platelets, as well as analysis of the membrane and cytosolic preparations from these cells, have shown that the 25 kDa component was of cytoplasmic origin. The mobility of the 28 kDa membrane component is decreased following neuraminidase treatment of intact blood cells, but these cells still react normally with the monoclonal antibody, indicating that sialic acids are not required for binding. The 28 kDa component is present on red cell membranes prepared from S-s-U-, En(a-) and Gerbich(-) individuals, demonstrating that it is a new sialoglycoprotein not derived from glycophorins A, B, C or D. The 28 kDa component was totally solubilized with 0.1% Triton X-100 from red cell membranes and behaves like the other red cell membrane sialoglycoproteins since it was extracted in the aqueous phase following chloroform/methanol/water or butanol/water partitionings. The 28 kDa component could be partially purified by h.p.l.c. gel permeation chromatography and preparative SDS/polyacrylamide-gel electrophoresis. The material finally obtained strongly inhibits the 12E7 monoclonal as well as human anti-Xga antibodies, suggesting either that the 28 kDa glycoprotein carries both antigens or that the 12E7 and Xga-active molecules copurified.

Evolution of homologous sequences on the human X and Y chromosomes, outside of the meiotic pairing segment

A sequence isolated from the long arm of the human Y chromosome detects a highly homologous locus on the X. This homology extends over at least 50 kb of DNA and is postulated to be the result of a transposition event between the X and Y chromosomes during recent human evolution, since homologous sequences are shown to be present on the X chromosome alone in the chimpanzee and gorilla.

Physical mapping of genes and sequences at the end of the human X chromosome short arm

Human-rodent somatic cell hybrids containing deleted and translocated human X chromosomes have been used to map genes and sequences in and around the pseudoautosomal region. The following order was found: (DXS69, DXS70, DXS143)-(DXS31, STS)-MIC2. This order is consistent with the known inheritance patterns of DXS31, STS and MIC2. Assuming that the translocations and deletions we have studied are not complex rearrangements, we conclude that the pseudoautosomal region consists of less than 5 X 10(6) bp of DNA.

Recombination between the X and Y chromosomes: implications for the relationship between MIC2, XG and YG

Three loci affect the levels of expression of the 12E7 antigen on red blood cells: MIC2, the pseudoautosomal structural gene for the 12E7 antigen; XG, an X-linked red cell blood group locus and YG, a polymorphic Y-linked locus. In this report we describe recombination between XG and MIC2 and an exchange between the X and Y chromosomes which included YG. These results have prompted us to propose a new model describing the genetic relationship between the XGa and 12E7 antigens.

Natural selection and Y-linked polymorphism

Several population genetic models allowing natural selection to act on Y-linked polymorphism are examined. The first incorporates pleiotropic effects of a Y-linked locus, such that viability, segregation distortion, fecundity and sexual selection can all be determined by one locus. It is shown that no set of selection parameters can maintain a stable Y-linked polymorphism. Interaction with the X chromosome is allowed in the next model, with viabilities determined by both X- and Y-linked factors. This model allows four fixation equilibria, two equilibria with X polymorphism and a unique point with both X- and Y-linked polymorphism. Stability analysis shows that the complete polymorphism is never stable. When X- and Y-linked loci influence meiotic drive, it is possible to have all fixation equilibria simultaneously unstable, and yet there is no stable interior equilibrium. Only when viability and meiotic drive are jointly affected by both X- and Y-linked genes can a Y-linked polymorphism be maintained. Unusual dynamics, including stable limit cycles, are generated by this model. Numerical simulations show that only a very small portion of the parameter space admits Y polymorphism, a result that is relevant to the interpretation of levels of Y-DNA sequence variation in natural populations.

The effect of recombination between the X and Y chromosomes of mammals

Recent molecular analysis has proved the hypothesis that parts of the human sex chromosomes are homologous and can recombine in male meiosis. The biological consequences of this recombination have been investigated by considering the joint effect of neutral mutation and random genetic drift on a locus that is closely linked to the non-homologous segments of the sex chromosomes, but which recombines with them in an appreciable frequency. Our model predicts that, in the absence of selection, allelic differentiation between genes carried on the X and on the Y chromosomes will develop only if the recombination rate is of the same order of magnitude as the mutation rate or smaller. Similarly, a mutation favourable in males but disadvantageous in females will increase in frequency on the Y chromosomes, while remaining rare on the X chromosomes, only if the recombination rate is smaller than the fitness advantage of the mutation. The X and Y chromosomes are, thus, not expected to show any genetic differentiation for almost all of their homologous parts. Divergence will occur only for loci that very rarely recombine between the sex chromosomes.

A pseudoautosomal gene in man

The X/Y homologous gene MIC2 was shown to exchange between the sex chromosomes, thus demonstrating that it is a pseudoautosomal gene in man. MIC2 recombines with the sex-determining gene(s) TDF at a frequency of 2 to 3 percent. It is the most proximal pseudoautosomal locus thus far described and as such is an important marker for use in studies directed towards the isolation of TDF.