Alternative titles; symbols
HGNC Approved Gene Symbol: CD99
Cytogenetic location: Xp22.33 Genomic coordinates (GRCh38): X:2,691,295-2,741,309 (from NCBI)
CD99 is a 32-kD T-cell surface glycoprotein involved in spontaneous rosette formation with erythrocytes (Bernard et al., 1988). The gene encoding CD99 (MIC2X) is located in the pseudoautosomal region (PAR) at the end of the short arm of the X and Y chromosomes (Goodfellow et al., 1983). See also MIC2Y (450000).
The monoclonal antibody 12E7 was raised against human leukemia T cells. It detects a 30,000 MW protein which is expressed on all human tissues tested with the possible exception of spermatozoa (Levy et al., 1979).
Dracopoli et al. (1985) described a monoclonal antibody, O13, that defines a cell surface antigen that is expressed on most cultured human cells but not on rodent cells. Glycoproteins of 25,000 and 30,000 MW were precipitated by O13. Either the X or the Y chromosome in cultured hybrid cells was sufficient for serologic reactivity with the antiserum. The gene encoding O13 maps to Xp22-pter and apparently escapes lyonization. All of these characteristics suggested that O13 was related or identical to 12E7 and that MSK5X and MSK57 (so-called because the workers were at Sloan-Kettering) were related or identical to MIC2X and MIC2Y.
Darling et al. (1986) cloned the MIC2X and MIC2Y genes and concluded that their sequences are closely related or identical.
Gelin et al. (1989) isolated a 1.11-kb cDNA from a lambda-gt11 expression library by screening with monoclonal antibodies directed against E2 antigen. The primary structure of E2, deduced from the nucleotide sequence of its gene, comprises 185 amino acids and is devoid of N-linked glycosylation sites. The protein displays an organization typical of an integral membrane protein. Nucleotide sequencing revealed that E2 is the MIC2 gene product.
Goodfellow et al. (1983) showed that the gene for the E2 antigen, called MIC2 (M = monoclonal; IC = Imperial Cancer Research Fund; 2 = order of discovery), maps to the band between Xp22.3 and Xpter, where the STS (300747) and Xg (300879) genes are located. Goodfellow et al. (1984) showed that the MIC2X locus, like Xg and STS, escapes lyonization. They identified a homologous locus on the Y chromosome (MIC2Y; 450000) in the euchromatin region Ypter-q11.1. This was the first instance of a clear Y-linked structural gene.
Curry et al. (1984) found that the STS, Xg, and MIC2X loci as well as the locus for X-linked chondrodysplasia punctata (302950) were apparently absent in males with deletion of Xp22.32.
By in situ hybridization, Buckle et al. (1985) showed that MIC2Y is located on the distal part of Yp, namely, Ypter-p11.2.
On the basis of an X/Y translocation in which STS activity was retained with the X chromosome (selected by fusion with an HPRT-deficient mouse cell line) but MIC2X was lost, Geller et al. (1986) concluded that MIC2X is distal to STS.
Pseudogene
Mangs and Morris (2007) stated that the sequence identified by Smith and Goodfellow (1994) as MIC2R (CD99L1) is a pseudogene that shares 78% sequence homology with MIC2 (CD99). Smith and Goodfellow (1994) had detected sequences related to exons 1, 4, and 5 of MIC2 on the X and Y chromosomes of humans and other primate species. Isolation of these sequences defined the MIC2R (MIC2-related) locus, which is associated with the second-most proximal CpG-rich island in the human pseudoautosomal region. Genomic sequences from the MIC2R locus showed that it is composed of a single sequence related to exon 1 and at least 4 tandem copies of sequences related to exons 4 and 5 of MIC2. Comparison of the 4 sequences related to exons 4 and 5 suggested that they are the result of sequential duplication of a 2.8-kb region during evolution. Smith and Goodfellow (1994) detected transcripts from the MIC2R locus in at least 10 adult and fetal tissues, and a number of different transcripts appear to be generated by alternative RNA splicing. Since none of the transcripts they analyzed contained a significant open reading frame, the function of the MIC2R locus remained unknown.
Gelin et al. (1989) found that Xg(a-) females (see 314700) have no E2 molecule on the surface of their red cells, in contrast with Xg(a+) individuals, but have the molecule in their cytoplasm, in the form of the 28-kD precursor. Thus, the MIC2 gene encodes a cell surface molecule involved in T-cell adhesion processes.
Kovar et al. (1990) noted that Ewing sarcoma and other PNE tumors express high amounts of a glycoprotein on their cell surface, which could be specifically detected by the monoclonal antibody HBA-71. They identified this glycoprotein as the product of the pseudoautosomal gene CD99. Kovar et al. (1990) presented evidence that CD99 is expressed at low levels in most, if not all, human cells and normal tissues. Because expression of CD99 is significantly enhanced in ES and PNET cells, they suggested that detection of the antigen by immunocytochemical analysis might be a useful tool in tumor diagnosis. Khoury (2005) stated that strong diffuse CD99 immunostaining constitutes a useful positive marker for the Ewing sarcoma family of tumors.
Goodfellow et al. (1987) presented evidence suggesting the existence of a pseudoautosomal locus, XGR (314705), that regulates expression of MIC2 and XG.
Using flow cytometry and Western and Northern blot analyses, Fouchet et al. (2000) provided a quantitative estimation of XG and CD99 on human erythrocytes. Their findings supported the hypothesis of genetic control of XG and CD99 expression by the hypothetical XGR locus.
Fouchet et al. (2000) examined coexpression of human XG and CD99 cDNAs in transfected mouse cells, either in double transfectants or in somatic hybrids from single transfectants. Their findings were consistent with transcriptional coregulation of XG and CD99 expression, because no influence of either protein on the surface production of the other was observed. In addition, Fouchet et al. (2000) found no evidence of association or complex formation between XG and CD99 on transfected mouse cells or human erythrocytes.
Using flow cytometric analysis, Pettersen et al. (2001) demonstrated that activation of a distinct domain of CD99 activates a caspase-independent death pathway in T cells. Ligation of FAS (TNFRSF6; 134637) and TRAIL (TNFSF10; 603598) death receptors was less effective than CD99 ligation in controlling transformed T cells.
Bixel et al. (2010) found that antibodies against mouse Cd99 or Pecam1 (173445) trapped neutrophils between endothelial cells in vitro. In contrast, electron and 3-dimensional confocal microscopy of inflamed cremaster demonstrated that antibodies against Cd99 or Cd99l2 (300846) or Pecam1 gene deletion led to accumulation of neutrophils in vivo between endothelial cells and basement membrane rather than between endothelial cells. Antibodies against Cd99 or Cd99l2 in combination with Pecam1 deficiency resulted in additive inhibitory effects on leukocyte extravasation in 2 different inflammation models. Bixel et al. (2010) concluded that CD99 and CD99L2 act independently of PECAM1 but at the same site during diapedesis, i.e., between endothelial cells and the basement membrane.
Yeh et al. (2018) noted that high and low erythroid expression of CD99 (CD99H and CD99L, respectively) is directly related to Xg(a) expression. Among females and males, Xg(a+) is associated with CD99H, and Xg(a-) females show an association with CD99L. However, Xg(a-) males may have either the CD99H or CD99L phenotype. Using next-generation sequencing of genomic areas relevant to XG and CD99 followed by a large-scale association study, Yeh et al. (2018) demonstrated an association between rs311103 in the XGR locus (314705.0001) and Xg(a)/CD99 phenotypes. The G and C alleles of rs311103 were associated with the Xg(a+)/CD99H and Xg(a-)/CD99L phenotypes, respectively. Reporter assays showed that the rs311103 genomic region with the G genotype had strong transcription-enhancing activity specifically in erythroid lineage cells that was absent with the C genotype. Follow-up analysis showed that GATA1 (305371) could bind specifically to the G allele of rs311103 and stimulate transcriptional activity. Yeh et al. (2018) concluded that rs311103 provides the genetic basis of the erythroid-specific Xg(a)/CD99 blood group phenotypes.
Polymorphism at the Xg locus and the Yg locus shows similar allele frequencies. This could be due to chance, to selection, or to recombination between the X and Y chromosomes (Burgoyne, 1982).
Tippett et al. (1986) presented family and sibship analysis to prove that the 12E7 quantitative polymorphism of red cells is controlled by the Y-borne locus, Yg, in addition to the X-borne locus, Xg. X-Y recombination was invoked to explain the apparent exception to Y-borne control in 1 family.
Goodfellow et al. (1986) stated that MIC2 recombines with TDF (testis-determining factor) at a frequency of 2 to 3%. MIC2 was the most proximal autosomal locus described to that time and a useful marker in studies directed toward isolation of TDF. The order of Y-specific sequences located proximal to the sex-determining gene(s), and therefore not pseudoautosomal, has been determined on the basis of their presence or absence in DNA from XX males, and the order of pseudoautosomal loci situated distal to TDF has been established through family studies such as those presented by Goodfellow et al. (1986).
During meiosis, pairing of the X and Y begins at the ends of the short arms. Cooke et al. (1985) found sequence homology in the pairing regions of the human X and Y. Because of a high order of polymorphism, they could do family studies which showed what they termed 'pseudoautosomal' inheritance, whereas Cooke et al. (1985) used repetitive sequences for this demonstration. Simmler et al. (1985) used a single-copy genomic DNA fragment which occurred in different allelic forms shared by both sex chromosomes. Homologous segments of the X and Y have been suspected because the 2 have a common ancestral origin; there is karyologic evidence for pairing and crossing-over and, in man, the Turner phenotype suggests deficiency of genetic material located on the second sex chromosome.
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