Formal genetics of the immunoglobulins systems

Human Gm, Km, and Am allotypes and their molecular characterization: a remarkable demonstration of polymorphism

Human immunoglobulin allotypes are antigenic determinants (or "markers") determined serologically, classically by hemagglutination inhibition, on the human immunoglobulin (IG) heavy and light chains. The allotypes have been identified on the gamma1, gamma2, gamma3, and alpha2 heavy chains (they are designated as G1m, G2m, G3m, and A2m allotypes, respectively), and on the kappa light chain (Km allotypes). Gm-Am allotypes are inherited in fixed combinations, or Gm-Am haplotypes, owing to the linkage of the human IGHC genes (IGHG3, IGHG1, IGHA1, IGHG2, IGHG4, IGHE, and IGHA2 from 5' to 3' in the IGH locus on chromosome 14). Gm and Am allotypes have been one of the most powerful tools in population genetics and very instrumental in molecular characterization of the human IGHC genes (gene conversion, copy number variation, gene order). They represent a major system for understanding immunogenicity of the polymorphic IG chains, in relation with amino acid and conformational changes. The correlation between G3m allotypes and amino acid changes has been possible with the sequencing of many alleles of the IGHG3 gene, from individuals from different populations and with known allotypes. In this chapter, we integrate genetics and sequence data and provide an updated overview of the Gm-Am haplotypes and Km allotypes. We propose, for the first time, a complete elucidation of the G3m allotypes, illustrated by the "IMGT G3m allele butterfly" concept that allows a graphical representation of the G3m alleles (variants of a gene expressing a given set of allotypes). Knowledge of allotypes is important in antibody engineering and humanization of monoclonal antibodies to improve immunotherapy.

A human immunoglobulin IGHG3 allele (Gmb0,b1,c3,c5,u) with an IGHG4 converted region and three hinge exons

The five human IGHG genes consist of three constant domain exons plus one of or four hinge exon(s), the quadruplicated hinge region being characteristic of the IGHG3 gene. Besides this structural difference, the IGHG genes are polymorphic, as demonstrated by the restriction fragment length polymorphism and, at the protein level, by the Gm allotypic antigenic determinants. In this paper, we report the sequence of the G3m(b0, b1, c3, c5,u) IGHG3 allele, typical of the Black African populations and of populations with Negroid admixture, found in a homozygous Tunisian designated as LAT. We demonstrate that this G3 allele contains only three hinge exons instead of four (the probable result of an unequal crossing over) and that IGHG3 genes with triplicated hinge exons (and therefore encoding shorter gamma 3 chains) are present in healthy individuals from different populations. Moreover, we show that the LAT G3m (b0, b1, c3, c5, u) coding sequence results from the conversion, in the CH3 exon, of the G3m (b0, b1, b3, b4, b5, u, v) allele, the most frequent IGHG3 gene in the Negroid populations, by the homologous region of a IGHG4 gene. The structural features of the LAT IGHG3 allele, which are the lack of one hinge exon and its conversion by the IGHG4 gene, demonstrate that both crossing-over and gene conversion events occur in the evolution of the human IGHG genes.

BamHI and SacI RFLPs of the human immunoglobulin IGHG genes with reference to the Gm polymorphism in African people. Evidence for a major polymorphism

In this paper, we extend the study of the IGHG gene RFLPs in black African persons and in some other individuals characterized by a Negroid admixture. We demonstrate a polymorphism that is much more important in black Africans, than in Caucasoids, mainly for the IGHG3 and G1 genes, the most 5' members of the IGHG multigene family. These genes encode for the IgG3 and IgG1 subclasses, which are of crucial biological importance.

Restriction fragment haplotypes in the human immunoglobulin IGHG locus and their correlation with the Gm polymorphism

Of the four human IgG subclass heavy chain (IGHG) genes, three--IGHG1, IGHG2 and IGHG3--have allelic forms which encode antigenic determinants called G1m, G2m and G3m allotypes for markers of the IgG1, IgG2 and IgG3 molecules. The alleles at these three closely linked loci are transmitted as Gm haplotypes, together with the IGHG4 and the IGHGP (or pseudo gamma) genes. Restriction Fragment Length Polymorphisms (RFLPs) have been described for all these IGHG genes, allowing us to define at least 15 RFLP alleles (N. Ghanem et al., Eur. J. Immunol. 1988. 18:1059). In this article, we determine unambiguously the Gm alleles and haplotypes and the associated RFLPs in French, Lebanese and Tunisian families, chosen for the various Gm phenotypes of their members, in order to correlate both these polymorphisms and to know the evolution of the IGHG multigene family. We were able to assign all the IGHG alleles to the Gm alleles and to their linked IGHG4 and IGHGP genes allowing us to define restriction fragment haplotypes of the IGHG gene family.

Gene conversion in human immunoglobulin gamma locus shown by unusual location of IgG allotypes

The constant region of the gamma 1, gamma 2 and gamma 3 heavy chains of the human IgG1, IgG2 and IgG3 immunoglobulins carries antigenic determinants or G1m, G2m and G3m allotypes, which are genetic markers of these subclasses. The exceptional presence on gamma 1 and gamma 2 chains of Gm allotypes usually located on the CH3 domain of gamma 3 shows an unexpected clustering of base changes and subsequent identity of short DNA sequences in the CH3 exon of the non-allelic gamma 1, gamma 2 and gamma 3 genes. Such clusters of substitutions are not easily explained on the classical basis of point mutations. A gene conversion, which substituted a segment of the gamma 1 or gamma 2 gene with the homologous region of the non-allelic gamma 3 gene, is more likely. Other examples of possible conversion involving the gamma genes are described. The conservation or the restoration of short sequences produced by the conversion events might be related to the biological properties of the constant region of the heavy chains.

Simultaneous absence of the human IgG1, IgG2, IgG4 and IgA1 subclasses: immunological and immunogenetical considerations

Simultaneous absence of the IgG1, IgG2, IgG4 and IgA1 immunoglobulins has been unambiguously demonstrated in a healthy 75-year-old woman by testing for allotypes, isoallotypes and for isotypes of these four subclasses. Only IgM, IgD, IgG3, IgA2 and IgE were present. The IgG3 levels were significantly increased. Family investigation showed inheritance of a haplotype Gm-;-;b A2m2. This person is homozygous for an extensive DNA deletion including the C gamma 1, C gamma 2, C gamma 4 and C alpha 1 genes.

Gene deletion and gene duplication within the cluster of human heavy-chain genes. Selective absence of IgG sub-classes

Individuals with selective absence of IgG1 and IgG2 were discovered by testing for allotypes and isotypes of the respective sub-classes. These individuals were homozygous for sub-class deleted Gm-Am haplotypes, as shown by allotype studies in two families (Gm--;..;g;A2m1/Gm--;n;b;A2m1 and Gm--;n;b;A2m1/Gm--;..;b;A2m1) and by a population study of New Guineans (Gm fa;--;b;A2m2). The individuals with IgG1 sub-class deficiency showed elevation of IgG2, IgG4 and in particular of IgG3. Gene deletion can result from unequal crossing over which renders a complementary chromosome with a duplication of a sub-class gene. In one family, duplication of gamma 3 genes was observed to have happened in one of a twin pair. Quanitation of sub-classes in families with gamma 1- and with gamma 3-duplicated haplotypes did not show increased levels of the gene involved.

Gm, Am and Km immunoglobulin allotypes of two populations in Tunisia

Gm, Am and Km allotypes were investigated in two Tunisian populations (236 samples from Mahdia and 142 samples from Sfax). These populations descend from immigrants and, therefore, the results were compared with those obtained in other populations living in the Near East and in North Africa. The subclass heavy chain allotypes G1m, G2m, G3m and A2m are inherited in fixed combinations. There were five main and four minor Gm-Am haplotypes that could be deduced from the phenotypes. This led to the conclusion that the populations studied are Caucasoids with some African admixture (about 10%) and a very low oriental contribution. Furthermore, there were 11 samples which showed 8 uncommon Gm-Am phenotypes. These could be explained by the assumption of five different uncommon Gm-Am haplotypes. Four of these may have arisen by equal crossing over of prevalent haplotypes. The fifth may be the result of unequal crossing over of prevalent haplotypes. The fifth may be the result of unequal crossing over, since it was proven, by family study, that more markers are transmitted together than are present in the prevalent haplotypes.

HLA antigens and IgG allotypes on the Island of Barra (Outer Hebrides)

The distribution of HLA antigens and IgG allotypes in a group of nearly 200 adults on the Island of Barra, shows some unusual features. The sample tested included only people age 65 and over. Marked differences in the frequency of some HLA antigens were noted, compared to those previously determined for the northern island of Lewis and Harris, with increase in the frequency of HLA-A5 and B7. The latter antigen was present in over 40% of the group, compared with a frequency of 27% in the Northern islands. The high frequencies of HLA-A2, the Aw19 group and B12 previously noted in Scottish populations were also present. The IgG allotype, Gm (f;b4) was markedly increased, being present in 67% of the group, in comparison with frequencies of 40--45% in previous Scottish studies. These results either suggest a sampling phenomenon, or may be an indicator of selective forces related to immunological mechanisms or the age distribution of the population studied.

Common and uncommon immunoglobulin haplotypes among Lebanese communities

Allotypes of IgG1, IgG2, IgG3, and IgA2 subclasses were investigated in seven Lebanese communities (three Moslem and four Christian). The Gm-Am haplotypes found were mainly those prevalent in Caucasians with a low frequency of haplotypes usually observed in Africans and Orientals. The difference between highlanders and lowlanders as expressed by G2m(23) was highly significant and suggested a possible adaptation to selective pressure related to the gamma2 genes, possibly due to endemic malaria in the past. Exceptional Gm-Am haplotypes were unambiguously determined by family studies. Some were characterized either by a deletion or a repression or, in contrast, by a partial or total duplication of gamma genes. Two others had uncommon combinations of allotypes: Gm17;23;5,10,11,13,14 A2m1, where G1m (17) was present without G1m (1); and Gm3;23;5,14 A2m1, where the CH3 allotypes G3m (10,11,13) were lacking.