cis-Interacting genes in the S region of the murine major histocompatibility complex

Phage Mu transposition immunity reflects supercoil domain structure of the chromosome

Transposition immunity is the negative influence that the presence of one transposon sequence has on the probability of a second identical element inserting in the same site or in sites nearby. A transposition-defective Mu derivative (MudJr1) produced transposition immunity in both directions from one insertion point in the Salmonella typhimurium chromosome. To control for the sequence preference of Mu transposition proteins, Tn10 elements were introduced as targets at various distances from an immunity-conferring MudJr1 element. Mu transposition into a Tn10 target was not detectable when the distance of separation from MudJr1 was 5 kb, and transposition was unencumbered when the separation was 25 kb. Between 5 kb and 25 kb, immunity decayed gradually with distance. Immunity decayed more sharply in a gyrase mutant than in a wild-type strain. We propose that Mu transposition immunity senses the domain structure of bacterial chromosomes.

Tissue-specific variation in C4 and Slp gene regulation

C4 and Slp are highly homologous mouse genes that differ in function and regulation. Allelic variants exist in quantitative regulation of C4 and in hormonal regulation of Slp. We have examined expression in several tissues, including liver and peritoneal macrophages which are the major sites of synthesis, using a probe that allows direct comparison of C4 and Slp mRNAs. Correctly-sized and initiated RNA, within an order of magnitude of liver levels, is found in mammary gland, lung, spleen, and kidney; lower levels are detectable in testis, brain, heart and submaxillary gland. By comparing expression in congenic mouse strains differing in C4 and Slp loci, regulation of these genes is seen to vary in different tissues. This provides a well-defined genetic system in which to examine cis-acting sequences and trans-acting factors that result in tissue-specific patterns of gene regulation.

The molecular genetics of components of complement

Rapid progress has been made in establishing linkages and in chromosome allocation of the genes of some 9 complement components. In the MHC, C2, Factor B, and two C4 or C4 related genes have been placed in some detail in both man and mouse. The gene coding for the cytochrome P-450 21-hydroxylase has been shown to be duplicated and immediately 3' to the two C4 genes, though it appears to be functionally and structurally unrelated to the complement components. Thus six genes have been mapped to this region where particular haplotypes are associated with increased susceptibility to a number of diseases, some of which are autoimmune in character. The complete gene structure of Factor B has been solved in man and rapid progress is being made with the C2 and C4 genes. The structural basis of the polymorphisms of these genes is being established. In C4, the polymorphism is exceptionally complex with varying numbers of loci and probably more than 50 allotypes occurring in man. A structural basis has also been found for the big differences in the biological activity of some of the C4 allotypes in man. Apart from the genes in the MHC, linkage has been found between the genes coding for C4bp, CR1, and Factor H. Remarkably there are sequence homologies between these proteins and C2 and Factor B, probably related to the ability to bind to one or other of the structurally similar proteins C3b and C4b. The complete cDNA sequences of C3 and C4 in mouse and man have given much information on the many posttranslational modifications of these proteins. A partial structure has been obtained for the C3 gene and the homology shown between C3, C4, C5, alpha 2-macroglobulin, and pregnancy zone protein. Although the amount of detailed information in the molecular genetics of complement components is accumulating rapidly, there appears to be a reasonable prospect that linkages and homologies will classify the data into a comprehensible form.

Molecular organization and in vitro expression of murine class III genes

These experiments demonstrate that at least two types of gene duplications have occurred during the evolution of the S region. The first type, which produced the C2 and factor B genes, involved a short segment of the chromosome encompassing a single gene. The related products have subsequently diverged yielding sequences which do not cross-hybridize. Further duplication of these genes has not been observed. The second type of duplication consisted of a much longer primordial sequence, spanning approximately 55 kb of genomic DNA and including at least two genes, C4/Slp and 21-hydroxylase. The duplicated sequences are separated by a segment of single copy sequence of as yet undefined length. These duplicated sequences have been relatively conserved. There is evidence that further duplication of this region is possible (as seen in the H-2w7 strain) although the exact nature of the increase in gene number has not been fully characterized. Detailed analysis of cosmid clones which span these two duplications has permitted the assignment of a new pair of loci to the S region, encoding 21-hydroxylase A and B. The advantage conferred by linkage of the gene encoding this adrenal steroid biosynthesis enzyme to the genes encoding complement components C2, factor B, and C4 is unclear, as is the advantage of the association of all of the class III genes with the remainder of the MHC. The availability of cloned sequences containing all of the class III genes permits further study of the factors which govern the tissue specificity of their expression and which confer androgen responsiveness on certain of the Slp alleles.

Multiple duplications of complement C4 gene correlate with H-2-controlled testosterone-independent expression of its sex-limited isoform, C4-Slp

Mouse liver cDNA clones related to the C4 and C4-Slp isoforms of the fourth component of complement differ by few nucleotide changes within a region of substantial divergence from human C4. It is suggested that the mouse C4 gene duplication is an evolutionarily recent event with respect to the time of mammalian radiation. This conclusion is reinforced by the presence of a single C4 gene in the Syrian hamster. Most H-2 haplotypes, including those characterized by an undetectable C4-Slp protein, possess two C4 gene copies which, in contrast to the neighboring factor B, show a marked restriction site polymorphism. The genetic variation of this region is emphasized by the presence in the mouse of a rare "polymorphism" for C4 gene number. Multiple C4-related gene copies characterize those exceptional wild-derived H-2 haplotypes, H-2w7, H-2w16, and H-2w19, that determine the expression of the C4-Slp protein in female animals.

Close linkage between mouse genes determining the two forms of complement component C6 and component C7, and cis action of a C6 regulatory gene

Two forms of mouse complement component C6, with molecular weights (Mrs) of 90 and 100 kilodaltons (kd), are present in the sera from certain inbred strains such as the CBA strain; other strains, such as the BALB/c and DBA/2 strains, have only the 90 kd C6A form. The present work was undertaken to determine whether the two Mr forms were the products of genes coding at separate loci. We screened sera from mice from a number of inbred strains by isoelectric focusing and found one strain, AKR, exhibiting allotypic structural variations of C6 forms. To distinguish the various types, we designated the 90 kd types from CBA and AKR mice C6A1 and C6A2, respectively, and the corresponding 100 kd types C6B1 and C6B2, respectively. Mice possessing only one Mr form were all typed as C6A1. Results of breeding experiments strongly suggested that the two Mr forms of C6 are coded for at two closely linked loci. Sera from a number of inbred strains were also screened for a complement C7 polymorphism by means of isoelectric focusing and functional overlay. C7 from all strains, excepting the AKR strain, produced identical C7 band patterns. AKR C7 produced a unique band pattern, and results of breeding experiments with AKR and BALB/c mice showed the C6 and C7 loci to be closely linked. In addition, we identified a regulatory gene for C6 production. The gene apparently requires androgen to facilitate C6 production in the majority of strains. In these strains C6 activity is virtually absent from female sera. However, we observed moderate levels of C6 activity in sera from IS/Cam females, indicating that, in this strain, male physiological androgen levels are not necessary for C6 production. IS/Cam possess one form of circulating C6 which appears identical with BALB/c C6A1, and therefore IS/Cam mice differ from AKR mice at both the C6 structural and regulatory loci. These two strains were thus suitable for use in breeding experiments to determine the manner of action of the regulatory gene. Results showed that it acted in a cis manner.

Genes for murine fourth complement component (C4) and sex-limited protein (Slp) identified by hybridization to C4- and Slp-specific cDNA

Murine fourth component of complement (C4) and sex-limited protein (Slp) are two closely related serum proteins whose structural genes lie in the S region of the murine H-2 complex. We have cloned two very similar (95% nucleotide sequence identity) cDNAs; they encode amino acid sequences that are distinct but that correspond equally well with the limited amino acid sequence available for murine and human C4. We have identified one of these as C4 cDNA and the other as Slp cDNA by comparing, in a RNA blot hybridization experiment, the extent of hybridization of the two cDNAs to liver mRNAs from inbred mouse strains expressing varying amounts of C4 and Slp proteins in their plasma. The C4 and Slp cDNAs were used in a Southern blot experiment to identify the C4 and Slp genes in the molecular map of the S region.

Left-handedness: association with immune disease, migraine, and developmental learning disorder

We report an experimental study designed to test the following hypothesis derived from clinical observations: There is an elevated frequency in left-handed individuals and in their families of immune disease, migraine, and developmental learning disorders. In two separate investigations the frequency of these conditions was compared in strongly left-handed subjects and in strongly right-handed controls. In each of the investigations we found markedly higher frequencies of immune disease in the left-handers than in the right-handers. The rate of learning disabilities was also much higher in the left-handers than in the right-handers in both investigations. In a second study the frequency of left-handedness was compared in patients with migraine or immune disease and in general population control subjects free of these disorders. There was a higher frequency of left-handedness in patients with migraine and myasthenia gravis than in controls. We present a brief outline of a hypothesis that may account for an increased frequency of immune disease in left-handers and in their families.