DNA polymorphisms and linkage relationship of the human complement component C6, C7, and C9 genes

Complement C7 deficiency presenting as recurrent aseptic meningitis

BACKGROUND

Complement deficiency states are rare inherited disorders that may predispose affected individuals to angioedema, collagen vascular disease, or infection due to encapsulated organisms, especially Neisseria meningitidis.

OBJECTIVES

To report the case of a 36-year-old man of Irish descent with recurrent culture-negative neutrophilic meningitis, to offer potential reasons for the inability to recover a causative pathogen, and to review the genetics and prevalence of complement deficiency states, the methods of screening for such deficiencies, the features of meningococcal infection as they relate to such deficiencies, and management strategies for clinicians caring for patients with such deficiencies.

METHODS

The patient presented in 1988 and again in 2002 with culture-negative neutrophilic meningitis. His second episode was characterized by a rash suggestive of meningococcal infection, prompting immunologic evaluation.

RESULTS

Immunologic evaluation revealed an undetectable CH50 level. Levels of C1, C2, and C5 through C9 were normal except for C7, which was undetectable. Further testing revealed that the patient's sister was also C7 deficient.

CONCLUSIONS

Complement component deficiencies are relatively rare; individuals with collagen vascular disease and systemic neisserial infection should be screened using either the CH50 or the APH-50 assay. Key to the management of a late-complement component-deficient host is counseling, education about meningococcal infection, and discussions about the potential benefits of chemoprophylaxis and immunoprophylaxis. The ability to detect the bacterial cause of meningitis in such patients is organism dependent and may be influenced by factors such as cerebrospinal fluid bacterial concentration and previous antibiotic drug exposure.

A novel PCR-based technique using expressed sequence tags and gene homology for murine genetic mapping: localization of the complement genes

The complement system is a cascade of serum proteins and receptors which forms a vital arm of innate immunity and enhances the adaptive immune response. This work establishes the chromosomal localization of four key genes of the murine complement system. Mapping was performed using a novel and rapid PCR restriction length polymorphism method which was developed to exploit the murine expressed sequence tag (EST) database. This technique circumvents the laborious cDNA or genomic cloning steps of other mapping methods by relying on EST data and the prediction of exon-intron boundaries. This method can be easily applied to the genes of other systems, ranging from the interests of the individual researcher to large-scale gene localization projects. Here the complement system, probably one of the most well-characterized areas of immunology, was used as a model system. It was shown that the C3a receptor C1r and C1s genes form an unexpected complement gene cluster towards the telomeric end of chromosome 6. The second mannose binding lectin-associated serine protease gene was mapped to the telomeric end of chromosome 4, which is distinct from other complement-activating serine proteases. These results provide new insights into the evolution of this group of proteins.

C7 deficiency in an Irish family: a deletion defect which is predominant in the Irish

Human deficiencies of terminal complement components are known to be associated with increased susceptibility to Neisseria meningitidis infection. Polymorphic DNA marker studies in complement deficient investigations allow identification of haplotypes associated with the deficiency and enable the possible identification of heterozygote carriers of the defect. We report studies of an Irish family in which the index case had suffered recurrent meningococcal disease and was found to be deficient in the seventh component of complement (C7). The availability of all family members enabled us to determine the segregating haplotypes. The defects in the family segregated with two very closely related C6 and C7 DNA haplotypes, one of which is known to be associated with the large Irish C7 DNA deletion defect. The index case and two C7 deficient siblings were found to be homozygous for this defect, a deletion that spans approx. 6.8 kbp and encompasses exons 7 and 8. The deletion defect of exons 7 and 8 of C7 has been found in homozygous form in another C7 deficient Irish individual, and is present in heterozygous form in C7 deficient members of a third Irish family. Therefore, this deletion defect occurs in five of the six deficient chromosomes of these three unrelated Irish families, raising the interesting question of how prevalent this defect may be within the Irish community.

Complement component C6 and C7 haplotypes associated with deficiencies of C6

Both complete C6-deficiency (C6*Q0) and subtotal C6-deficiency (C6*SD) have been described as simple recessive traits and C6*SD has been described in combination with subtotal deficiency of the C7 coded at an adjacent locus. The trace of C6 protein found in both C6*SD traits is phenotypically indistinguishable, being smaller than normal C6 and having different isoelectric properties. A defect has been found in the C6 gene which plausibly explains the C6*SD phenotype, and this defect is also common to both C6*SD traits. We present data from seven DNA markers of the C6 and C7 genes which show that although at least four haplotypes are associated with C6*Q0, most South African C6*Q0 patients carry a common defective haplotype. The most common haplotype associated with C6*Q0 has been observed only once among unaffected haplotypes of relatives. In one family, the cases of C6*SD share a complete haplotype with both cases of combined deficiency and are probably heterozygous for this condition and complete deficiency of C6. In another family, the C6*SD is on a slightly different haplotype and C7 is normally expressed. Thus, the C6 defect is not sufficient on its own to explain the C7 deficiency in the combined deficient haplotype. The haplotype associated with the combined deficiency is found not only in normal control subjects, but also in one case of complete C6 deficiency. In this case the molecular defect seen in combined or C6*SD cases is absent.

DNA polymorphisms of the complement C6 and C7 genes

The linked C6 and C7 loci are rich in genetic markers, both at the protein and DNA levels. There are now seven common DNA polymorphisms distributed over about 300 kbp of chromosome 5p12-14. We report a new TaqI RFLP for C7 and a method for typing a C7 variant (T368S) hitherto known only from cDNA clones. We have re-investigated the published RFLPs to provide information on their frequency in North European Caucasian (predominantly British and Irish) subjects and have revised some of the published parameters, especially the sizes of polymorphic restriction fragments. Their precise locations within the genes are also reported: the three markers for C6 are in exon 3, intron 3 and adjacent to exon 17 and the four markers for C7 are in introns 15 and 13 and in exons 13 and 9. The gene frequencies of the second commonest allele of all seven markers lie in the range 0.2 to 0.37, except C6 A/B in the Japanese, where the frequencies of both common alleles are about 0.45. We have estimated the gene frequencies for the DNA polymorphisms which correlate with C7 M/N phenotype and for the C6 A/B phenotype and find them to be the same as the phenotypic estimates in Caucasians and in the Japanese respectively. The markers provide the possibility of defining 128 haplotypes, many (28) of which have been observed. Allelic associations in these genes are generally surprisingly weak.

Genetic detection of the silent allele (*Q0) in hereditary deficiencies of the human complement C6, C7, and C9 components

DNA polymorphisms (RFLPs) of the human complement component C6, C7, and C9 genes were studied in three C7-deficient (C7D) families, one C6-deficient (C6D) family, and one C9-deficient (C9D) family. The 3 loci are closely linked on human chromosome 5. The haplotypes carrying the "silent" allele (C7*Q0, C6*Q0, and C9*Q0) were defined in each family, allowing for the detection of carriers among asymptomatic relatives. This paper describes familial studies on a type of hereditary trait, characterized by recurrent Neisseria infections in individuals homozygous for "silent" alleles at the C6, C7, or C9 loci.

A novel human complement component C7 phenotype detected in South Africa and proposed designation of the allele as C7*10

A novel allotype of the seventh component of human complement was detected by isoelectric focusing. The band pattern showed a complete band-interval shift towards the cathode compared with the C7 1 allotype and it is proposed that the designation is C7 10. The allele C7*10 segregated in two South African Cape Coloured families and had a gene frequency of 0.006. It was also found to segregate together with a C6*rare allele.

Linkage between complement components 6 and 7 and glutamic pyruvate transaminase in the marsupial Monodelphis domestica

The sixth and seventh components of complement were found to be polymorphic and tightly linked in the laboratory opossum (Monodelphis domestica), as they are in eutherian mammals. In addition, strong evidence for linkage of the C6-C7 haplotype to the gene for glutamic pyruvate transaminase (GPT) was obtained for females but not for males. This result, combined with previous observations, established as a generality that recombination is severely reduced in females of this species by comparison with males. It also establishes synteny of C6-C7 and GPT in a marsupial species, as exists in mice. Because these loci are not syntenic in humans, the results imply that this synteny is ancestral to the separation of marsupials and eutherians and that it was broken relatively recently in the mammalian lineage leading to human beings. The newly described C6 and C7 polymorphisms provide additional power for developing a linkage map for M. domestica and for localizing genes that confer susceptibility to diseases for which this species is used as a model.

C7 M/N protein polymorphism typing applied to inherited deficiencies of human complement proteins C6 and C7

C7 M/N typing, the determination of the complement component C7 M/N phenotypes, was successfully used in family studies to trace haplotypes bearing C7 deficiency genes. Furthermore, it was shown to be preferable to C7 allotyping based on isoelectric focusing (IEF) since it distinguishes two common alleles (C7*M and C7*N), whereas one common C7 IEF allele (C7*1) predominates in most populations. It is also the more sensitive method, as it enabled detection of very low amounts of abnormal C7 molecules in the third generation of a combined subtotal C6/C7-deficient subject and thus confirmed that this partial deficiency gene is not silent in heterozygotes. In this respect C7 M/N typing is even more informative than DNA restriction fragment length polymorphism typing which will assess the presence but not necessarily the functional status of a gene. C6 and C7 genes are tightly linked and therefore C7 M/N typing was also applied to tracing C6 deficiency genes in families. C6/C7 haplotype analysis of South African C6-deficient (C6Q0) subjects revealed a strong allelic association of C6*Q0 and C7*M.