Genetics of complement deficiencies associated with lupus-like syndromes

The genetic basis of systemic lupus erythematosus: What are the risk factors and what have we learned

The genome-wide association study is a free-hypothesis approach based on screening of thousands or even millions of genetic variants distributed throughout the whole human genome in relation to a phenotype. The relevant role of the genome-wide association studies in the last decade is undisputed because it has permitted to elucidate multiple risk genetic factors associated with the susceptibility to several human complex diseases. Regarding systemic lupus erythematosus (SLE) this approach has allowed to identify more than 60 risk loci for SLE susceptibility across populations to date, increasing our understanding on the pathogenesis of this disease. We present the latest findings in the genetic of SLE across populations using genome-wide approaches. These studies revealed that most of the genetic risk is shared across borders and ethnicities. Finally, we focus on describing the most important risk loci for SLE attempting to cover the genetic findings in relation to functional polymorphisms, such as missense single nucleotide polymorphisms (SNPs) or regulatory variants involved in the development of the disease. The functional studies try to identify the causality of some GWAS-associated variants, many of which fall in non-coding regions of the genome, suggesting a regulatory role. Many loci show an environmental interaction, another aspect revealed by the studies of epigenetic modifications and those associated with genetic variants. Finally, new-generation sequencing technologies can open other paths in the research on SLE genetics, the role of rare variants and the detailed identification of causal regulatory variation. The clinical relevance of the genetic factors will be shown when we are able to use them or in combination with other molecular measurements to re-classify a heterogeneous disease such as SLE.

The combination of complement deficiency and cigarette smoking as risk factor for cutaneous lupus erythematosus in men; a focus on combined C2/C4 deficiency

BACKGROUND

Although deficiencies in the early components of the complement system were among the first identified genetic risk factors for systemic lupus erythematosus (SLE), only a few studies addressed their significance in patients with cutaneous LE (CLE). Among environmental factors, it was postulated that cigarette smoking might intervene in the pathogenesis of LE.

OBJECTIVES

To describe the clinical and biological features of patients with CLE and a complement deficiency. A secondary objective was to assess cigarette smoking in patients with CLE.

PATIENTS AND METHODS

A retrospective study including all patients diagnosed as having LE between 1995 and 2003 in the Dermatology Department of Strasbourg University Hospital. Patient charts were reviewed and those patients in whom a C4 and/or C2 deficiency was diagnosed were included. Two patients with a combined C2/C4 deficiency were analysed in detail.

RESULTS

There were 48 females and 37 males (F/M ratio = 1.3), with a mean age of 41 years at diagnosis; 73% of the patients had chronic LE and 27% subacute CLE. Among 32 screened patients, 24 patients with a mean age of 36 years had a complement deficiency; 17 had a C4A deficiency, five a C4B deficiency and two a combined C4A/C2 deficiency. A high proportion (58%) of these patients was male; 82% of the patients were smokers. This was especially true in males: 94% were smokers compared with 69% of females.

CONCLUSIONS

Partial deficiency of C4, C2 or C4 and C2 is a common finding in patients with CLE. Most male patients with CLE are smokers. It is thus suggested that the combination of cigarette smoking and complement deficiency could be a risk factor for LE in men.

Points of control in inflammation

Inflammation is a complex set of interactions among soluble factors and cells that can arise in any tissue in response to traumatic, infectious, post-ischaemic, toxic or autoimmune injury. The process normally leads to recovery from infection and to healing, However, if targeted destruction and assisted repair are not properly phased, inflammation can lead to persistent tissue damage by leukocytes, lymphocytes or collagen. Inflammation may be considered in terms of its checkpoints, where binary or higher-order signals drive each commitment to escalate, go signals trigger stop signals, and molecules responsible for mediating the inflammatory response also suppress it, depending on timing and context. The non-inflammatory state does not arise passively from an absence of inflammatory stimuli; rather, maintenance of health requires the positive actions of specific gene products to suppress reactions to potentially inflammatory stimuli that do not warrant a full response.

Autoimmunity, polyclonal B-cell activation and infection

It is widely believed that autoimmunity is an integral part of the immune system, and that genetic, immunologic, hormonal, environmental and other factors contribute to the pathogenesis of autoimmune disease. Thus, autoimmune disease may represent an abnormal expression of immune functions instead of loss of tolerance to self, and it can be organ specific or systemic in its manifestations. We review the various factors that contribute to the development of autoimmune disease; we also review the mechanisms of polyclonal B-cell activation, with emphasis on the role of infectious agents. We consider systemic lupus erythematosus in humans and in experimental animals as prototypic autoimmune disease, and we summarize data to indicate that polyclonal B-cell activation is central to the pathogenesis of systemic autoimmune disease. The effect of polyclonal B-cell activation, brought about by injections of a B-cell activator-lipopolysaccharide from Gram-negative bacteria-is sufficient to cause autoimmune disease in an immunologically normal host. In fact, autoimmune disease can be arrested if excessive polyclonal B-cell activation is suppressed; alternatively, autoimmune disease can be exacerbated if polyclonal B-cell activation is enhanced. We explore the mechanism of tissue injury when autoimmune disease is induced or exacerbated, and we consider the pathogenic roles of autoantibodies, immune complexes, complement, the blood cell carrier system, and the mononuclear phagocyte system. Although polyclonal B-cell activation may be the mechanism whereby various factors can cause or exacerbate systemic autoimmune disease, polyclonal B-cell activation may cause autoimmune disease on its own.

Deficiency of C4A is a genetic determinant of systemic lupus erythematosus in three ethnic groups

Systemic lupus erythematosus (SLE) has shown associations with the major histocompatibility complex (MHC) class II DR antigens and class III complement components C2 and C4 in previous studies. The primary susceptibility locus has been difficult to identify, however, on account of linkage disequilibrium within the MHC. We have studied C4A and C4B distributions in 63 Caucasoid, 75 Chinese and 51 Japanese SLE patients. All three populations showed a statistically significant increase in C4A*Q0 (null) alleles when compared with 323 ethnically matched controls. We conclude that complete or partial deficiency of C4A is a genetic determinant of SLE common to these three ethnically distinct populations.

SLE-like and sicca symptoms in late component (C9) complement deficiency

Hereditary deficiencies in early and late complement components are well known to predispose to SLE-like syndromes or recurrent infection. Hitherto reported C9 deficient cases have usually been healthy subjects, however, and it is not considered that C9 deficiency is associated with any specific disease. We describe a completely C9 deficient patient with possible Sjögren's syndrome and discuss the relationship.

Inherited disorders of complement

Isolated complement component deficiencies are uncommon. Deficiencies of all eleven components and two inhibitors of the classical pathway have been described. Complete absence of the components of the alternative pathway has not been described. The consequences of a single defect in complement are often predictable from an understanding of the biologic activities associated with activation of the complement system. Deficiency of C1 esterase inhibitor gives rise to the disease, hereditary angioedema; deficiency of the early components of the classical pathway are associated with lupus erythematosus; C3 and C3 inactivator deficiencies with pyogenic infections; C5 dysfunction with Leiner's disease; deficiencies of the terminal components with recurrent Neisseria bacteremia; and C9 deficiency with normal health. The complement system and its associated biologic activities are reviewed. The present knowledge of the inherited complement deficiencies and associated diseases, with particular emphasis on the dermatologic manifestations, genetics, and diagnosis, is summarized.

Inherited complement component deficiencies in membranoproliferative glomerulonephritis

Anecdotal reports of complement component deficiencies in patients with immune complex disease led to a systematic study of the levels of seven complement components in serum specimens from 178 patients with glomerulonephritis and 163 normal subjects. Deficiencies were found with significantly higher frequency (22.7%) among 44 patients with membranoproliferative glomerulonephritis (MPGN) types I and III, than among the normal subjects (6.7%, P less than 0.002) or among 134 patients with other glomerulonephritides (5.2%, P less than 0.001). The component deficiencies in MPGN were partial in nine patients and subtotal in one. They could not be ascribed to acquired hypocomplementemia or to a nephrotic syndrome. They were present over long periods, were found in family members, and involved C2, C3, factor B, C6, C7, and C8. Six were presumably the result of null structural genes, two were associated with a structurally abnormal component, and two were of unknown cause. The results give evidence that partial deficiency of one or more complement components is a factor predisposing to MPGN.

An inherited defect in the C3 convertase, C3b,Bb, associated with glomerulonephritis

The control of the amplification C3 convertase, C3b,Bb, of the serum complement system has been found to be defective in five members of a family spanning three generations. One of the five has membranoproliferative glomerulonephritis (MPGN) type III and another has mild idiopathic rapidly progressive glomerulonephritis. The defect is manifested by low serum concentrations of C3 and usually factor B with normal levels of the proteins which control the convertase, H and I. C3 nephritic factor (C3NeF) was not demonstrable. Enhanced C3 conversion was produced by the incubation of their serum at 37 degrees C for 30 min. This conversion was further accelerated by incubation after increasing the serum magnesium concentration by increments ranging from 0.25 to 1.9 mM. Incremental additions of H to serum depleted of H indicated that the amplification convertase of affected family members required more H for its inhibition than did that of normal subjects. This requirement was reduced by the addition of purified normal C3 but not by the addition of purified C3 of the propositus. It is postulated that affected family members are heterozygous for a gene producing an abnormal C3 which, as a constituent of the amplification convertase, C3b,Bb, confers resistance to H. Investigation of this apparently nephritogenic defect may provide insight into the pathogenesis of these glomerulonephritides.

Complement studies of sera and other biologic fluids

The complement system consists of at least 15 proteins whose sequential activation, split products, and interaction with other plasma proteins and cells are important in inflammation. Measurement of complement is useful in many rheumatic and immunologic diseases. Most attention has focused on low levels usually due to immune complex disorders. A logical starting point is measurement of the total hemolytic complement, CH50; if this is low, one can see which component is involved. If only one component concentration is decreased, an inherited defect may be present; multiple low levels usually reflect an acquired process. Serial levels of CH50, C4, and C3 are particularly useful in monitoring patients with systemic lupus erythematosus and vasculitis. Complement may also be involved in effusions.

Some recent advances in HLA and skin diseases

During the past 15 years, much has been learned about the human major histocompatibility complex. These new findings have been applied to the whole field of medicine, including transplantation immunology, blood transfusions, association with disease, human genetics, and the mechanism of human immune responses. In clinical medicine, the association between the major histocompatibility complex and various diseases has attracted the greatest interest. In this paper, we summarize the basic information on the human histocompatibility complex and its association with dermatologic disorders. Data from the world literature are reviewed and tabulated. Our own investigations on psoriasis, atopic dermatitis, and epidermolysis bullosa are discussed in detail.

Familial discoid lupus erythematosus associated with heterozygote C2 deficiency

Two siblings with chronic discoid lupus erythematosus and several family members were found with heterozygous C2 deficiency. An association with histocompatibility markers HLA-B18 and HLA-Dw2 was demonstrated, and the slow allotype of factor B was present. Linkage studies in this family suggested a close linkage between the C2 deficiency gene and genes coding for B18, Dw2, and BfS antigens. One HLA-ACB/DBf recombinant was observed showing closer linkage between HLA-D and Bf than between HLA-B and Bf.

Linkage relationship of C2 deficiency, HLA and glyoxalase I loci

Immunogenetic analysis of a homozygous C2-deficient individual and family members demonstrated linkage of HLA-A25, B18 and C2o. HLA-D typing showed that 5 members typed with homozygous Dw2 typing cells from an individual with C2 deficiency but not with Dw2 typing cells from 2 individuals with normal C2. The homozygous C2-deficient propositus and brother were HLA-A and B homozygous but heterozygous at the HLA-D and glyoxalase I loci. Therefore, in this family, the C2o gene is linked with two distinct haplotypes: HLA-A25, B18, Dw2, GLO1 and HLA-A25, B18, D unknown, GL02. These results could be explained by an ancestral recombinant event, which occurred between the C2o locus and HLA-D locus in which C2o segregated with HLA-B. This would suggest that the locus for the C2o gene maps between HLA-B and HLA-D on the sixth chromosome.

Serum complement abnormalities in the antinuclear antibody-positive relatives of children with systemic lupus erythematosus

Serum C3, C4 and total hemolytic complement (CH50) levels were measured for 21 children with systemic lupus erythematosus (SLE) and 81 first degree relatives. The mean serum C4 and CH50 levels of the 12 relatives with antinuclear antibodies (ANA( were depressed to levels equal to those of the index cases. A similar depression was not found for C3, nor was there a depression of C3, C4, or CH50 in the relatives without ANA. If preexistent depression of C4 levels can be documented in the ANA-positive relatives of index cases, it may provide an explanation for the inherited predisposition to SLE in some families.