Genes and genetics of murine systemic lupus erythematosus

Introduction

Systemic lupus erythematosus (SLE) is inherited as a multifactorial trait with genetic predisposition playing a major role. Consequently, there has been considerable interest in defining the genetics of this disease. Progress in this area has advanced tremendously over the past few years, resulting in the identification of more than 100 genes associated with lupus risk variants. This effort has been greatly facilitated by studies in the mouse, which because of a seemingly natural susceptibility to lupus, an immune system and genomic composition similar to humans, and the relative ease of manipulating its genes and immune system, has made it the de facto model species of choice. Accordingly, there has been an increasing body of complementary information on the genetics of lupus in mouse models with over 190 genes associated with increased susceptibility and at least 100 other genes that when modified inhibit disease. This chapter will provide an overview of the model systems used in defining genetic predisposition to lupus and briefly describe the major mechanistic pathways implicated.

Mouse models of lupus used in genetic studies

Models of lupus can be classified into three general types: naturally occurring, genetically modified, and induced. In the first group, lupus arises spontaneously as a polygenic trait sometimes associated with a major risk variant, for example, the Fas ^lpr in MRL- Fas ^lpr mice and the Yaa (TLR7 duplication on the Y-chromosome) BXSB males. The modified group consists of genetically manipulated mice that spontaneously develop lupus because of an engineered mutation. Their background can be either nonautoimmune or lupus predisposed with the latter used to detect less potent lupus variants. Genetically modified lupus mice provide the means to define for a single genetic change the relevant pathways and cell types critical for disease pathogenesis.

The spontaneous group includes strains, recombinant inbred (RI) mice, and related congenics and is genetically heterogeneous ( Table 34.1 ). The most commonly studied strains or crosses are the MRL- Fas ^lpr , female (NZBxNZW) F1 hybrid, and male BXSB, which all develop hypergammaglobulinemia, antinuclear antibodies, and glomerulonephritis (GN). The MRL- Fas ^lpr also develops a mild arthritis and accumulates CD4 ⁻ CD8 ^– (double negative, DN) T cells, NZB acquires autoimmune hemolytic anemia, and a marked CD11b ⁺ MHCII ^- monocytosis is found in BXSB mice. (NZWxBXSB)F1 males produce high autoantibody titers to phospholipids and platelets and develop antiphospholipid syndrome and immune thrombocytopenia. Importantly, the distinct characteristics of individual lupus models clearly demonstrate the central role genetics can play in lupus susceptibility. Notably, each exhibits a unique lupus phenotype with consistent disease course, autoantibody profiles, and types of end-organ pathology.

Several RI lines, generated from autoimmune and normal strains, also develop spontaneous lupus. These include RI lines derived from NZB and NZW strains (NZM2410 and NZM2328) and the BXD2 line derived from the nonautoimmune C57BL/6 (B6) and DBA/2 strains. The latter is unique in developing severe erosive arthritis associated with rheumatoid factor and anti-cyclic citrullinated peptide (CCP) in addition to lupus and is an example of the presence of significant lupus genetic variants in so-called normal strains. Interval-specific congenic mice containing susceptibility loci have been instrumental in defining the contribution of specific loci and in identifying candidate genes. A notable example is the B6. Sle set of congenics that contain individually or in combination the full length or subintervals of the three Sle1–3 loci.

The genetically modified group, generated by transgenic, knockout, knockin, or mutagenic modification of genes in nonautoimmune and autoimmune background strains, is already the largest and continues to expand in part because many are serendipitously discovered to exhibit features of lupus. Genetic manipulation models yield not only insights into the potential contribution of individual genes, alone or combined with other predisposing alleles, but also a way to directly examine human SLE risk variants. Importantly, compilation of these genes provides a scaffold for constructing a model of the central pathways in lupus immunopathology.

The induced models allow more rapid testing of genetic variants and modified genes for their effects on systemic autoimmunity. The most common models used for this purpose include exposure of mice to tetramethylpentadecane (pristane) or mercury and chronic graft-versus-host disease. Susceptibility in the induced models is strain-dependent and genetic studies have produced results concordant with the spontaneous-occurring models.

Predisposing loci and genes in natural-occurring lupus models

Over 80 loci distributed over autosomal and sex chromosomes have been identified in natural-occurring models by directed and genome-wide approaches. Some loci, identified in different strains, appear to represent the same variant, whereas most are unique. Susceptibility in each model therefore appears to be most likely the consequence of a few, but different, loci rather than a large number of common ones. Many of these loci have been confirmed in interval congenic mice, including Sle1 , Cgnz1 , Nba2 , Bxsb1–4 , Sle16 (chromosome (chr) 1); Sle18 (chr 3); Sle2, Adnz1, Lbw2 , and Lmb1 (chr 4); Lmb2 (chr 5); Sle3, Sle5, Nba5, Lmb3 (chr 7); Lmb4 (chr 10); Ssb2 (chr 12); Sgp3 (chr 13); and Sles1 (chr 17) (reviewed in Ref.[7]).

Sle16 , a 129-derived chromosome 1 locus, will be briefly mentioned because of its implication for studies of lupus in genetically modified mice. Congenic mice containing the 129- Sle16 interval on the B6 background despite being generated from nonautoimmune strains were found to develop autoantibodies and even GN. As gene knockout mice are commonly generated using 129 embryonic stem cells followed by backcrossing to the B6 strain, the resulting 129/B6 mixed background can sometimes develop lupus independent of the modified gene. Awareness of this problem has led to greater attention to reducing the possibility of lupus arising from background genes and the use of B6 embryonic stem cells.

Several lupus risk variants have been identified in naturally occurring models ( Table 34.2 ). These include genes involved in apoptosis ( Fas, Fasl ), lymphocyte activation ( Ly108, Fcgr2b, Cr2 ), transcription regulation ( Pbx-1d ), oxidative metabolism ( Essrg ), the cell cycle ( Cdkn2c ), actin dynamics ( Coro1a ), TLR-mediated cell activation ( TLR7, Yaa ), and antigen presentation ( H-2 ).

Several general conclusions about the genetics of mouse models of lupus can be deduced. Notably, the contribution of individual lupus variants on disease manifestations is context dependent. As an example, for even highly penetrant lupus alleles, such as Fas ^lpr and Yaa , other susceptibility genes are required to determine the type of autoimmune disease, i.e., nephritis or autoimmune hemolytic anemia, and to develop tissue pathology. Most lupus variants have small effect sizes and many require the presence of other risk alleles for autoimmunity to develop, making characterization a challenge. Additive and epistatic interactions are common and a single locus often consists of a cluster of loci. Candidate gene selection is complicated by the large number of genes that could potentially influence lupus. Defining how a risk variant promotes autoimmunity is often difficult because of expression in multiple potentially relevant immune cell types. Finally, like human SLE, risk variants are often located in noncoding regions that manifest as alterations in gene expression, splicing, or other functions. Taken together, they support the presence of a large heterogeneous pool of lupus variants.