**3. Genetic and epigenetic basis of disease in ANCA vasculitis**

Consistent with improved mechanistic studies the last decade has witnessed significant advances in our understanding of the role of both the genetic and epigenetic factors driving AAV. While a detailed description and discussion of these factors is beyond the scope of this chapter it would be remiss not to discuss several recent key studies. It is important to note that all results discussed in this section are from clinical studies. It should also be noted that while the varying genetic background of commonly used laboratory rodents may contribute to a particular pattern and severity of disease in experimental AAV, the relevance and correlation of this to human disease is less clear.

A genetic basis for AAV has long been suspected, however this was recently confirmed by a publication which demonstrated a relative risk of 1:56 for first degree relatives of patients with GPA.[25] This rate is similar to that seen in other autoimmune diseases with an established genetic component which contributes to injury. This study followed on from previous studies which had suggested a genetic link into AAV. Many of the candidate genes identified as being over represented in vasculitis patients are associated with genes which encode proteins involved in the immune system. These include several genes encoded in the human leukocyte antigen (HLA) as well as genes encoding protein tyrosine phosphatase non-receptor type 22 (*PTPN22*), cytotoxic T-lymphocyte antigen 4 (*CTLA4*), Interleukin (*IL*)-2, *PRTN3* which encodes PR3, α1 anti-trypsin (*AAT*), complement related genes, *CD18*, *IL-10*, *CD226* as well as the Fc gamma receptors; *FCGR2A*, *FCGR3B* (for both copy number high and copy number low). For a detailed review of the individual genes linked with clinical disease, the authors recommend the review by Willcocks and colleagues, whose work with Ken Smith has been instrumental in advancing knowledge in this field.[26] It is important to acknowledge that while genetic variation of these genes has been associated with an increased incidence of AAV, many of these genes display aberrant expression in several autoimmune diseases. This is not surprising considering several of these genes encode proteins critical for maintenance of the immune system, including the function of innate immune cells, T lymphocytes, B lymphocytes and regulatory cells. There are several limitations to these studies. Some studies which linked aberrant gene expression with AAV included patients with only one form of the disease (i.e. GPA, MPA, RLV or AGA), while other studies were less specific and included all patients who had detectable ANCA levels. Furthermore several of these associations were not confirmed when assessing disease in different population groups and hence results from these early studies suggested that there was, at best, a modest link between genetic background and disease.[26-27]

In this chapter we will focus on the pathogenesis of the ANCA associated vasculitides, focussing on AAV attributable to MPA and GPA. We will pay attention to the develop‐ ment of autoimmunity and concentrate on end organ injury in the kidney, a critical tar‐ get of the small vessel vasculitides. Interestingly, while GPA and MPA share many diagnostic and clinical features and patients with these diseases have been grouped to‐ gether in many clinical trials, more recent evidence including a landmark genetic study, suggests that GPA and MPA represent two different diseases. While we will discuss GPA and MPA separately, there is stronger experimental evidence linking MPO with disease. This includes several small animal studies which have confirmed pathogenic roles for cellular and humoral autoimmunity, directed against MPO, which closely re‐ semble human disease. Our discussion will focus on the disease pathogenesis of AAV and attempt to define future directions for study which ultimately may lead to therapeu‐ tic interventions. Information has been made available from human studies assessing mechanisms of disease as well as experimental studies, utilizing rodent models of vascu‐ litis. Further insights into disease pathogenesis can be gained from clinical trials, includ‐

**3. Genetic and epigenetic basis of disease in ANCA vasculitis**

Consistent with improved mechanistic studies the last decade has witnessed significant advances in our understanding of the role of both the genetic and epigenetic factors driving AAV. While a detailed description and discussion of these factors is beyond the scope of this chapter it would be remiss not to discuss several recent key studies. It is important to note that all results discussed in this section are from clinical studies. It should also be noted that while the varying genetic background of commonly used laboratory rodents may contribute to a particular pattern and severity of disease in experimental AAV, the relevance and correlation

A genetic basis for AAV has long been suspected, however this was recently confirmed by a publication which demonstrated a relative risk of 1:56 for first degree relatives of patients with GPA.[25] This rate is similar to that seen in other autoimmune diseases with an established genetic component which contributes to injury. This study followed on from previous studies which had suggested a genetic link into AAV. Many of the candidate genes identified as being over represented in vasculitis patients are associated with genes which encode proteins involved in the immune system. These include several genes encoded in the human leukocyte antigen (HLA) as well as genes encoding protein tyrosine phosphatase non-receptor type 22 (*PTPN22*), cytotoxic T-lymphocyte antigen 4 (*CTLA4*), Interleukin (*IL*)-2, *PRTN3* which encodes PR3, α1 anti-trypsin (*AAT*), complement related genes, *CD18*, *IL-10*, *CD226* as well as the Fc gamma receptors; *FCGR2A*, *FCGR3B* (for both copy number high and copy number low). For a detailed review of the individual genes linked with clinical disease, the authors recommend the review by Willcocks and colleagues, whose work with Ken Smith has been instrumental in advancing knowledge in this field.[26] It is important to acknowledge that while genetic variation of these genes has been associated with an increased incidence of AAV,

ing those with negative results.

36 Updates in the Diagnosis and Treatment of Vasculitis

of this to human disease is less clear.

In a genome wide association study with over 10 000 patients (including controls), not only was a genetic component confirmed but the antigenic specificity for AAV, i.e. for MPO or PR3 was found to have distinct genetic associations. For patients with ANCA directed against PR3, there was a strong genetic association with *HLA-DP and* genes encoding *α1-AT-SERPINA1* and *PTN3*. Conversely patients with antibodies directed against MPO showed a strong association with *HLA-DQ*.[28] The observation that there were different genetic associations for MPO-ANCA and PR3-ANCA strengthens the proposal that these diseases represented two different clinical entities. Furthermore the stronger genetic component to PR3 related disease identified in earlier studies was substantiated.

An epigenetic basis for disease has also been proposed. Neutrophil levels of the chroma‐ tin modification protein complex, H3K27me3, required for gene silencing were decreased in patients with AAV, at both the MPO and PR3 loci. This phenomenon was dependent on the transcription factor encoding gene, RUNX3. Interestingly RUNX3 message was found to be decreased in patients with AAV compared to healthy controls. These studies provided the first evidence that epigenetic modifications present in AAV patients could impair gene silencing and result in aberrant expression of the target auto-antigens, MPO and PR3.[29] These recently published genetic and epigenetic studies have added consid‐ erably to our understanding of AAV.
