The Concept of Immunogenetics.

Genetics plays a major role in shaping the immune responses in both physiological and pathological states such as psoriasis, alopecia areata, and other immune-mediated dermatological conditions. The genes encoding the elements of the immune system and its regulators are among the most polymorphous loci in the genome. Subtle variations in these genes can thus alter the balanced defensive responses of the immune system and make an individual liable to diseases and environmental triggers. Immunogenetics deals with finding the precise set of liability genes involved in the pathogenesis of specific complex diseases. In this chapter, we will briefly discuss the basic principles of genetic polymorphisms, the methods used in scanning these polymorphisms, and the strategies employed to find the role of these polymorphisms in complex diseases.

The Immunogenetics of Alopecia areata.

Alopecia areata (AA) is an autoimmune disease that targets the hair follicles (HF) and results in non-scarring hair loss. AA results from the collapse of the HF's immune privilege due to a combination of environmental and genetic factors that either change the local HF dynamics or dysregulate the central immune tolerance. Multiple genetic studies have attempted to identify AA susceptibility genes through candidate gene approaches and genome-wide analysis. These studies were able to show an association between AA and multiple immune-related genes such as those encoding cytokines, chemokines, molecules involved in regulatory T-cell functions, and adaptor molecules along with genes involved in autophagy, melanogenesis, and hair cycling pathways. This chapter aims to explore these genes and their contribution to the pathogenesis of the AA.

The Immunogenetics of Systemic Sclerosis.

Systemic sclerosis (SSc) is a rare disease with a prevalence ranging from 7 to 700 cases per million. Like with most autoimmune diseases, both environmental and genetic factors are involved in the pathogenesis of the SSc. Though the incidence of SSc in the family members of those affected and the concordance rate in twins is very low, inheritance is still the strongest risk factor of SSc. Thus, multiple studies have been conducted to identify the genes responsible for this inheritance including candidate gene association studies and genome-wide analyses. Variations and mutations in the genes encoding cytokines, adhesion molecules, and signaling proteins involved in the interaction between endothelial cells, fibroblasts, and immune cells have been found to be associated with SSc susceptibility. In this chapter, these genes and their contribution to the pathogenesis of the SSc are discussed in detail. These genes are categorized into five major groups of HLA genes, genes involved in the innate immune responses, genes affecting adaptive immune responses, genes with a role in the fibrogenesis pathways, and apoptosis, autophagy, and pyroptosis-related genes.

Immunogenetics of Lupus Erythematosus.

Lupus erythematosus (LE) is a heterogeneous disease with a wide range of manifestations ranging from localized lesions in cutaneous lupus erythematosus (CLE) to severe disseminated disease in systemic lupus erythematosus (SLE).Lupus results from a complex interaction between genetic and epigenetic backgrounds and environmental triggers that cause loss of tolerance to self-antigens and the formation of autoantibodies. Genetic susceptibility plays a key role in the pathogenesis of lupus erythematosus. In most cases, multiple common alleles with modest effect sizes are combined to result in the polygenic inheritance of the disease but monogenic variants of lupus have also been described. Genes from the innate and adaptive immune system along with genes involved in apoptosis and immunoglobulin clearance have been linked to SLE. This chapter aims to explore the functions of these genes and their contribution to the pathogenesis of the disease.

The Overlap between Genetic Susceptibility to COVID-19 and Skin Diseases.

Coronavirus disease 2019 (COVID-19) mainly affects the respiratory system, but the involvement of other organ systems has also been commonly reported. Acute acro-ischemia or chilblain like lesions were among the first recognized dermatological presentations of COVID-19. Though the occurrence of such lesions has been attributed to the similar interferon-1 mediated immune response in both COVID-19 and systemic lupus erythematosus, we propose another possible explanation based on a common genetic background. In a recent genome-wide association study, the 3p21.31 region was found to be associated with COVID-19 severity. This region also contains the TREX1 gene. Missense mutations of the TREX1 gene are responsible for familial chilblain lupus and its genetic polymorphisms have been implicated in the pathogenesis of systemic lupus erythematosus. Based on this observation, herein we have reviewed other COVID-19 risk loci for potential overlap with dermatological conditions.

Macrophage migration inhibitory factor polymorphism (rs755622) in alopecia areata: a possible role in disease prevention.

Alopecia areata (AA) is an organ-specific autoimmune disease that targets the bulb of the hair follicles and results in non-scarring hair loss that can range from patchy lesions to involvement of the entire scalp. AA develops when the hair follicles lose their physiologic state of immune privilege. One of the key factors that help in maintaining this immune privilege by suppressing natural killer cells is macrophage migration inhibitory factor (MIF). Surprisingly, MIF is also known to provoke autoimmunity by upregulating cytokines. To address this dilemma and understand the exact nature of the involvement of MIF in disease pathogenesis we investigated the association of MIF gene polymorphisms (- 173 G > C, rs755622) with AA by conducting a case-control study of 274 subjects. We observed that the frequency of the C allele in the patients was significantly lower than the control group (0.15, 0.23, respectively, p = 0.01) and the combined frequencies of the CC and GC genotypes (dominant Mendelian pattern) had the most prevalent difference between the two groups (odds ratio 0.60, 95% confidence interval 0.36-0.99; p = 0.048).Since the C allele is associated with higher MIF transcription levels, this could infer that MIF is more likely to attribute to the preservation of the immune privilege rather than acting as a proinflammatory factor.

The Association between Genetic Variation in Wnt Transcription Factor TCF7L2 (TCF4) and Alopecia Areata.

Background: Alopecia areata (AA) is a non-scarring hair loss with a polymorphous presentation ranging from patchy lesions to involvement of the entire scalp. The disease is the consequence of an autoimmune attack on hair bulbs that results in a premature transition of hair follicles to catagen and telogen. Thus the Wnt/ß-catenin signaling pathway that regulates the hair cycling might be involved in the pathogenesis of AA. Genetic variations in the components of Wnt/ß-catenin could greatly alter their adaptive mechanisms against an immunologic attack. Objectives: Our aim was to investigate the association between AA and genetic polymorphisms in the TCF7L2 gene, one of the most important components of the Wnt/ß-catenin pathway. Methods: This is a case-control study of 145 patients with AA and 152 healthy controls. Genotyping of the TCF7L2 gene (rs7903146) was performed via the ARMS-PCR method (amplification refractory mutation system- polymerase chain reaction). The allele and genotype distribution was compared between the two groups. Results: The frequency of the T allele (0.38 vs. 0.28, odds ratio = 1.56, 95% CI = 1.09-2.17, p = 0.013) and TT + CT genotypes (0.68 vs. 0.53, odds ratio = 1.88, 95% CI = 1.17-3.02, p = 0.008) were significantly higher in AA patients. Conclusions: This study indicates that the TCF7L2 gene variant is associated with AA. Its contribution to disease pathogenesis could either be through a hair cycling defect or dendritic cell dysregulation.