Apeced (APS1)

Excerpt from the Master´s thesis by Mikaela Mutru, (Med. Student)


Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED, OMIM 240300), also known as autoimmune polyendocrinopathy syndrome type 1 (APS-1), is a rare disorder caused by mutations in the autoimmune regulator (AIRE) gene located on locus 21q22.3 (2-4). Over 60 different disease-causing mutations have been identified so far, and the syndrome is generally recessively inherited except for a single dominant inheritance pattern with incomplete penetrance that has been recognized in an Italian family (5,6). 


The gene defect results in a combination of endocrine and non-endocrine disorders and a susceptibility to chronic mucocutaneous candidiasis (CMC); the classic triad of components being hypoparathyroidism, Addison’s disease and CMC. Possible additional features include hypothyroidism, gonadal insufficiency, type I diabetes, keratitis, vitiligo, intestinal dysfunction and many others. The exact combination, onset time and severity of the disorders vary greatly from patient to patient as does the percentage of people affected by APECED between populations;  the prevalence is particularly high in Finns (1:25 000), Sardinians (1:14 000) and Iranian Jews (1: 9000). (7-10)


The AIRE gene contains 14 exons that encode a 545-amino acid protein with several motifs characteristic of transcriptional factors (e.g. a nuclear targeting signal, two PHD-type zinc fingers and a proline-rich region) (2,3).  The exact mechanism by which it regulates gene expression is still unclear though several theories have been suggested: working as a complex with other transcription factors such as CREB-binding protein (CBP), promoting polyubiquitylation of transcription factors by E3 ubiquitin ligase activity, and binding of hypomethylated histone 3 tails (11-13). AIRE is expressed mainly in the thymus by medullary thymic epithelial cells (mTECs) but also by cells of monocyte/dendritic cell line and to some extent in peripheral lymphoid organs such as lymph nodes, spleen and fetal liver (2,14). 


It is generally thought that the main function of AIRE is to partake in the negative selection of developing lymphocytes in the thymus by upregulating expression of tissue-restricted antigens (TRAs) in mTECs. Thus a mutation that renders AIRE dysfunctional enables the  release of autoreactive lymphocytes out of thymus, and these cells are primarily responsible for the development of the varied disorders seen in APECED (15,16). Discovery of TRAs expressed in mTECs independent of AIRE, lack of disorders typical to APECED in AIRE-deficient thymomas and the unproven mechanism of AIRE’s gene regulation have caused competing and complementing theories to be put forth (15,17,18). These include AIRE’s influence on the late differentiation of mTECS – causing lack of TRA expression and disruption of thymic microenvironment when deficient – and the involvement of AIRE-positive cells of the peripheral lymphoid tissue in establishing peripheral tolerance (17-19). Dysfunction of the regulatory T cell population is also seen in APECED patients, and might contribute to the inability to prevent harmful autoimmune reactions (20,21).


 Like the role of AIRE in creating self-tolerance, the exact pathogenic mechanisms of APECED remain in dispute; a plausible theory is that the initial damage to the affected organs would be due to autoreactive T-cell-mediated immune response. This would contribute to increased presentation of tissue-specific antigens by antigen-presenting cells like dendritic cells, causing further activation of CD4+ T-cells and autoantibody production by B-cells. The tissue destruction would lead to inflammation and release of cytokines, which in APECED patients leads to production of anticytokine antibodies. The anticytokine antibodies would be a secondary balancing reaction; a protective measure against the autoimmune reaction. (16) It has been argued though that this theory cannot explain the high prevalence (practically 100%) and early appearance (can precede even the first perceived symptoms and organ-specific antibodies) of the high-titer anticytokine antibodies (7,19).


Autoantibodies in APECED

The causal relation between the self-reactive antibodies seen in APECED patients and the array of disorders is still somewhat unclear; the antibodies might have a pathogenic effect or merely be a mark of ongoing tissue destruction carried out by T-cell mediated immune response (19). They are, however, a distinct part of the syndrome and in many cases specific enough to be used as a diagnostic measure (7). The autoantibodies fall into two categories: antibodies against cytokines, mainly type I interferons and interleukins related to Th-17 -cells, and organ-specific antibodies (e.g. against enzymes in endocrinal pathways) (22).


 The most common anticytokine antibodies in APECED are directed against type I interferons and Th17-related interleukins. Their presence can precede even the first clinical symptoms and appear in very high titers, up to and exceeding 1: 1,000,000. In APECED patients antibodies targeting IFN-ω, IFN-α and IFN-β have prevalences of 100%, 95% and 22%, respectively. Transient, low-titer autoantibodies against type III interferon (IFN-λ) have also been detected in some patients. IFN-α, IFN-β and IFN-ω production in cells like lymphocytes and dendritic cells is induced by viral infection and stimulation of Toll-like receptors, but no particular susceptibility to viral infections is seen in APECED patients. This could be due to the autoantibodies’ neutralizing ability being weaker in vivo than shown by in vitro assays, compensation by other interferons rarely targeted by autoantibodies (e.g. IFN-γ) or the circulating autoantibodies’ (IgG, IgM) lack of access to their targets when the interferons are produced locally and in high concentrations as a response to a viral infection. (22,23) The pathogenicity of the interferon-autoantibodies is uncertain despite their prevalence, but in theory autoantibodies against IFN-α might have a protective influence against systemic lupus erythematosus (SLE) (16).


 In contrast to anti-IFN antibodies, antibodies against interleukins 17 and 22 (IL-17A, IL-17F and IL-22) correlate strongly with a single component of APECED, chronic mucocutaneous candidiasis (CMC). CMC is often the first disorder to appear in APECED, and its prevalence approaches 100% with age – the prevalence of antibodies against IL-22, IL-17F and IL-17A are 91%, 75% and 41%, respectively. IL-17 and IL-22 are produced by Th17-cells and have potent antifungal properties, and reduction in their production correlates with the presence of autoantibodies in APECED patients. (23) Further proof for the link between CMC and autoantibodies against IL-17 and IL-22 comes from thymoma patients, in whom these autoantibodies are found rarely and usually only in association with CMC (24). AIRE dysfunction might also contribute to the CMC susceptibility more directly, as it has been found to take part in dectin-1 signaling pathway which is activated in phagocytes as a response to fungal and mycobacterial infections (25). Similarly to the possible protection anti-IFN-α antibodies might give against SLE, it has been suggested that autoantibodies against Th17-related interleukins might protect against psoriasis in APECED patients (16).


 The organ-specific antibodies so far found in APECED patients are almost always targeted towards intracellular molecules (26). The degree to which the discovered autoantibodies correlate with the failure of the targeted organ varies, but some of them are of clinical importance because their appearance often precedes detectable organ failure (27). For example in APECED-related Addison’s disease, three steroidogenic enzymes have been recognized as autoantibody targets: p450c21, p450c17 and p450sc (28,29). In a study by Söderbergh et al (28) 79% of APECED patients had Addison’s disease, and 84% of these patients’ sera contained autoantibodies towards one or several of the three enzymes – and in another study by Betterle et al (30) almost all of adrenocortical autoantibody-positive APECED-patients without Addison’s disease developed it within a few years. In addition, two of the enzymes, p450c17 and p450scc, are specific autoantibody targets for APECED-related Addison’s disease, and are not found in relation to solitary autoimmune Addison’s disease or Addison’s disease as it appears in autoimmune polyendocrine syndrome type 2 (APS2) (16).


 As for the other endocrinal disorders seen in APECED, autoantibody targets have been found for hypoparathyroidism, hypothyroidism, gonadal failure, type 1 diabetes mellitus and pituitary failure. Some have also been discovered in relation to the non-endocrine components of APECED: atrophic gastritis and pernicious anemia, autoimmune hepatitis, intestinal dysfunction, nephropathy, vitiligo, alopecia areata and pulmonary disease. Interestingly, there is some overlap between the antibodies and targeted organs, and many of the antibodies are not very sensitive for the correlating disorders even when taking into account that they may appear before detectable organ failure. There are still several APECED components such as keratitis, nail dystrophy, enamel hypoplasia and asplenia, where a plausible autoantibody target or pathogenetic mechanism is yet to be discovered. (16,23,28)


 Despite the fact that many of the autoantibodies found in APECED patients target enzymes with critical roles in neurotransmitter pathways (e.g. glutamic acid decarboxylase (GAD) involved in γ-aminobutyric acid (GABA) synthesis), neurologic manifestations of the syndrome are rare (7,31). Besides pituitary failure (e.g. growth hormone deficiency, diabetes insipidus) that is seen in a few percent of the patients (30), and neurological symptoms due to dysfunction of other endocrine organs, only solitary cases of neural involvement have been reported. Most notably these include a total of 6 patients with progressive limb-girdle myopathy (32) and a unique case of cerebellar dysfunction with gait ataxia where autoantibodies directed against Purkinje cells and neurons in the brain stem could be demonstrated in the patient’s serum (33). In a study by Fetissov et al (31), the sera of some APECED patients with autoantibodies against enzymes involved in neurotransmitter synthesis were shown to stain a variety of neurons in rodent and primate brain. A few of the patients in this study displayed neurological symptoms similar to Parkinson's disease, stiff-person syndrome or Tourette syndrome (31).





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(29) Krohn K, Uibo R, Aavik E, Peterson P, Savilahti K. Identification by molecular cloning of an autoantigen associated with Addison's disease as steroid 17 alpha-hydroxylase. Lancet 1992 Mar 28;339(8796):770-773.


(30) Betterle C, Greggio NA, Volpato M. Clinical review 93: Autoimmune polyglandular syndrome type 1. Journal of Clinical Endocrinology & Metabolism 1998 Apr;83(4):1049-1055.


(31) Fetissov SO, Bensing S, Mulder J, Le Maitre E, Hulting AL, Harkany T, et al. Autoantibodies in autoimmune polyglandular syndrome type I patients react with