Two novel mutations of the nicastrin gene in Chinese patients with acne inversa

Comprehensive Catalog of Variants Potentially Associated with Hidradenitis Suppurativa, Including Newly Identified Variants from a Cohort of 100 Patients

Hidradenitis suppurativa (HS) is a chronic skin disease characterized by painful, recurrent abscesses, nodules, and scarring, primarily in skin folds. The exact causes of HS are multifactorial, involving genetic, hormonal, and environmental factors. It is associated with systemic diseases such as metabolic syndrome and inflammatory bowel disease. Genetic studies have identified mutations in the γ-secretase complex that affect Notch signaling pathways critical for skin cell regulation. Despite its high heritability, most reported HS cases do not follow a simple genetic pattern. In this article, we performed whole-exome sequencing (WES) on a cohort of 100 individuals with HS, and we provide a comprehensive review of the variants known to be described or associated with HS. 91 variants were associated with the γ-secretase complex, and 78 variants were associated with other genes involved in the Notch pathway, keratinization, or immune response. Through this new genetic analysis, we have added ten new variants to the existing catalogs. All variants are available in a .vcf file and are provided as a resource for future studies.

Genetic factors associated with hidradenitis suppurativa, a literature review

Background

Hidradenitis suppurativa (HS) is a chronic inflammatory disease characterized by deep-seated, painful lesions most frequently occurring in intertriginous areas of the skin. HS leads to poor quality of life in affected individuals and is difficult to diagnose and treat.

Objective

Understanding the genetics associated with familial inheritance may lead to a better understanding of the pathogenesis of this debilitating disease.

Methods

Articles published until March 9, 2023, were identified in PubMed using the following search terms: hidradenitis suppurativa and gene* or acne inversa and gene*.

Results

The rate of monogenic mutations associated with HS is less than 7%, with the most common genetic mutations reported in sporadic and familial HS cases being in NCSTN and less frequently in PSENEN. Individuals with mutations in the gamma-secretase complex tended to have more severe HS and an early age of onset.

Limitations

This study was limited to the case studies available in PubMed, the majority of which used targeted gene panels to detect genetic mutations.

Conclusion

Approximately 30% of individuals diagnosed with HS report having a positive family history; however, very few studies demonstrate monogenic familial transmission of HS. The case studies of syndromic HS reported a variety of genetic mutations associated with HS, some of which were familial, while others were sporadic, suggesting that other pathways may be involved in the pathogenesis of HS and other potential mutations that have yet to be evaluated. More research is needed to understand the genetic mutations in HS.

Hidradenitis suppurativa: Detangling phenotypes and identifying common denominators

Hidradenitis suppurativa (HS) is a chronic inflammatory skin disease with a severe impact on patients' quality of life through its recurrent and painful nature, as well as its comorbidity burden. The shift in the pathogenic paradigm from a condition of the apocrine glands to an autoinflammatory disease associated with follicular destruction has rendered its understanding difficult, as there are still large gaps in pinpointing the underlying mechanisms, which cannot currently explain the existing clinical variation and as a result, translate into suboptimal therapy. Multifactorial involvement is hypothesized, with an implication of genetic mutations, microbiome dysbiosis, cytokine upregulation and environmental factors. Clinical observation is fundamental for diagnosis, however, the marked heterogeneity in presentation leads to delays in detection and challenges in treatment selection, showcasing clear limits in defining the link between genetic aspects of HS, the role of epigenetic factors and its pathogenic pathways. There have been attempts to formulate phenotypes that could aid in prognostication and management, however, current classification schemata show significant overlap and no validation through longitudinal studies. In this context, nomenclature poses a great challenge due to the lack of global agreement in the definition of lesions, which should be addressed by future research to enable simplified recognition and allow for more precise severity scoring. This could be complemented by the addition of extra dermatologic findings or paraclinical assessment in constructing phenotypes. The development of valid, predictive and reliable classifications of HS may lead to an improvement in comprehending its pathophysiology, favouring a more personalized approach in management. This could be achieved through consensus in the characterization of clinical features and data gathering, as well as validation attempts for described phenotypes. Ultimately, the genotype-endotype-phenotype correlation in HS requires targeted, systematic inquiries and should be addressed more largely to broaden the perspective on this debilitating entity.

Rare missense variants in the SH3 domain of PSTPIP1 are associated with hidradenitis suppurativa

Hidradenitis suppurativa (HS) is a chronic, debilitating skin disease for which few treatment options are available. While most HS is sporadic, some rare kindred show a high-penetrance, autosomal-dominant inheritance. We wanted to identify rare variants that could contribute to HS risk in sporadic cases using candidate gene sequencing. We ultimately identified 21 genes for our capture panel. We included genes of the γ-secretase complex (n = 6) because rare variants in these genes sometimes cause familial HS. We added Notch receptor and ligand genes (n = 13) because γ-secretase is critical for processing Notch receptor signaling. Clinically, some people with PAPA (pyogenic arthritis, pyoderma gangrenosum, and acne) syndrome, a rare inflammatory disease, have concurrent HS. Rare variants in PSTPIP1 are known to cause PAPA syndrome, so we included PSTPIP1 and PSTPIP2 in the capture panel. We screened 117 individuals with HS for rare variations and calculated the expected burden using Genome Aggregation Database (gnomAD) allele frequencies. We discovered two pathogenic loss-of-function variants in NCSTN. This class of NCSTN variant can cause familial HS. There was no increased burden of rare variations in any γ-secretase complex gene. We did find that individuals with HS had a significantly increased number of rare missense variants in the SH3 domain of PSTPIP1. This finding, therefore, implicates PSTPIP1 variation in sporadic HS and further supports dysregulated immunity in HS. Our data also suggests that population-scale HS genetic research will yield valuable insights into disease pathology.

Variations in genetics, biology, and phenotype of cutaneous disorders in skin of color - Part I: Genetic, biologic, and structural differences in skin of color

Skin of color (SOC) populations include those who identify as Black/African, Hispanic/Latinx, Asian/Pacific Islander, American Indian/Native Alaskan, Indigenous Australian, Middle Eastern, biracial/multiracial, or non-White; this list is far from exhaustive and may vary between and within cultures. Recent genetic and immunological studies have suggested that cutaneous inflammatory disorders (atopic dermatitis, psoriasis, and hidradenitis suppurativa) and malignancies (melanoma, basal cell carcinoma, and cutaneous T-cell lymphoma) may have variations in their immunophenotype among SOC. Additionally, there is growing recognition of the substantial role social determinants of health play in driving health inequalities in SOC communities. It is critically important to understand that social determinants of health often play a larger role than biologic or genetic factors attributed to "race" in health care outcomes. Herein, we describe the structural, genetic, and immunological variations and the potential implications of these variations in populations with SOC. This article underscores the importance of increasing the number of large, robust genetic studies of cutaneous disorders in SOC to create more targeted, effective therapies for this often underserved and understudied population. Part II of this CME will highlight the clinical differences in the phenotypic presentation of and the health disparities associated with the aforementioned cutaneous disorders in SOC.

The Genomic Architecture of Hidradenitis Suppurativa-A Systematic Review

Hidradenitis suppurativa is a chronic, suppurative condition of the pilosebaceous unit manifesting as painful nodules, abscesses, and sinus tracts mostly in, but not limited to, intertriginous skin. Great strides have been made at elucidating the pathophysiology of hidradenitis suppurativa, which appears to be the product of hyperkeratinization and inflammation brought about by environmental factors and a genetic predisposition. The identification of familial hidradenitis suppurativa has sparked research aimed at identifying underlying pathogenic variants in patients who harbor them. The objective of this review is to provide a broad overview of the role of genetics in various aspects of hidradenitis suppurativa, specifically the pathophysiology, diagnosis, and clinical application.

Evolutionary distinct roles of γ-secretase subunit nicastrin in zebrafish and humans

BACKGROUND

Mutations in the genes that encode the human γ-secretase subunits Presenilin-1, Presenilin Enhancer Protein 2, and Nicastrin (NCSTN) are associated with familial hidradenitis suppurativa (HS); and, regarding Presenilin Enhancer Protein 2, also with comorbidity for the hereditary pigmentation disorder Dowling-Degos disease.

OBJECTIVE

Here, the consequences of targeted inactivation of ncstn, the zebrafish homologue of human NCSTN, were studied.

METHODS

After morpholino (MO)-mediated ncstn-knockdown, the possibilities of phenotype rescue through co-injection of ncstn-MO with wildtype zebrafish ncstn or human NCSTN mRNA were investigated. Further, the effects of the co-injection of a human missense, nonsense, splice-site, and frameshift mutation were studied.

RESULTS

MO-mediated ncstn-knockdown resulted in a significant reduction in melanophore morphology, size and number; and alterations in their patterns of migration and distribution. This phenotype was rescued by co-injection of zebrafish ncstn RNA, human NCSTN RNA, or a construct encoding the human NCSTN missense mutation p.P211R.

CONCLUSION

Human NCSTN mutations encoding null alleles confer loss-of-function regarding pigmentation homeostasis in zebrafisch. In contrast, the human missense mutation p.P211R was less harmful, asserting sufficient residual ncstn activity to maintain pigmentation in zebrafish. Since fish lack the anatomical structures affected by HS, our data suggest that the zebrafish ncstn gene and the human NCSTN gene have probably acquired different functions during evolution. In fish, one major role of ncstn is the maintenance of pigmentation homeostasis. In contrast, one of the roles of NCSTN in humans is the prevention of inflammatory processes in the adnexal structures of the skin, as seen in familial HS.

Insights from γ-Secretase: Functional Genetics of Hidradenitis Suppurativa

Hidradenitis suppurativa (HS) is a chronic, relapsing, and remitting inflammatory disease of the skin with significant heritability and racial disposition. The pathogenesis of HS remains enigmatic, but occlusion of the terminal hair follicle and dysregulation of the local innate immune response may contribute to pathogenesis. Genetic predisposition might also contribute to disease susceptibility and phenotypic heterogeneity because mutations in γ-secretase have been found to underlie a minor but characteristic subset of patients with HS. In this review, we synthesized the current data on γ-secretase in HS, evaluated its importance in the context of disease pathobiology, and discussed avenues of future studies.

Hidradenitis suppurativa in East and Southeast Asian populations: a systematic review and meta-analysis

Hidradenitis suppurativa (HS) is a chronic inflammatory skin condition that presents with recurrent inflammatory nodules and draining tunnels in the skin. Most HS studies have focused on Western populations, and the understanding of how HS characteristics differ in specific Asian ethnicities is poor. We conducted the first systematic review and meta-analysis to characterize HS patients from East and Southeast Asia. PubMed, Embase, and Ovid MEDLINE databases were searched from inception to June 12, 2020. English-language case-series, cross-sectional, observational, and randomized controlled trial studies investigating HS in East and Southeast Asian populations were screened by titles, abstracts, and articles in duplicate. Of 136 citations, 10 studies were included in the meta-analysis. Data on gender distribution, lesion distribution in the axilla and gluteal regions, and family history were extracted in duplicate. A random effects model was used for the meta-analysis. A total of 30,125 HS patients were included in the analysis. Most patients were male (66%, 95% CI = 60-72%). About half of Asian patients with HS develop lesions in the axilla (52%, 95% CI = 33-72%) and the buttocks (48%, 95% CI = 38-57%). Only a small subset had positive family history of HS (5%, 95% CI = 2-8%). We report an up-to-date characterization of HS in East and Southeast Asian populations and highlight differences in their Western counterparts. These results will hopefully improve understanding for how HS may manifest, lead to more personalized treatments for Asian patients with HS, and usher in a proper patient-centered approach to treating the disease.

Yin and Yang: A disrupted skin microbiome and an aberrant host immune response in hidradenitis suppurativa

The skin microbiome plays an important role in maintaining skin homeostasis by controlling inflammation, providing immune education and maintaining host defense. However, in many inflammatory skin disorders the skin microbiome is disrupted. This dysbiotic community may contribute to disease initiation or exacerbation through the induction of aberrant immune responses in the absence of infection. Hidradenitis suppurativa (HS) is a complex, multifaceted disease involving the skin, innate and adaptive immunity, microbiota and environmental stimuli. Herein, we discuss the current state of HS skin microbiome research and how microbiome components may activate pattern recognition receptor (PRR) pathways, metabolite sensing pathways and antigenic receptors to drive antimicrobial peptide, cytokine, miRNA and adaptive immune cell responses in HS. We highlight the major open questions that remain to be addressed and how antibiotic therapies for HS likely influence both microbial burden and inflammation. Ultimately, we hypothesize that the two-way communication between the skin microbiome and host immune response in HS skin generates a chronic positive feed-forward loop that perpetuates chronic inflammation, tissue destruction and disease exacerbation.

Investigation of gamma secretase gene complex mutations in German population with Hidradenitis suppurativa designate a complex polygenic heritage

BACKGROUND

Hidradenitis suppurativa (HS) is a chronic inflammatory disease affecting apocrine gland-bearing skin in the axilla, groin and under the breasts. Mutations of the gamma secretase gene complex, which is essential in the activation of Notch signalling pathways, were shown in some families with HS and in a few sporadic cases. Although an imbalance in Notch signalling is implicated in the pathogenesis, the exact mechanism of HS development is yet unknown.

OBJECTIVES

We aim to investigate the genetic basis of HS by determining the presence of mutations of gamma secretase gene complex in a cohort of HS patients and by searching for a disease-causing pathogenic variant in a multi-generational HS family using parametric linkage analysis.

METHODS

Thirty-eight patients clinically diagnosed with HS were included in this study. All exons and exon-intron boundaries of the genes encoding gamma secretase complex consisting of six genes: APH1A, APH1B, PSENEN, NCSTN, PSEN1 and PSEN2 were sequenced by Sanger technique. Genetic mapping with parametric linkage analysis for the patients in the family was performed with eight affected and four healthy individuals. The logarithm of odds was calculated.

RESULTS

In a sporadic patient with early-onset, severe lesions in axilla and groin, a novel single-nucleotide deletion causing frameshift in exon 1 of the NCSTN gene was identified ((NM_015331.3): c.38delG, p.(Gly13Glufs*15)). The LOD score of 1.5 was never exceeded in any region of the genome, pointing towards intricate multi-genic inheritance pattern within the affected family.

CONCLUSIONS

The gamma secretase gene complex mutations were rare in our cohort (3.2%). Besides, our analysis indicates a possible complex multi-genic inheritance in a seemingly autosomal dominantly inherited large HS family. Genetics of both familial and sporadic HS may be complicated in most cases, and the role of other potential genes such as autoinflammatory and modifier genes as well as environmental factors may influence the pathogenesis.

γ-Secretase Genetics of Hidradenitis Suppurativa: A Systematic Literature Review

BACKGROUND

Acne inversa/hidradenitis suppurativa (HS) is a chronic, recurrent inflammatory disease of the skin that can significantly affect patients' quality of life. The etiology and pathogenesis of HS are unclear and gene mutations might play a role.

SUMMARY

The primary focus of the review is on aggregating the gene mutations reported, summarizing the structure of γ-secretase and analyzing and speculating about the mechanism and the underlying relations between gene mutation and functional changes of protein. The systematic literature review was done by searching the PubMed, Embase, and Web of Science databases. γ-Secretase is an intramembrane protease complex responsible for the intramembranous cleavage of more than 30 type-1 transmembrane proteins including amyloid precursor protein and Notch receptors. The protein complex consists of four hydrophobic proteins: presenilin, presenilin enhancer-2 (PSENEN), nicastrin, and anterior pharynx defective 1 (APH1). To date, 57 mutations of γ-secretase genes have been reported in 70 patients or families worldwide, including 39 in NCSTN, 14 in PSENEN, and 4 in PSEN1, of which 17 are frameshifts, 15 result in nonsense mutations, 13 in missense mutations, and 12 are splice site mutations. Given the structure of γ-secretase and analysis of related mutation loci of NCSTN, PSENEN, and PSEN1, mutations in γ-secretase genes could affect activation of presenilin, prevent substrate binding, and hinder intramembrane cleavage of select proteins.

Hidradenitis Suppurativa: Comprehensive Review of Predisposing Genetic Mutations and Changes

Hidradenitis suppurativa (HS) is a chronic inflammatory skin disorder. A genetic component in the pathogenesis is highly likely considering that ~30% to 40% of patients with HS report a family history of the disease. The genetic mutations related to HS that have been reported to date suggest HS can be inherited as a monogenic trait because of a defect in either the Notch signaling pathway or inflammasome function, or as a polygenic disorder resulting from defects in genes regulating epidermal proliferation, ceramide production, or in immune system function. This review provides a summary of genetic mutations reported in patients diagnosed with HS and discusses the mechanisms by which these genes are involved in its pathogenesis.

Analysis of hidradenitis suppurativa-linked mutations in four genes and the effects of PSEN1-P242LfsX11 on cytokine and chemokine expression in macrophages

Hidradenitis suppurativa (HS), or acne inversa, is a chronic inflammatory skin disorder characterized clinically with acne-like lesions in apocrine gland-bearing skin, follicular occlusion and recurrent inflammation. Thirty-four unique mutations in patients with HS have been found in three genes encoding the γ-secretase complex: nicastrin (NCSTN), presenilin 1 (PSEN1), presenilin enhancer 2 (PSENEN) and in POGLUT1, an endoplasmic reticulum O-glucosyltransferase involved in Notch signaling. We have carried out a system review and have performed a functional analysis of the 34 unique reported HS-linked mutations in NCSTN, PSEN1, PSENEN and POGLUT1. We have also examined the effects of the HS-linked PSEN1-P242LfsX11 mutation on cytokine and chemokine expression in macrophages. Mutations in NCSTN are predicted to cause loss of function, to result in loss of transmembrane (TM) domain, to affect NCSTN substrate recruitment sites, to cause loss or creation of new ligand binging sites and to alter post-translational modifications and disulfide bonds. PSEN1-P242LfsX11 occurs at the opposite side of TM5 from Alzheimer's disease-linked PSEN1 mutations. All of the PSENEN mutations occur on TM regions that are predicted to disrupt membrane function. POGLUT1 mutations lead to an early termination of protein synthesis and are predicted to affect ligand binding function. In addition, PSEN1-P242LfsX11 mediates cytokine and chemokine expression and prolongs tumor necrosis factor α production on the inflammatory processes in THP-1 cells and phorbol-12-myristate-13-acetate-differentiated macrophages in response to lipopolysaccharide stimulation. These in silico analyses are instructive for functional studies of the HS-linked mutations. The PSEN1-P242LfsX11 mutation mediates cytokine and chemokine expression in macrophages.

Dysregulation of Neuronal Iron Homeostasis as an Alternative Unifying Effect of Mutations Causing Familial Alzheimer's Disease

The overwhelming majority of dominant mutations causing early onset familial Alzheimer’s disease (EOfAD) occur in only three genes, PSEN1, PSEN2, and APP. An effect-in-common of these mutations is alteration of production of the APP-derived peptide, amyloid β (Aβ). It is this key fact that underlies the authority of the Amyloid Hypothesis that has informed Alzheimer’s disease research for over two decades. Any challenge to this authority must offer an alternative explanation for the relationship between the PSEN genes and APP. In this paper, we explore one possible alternative relationship – the dysregulation of cellular iron homeostasis as a common effect of EOfAD mutations in these genes. This idea is attractive since it provides clear connections between EOfAD mutations and major characteristics of Alzheimer’s disease such as dysfunctional mitochondria, vascular risk factors/hypoxia, energy metabolism, and inflammation. We combine our ideas with observations by others to describe a “Stress Threshold Change of State” model of Alzheimer’s disease that may begin to explain the existence of both EOfAD and late onset sporadic (LOsAD) forms of the disease. Directing research to investigate the role of dysregulation of iron homeostasis in EOfAD may be a profitable way forward in our struggle to understand this form of dementia.

Nicastrin mutations in familial acne inversa impact keratinocyte proliferation and differentiation through the Notch and phosphoinositide 3-kinase/AKT signalling pathways

BACKGROUND

Acne inversa (AI) is a chronic inflammatory skin disease with an autosomal dominant inheritance pattern. Mutations of the gene encoding nicastrin (NCSTN), a cofactor subunit of γ-secretase, are responsible for familial AI. However, whether deficiency of nicastrin is functionally implicated in the biological behaviours of human keratinocytes and related molecular mechanisms remains unknown.

OBJECTIVES

To study alterations of biological traits and related signalling pathways modulated by nicastrin knockdown in keratinocytes.

METHODS

A human immortalized keratinocyte cell line (HaCaT) was treated with efficient small interfering (si)RNA-targeted NCSTN. Cell proliferation was measured by CCK-8 assay; cell-cycle and cell apoptosis analyses were detected by flow cytometry. Microarray analysis was applied to uncover impacts of NCSTN silencing on whole-genome expression of HaCaT cells. Altered signalling pathways were further confirmed by real-time polymerase chain reaction, Western blotting and immunohistochemistry in both HaCaT cells and lesions of a patient with AI with NCSTN mutation.

RESULTS

NCSTN knockdown in HaCaT cells impaired γ-secretase activity, leading to increased cell proliferation and S-phase population. Microarray data also showed that numerous genes and pathways implicated in proliferation and differentiation of keratinocytes were statistically changed. Among these genes, expression levels of several Notch pathway molecules, known as γ-secretase substrates, were validated to be significantly attenuated in both nicastrin-silencing HaCaT cells and the lesion of the patient. Furthermore, a remarkable elevation of expression of phosphoinositide 3-kinase (PI3K), AKT and its activated form pAKT was illustrated in siRNA-treated HaCaT cells.

CONCLUSIONS

Deficiency of the NCSTN in familial AI may regulate proliferation and differentiation of keratinocytes mainly through the Notch and PI3K/AKT signalling pathways.

Aerobic and Anaerobic Bacteriology of Hidradenitis Suppurativa: A Study of 22 Cases

INTRODUCTION

Hidradenitis suppurativa (HS) is a chronic inflammatory skin disease of unclear etiology. The role of bacteria in the pathogenesis of disease remains controversial.

MATERIALS AND METHODS

Specimens were obtained from 22 HS patients by direct percutaneous needle aspiration. The collected material was cultured in aerobic and anaerobic conditions, and sensitivity tests were performed.

RESULTS

Of the 22 patients, 32% were culture negative and 68% were culture positive. A total of 16 isolates was obtained, 14 aerobic and 2 anaerobic. Aerobic bacteria were present in 86% of the specimens, whereas only anaerobic bacteria were isolated in 7%. The predominant aerobic species were Proteus mirabilis, Staphylococcus haemolyticus and Staphylococcus lugdunensis. The isolated anaerobic bacteria were Dermacoccus nishinomiyaensis and Propionibacterium granulosum.

CONCLUSION

A variety of aerobic and anaerobic bacteria was isolated from the HS lesions of our patients. In contrast to previous studies, fewer patients were found to be culture positive, and Staphylococcus aureus was isolated in only 1 of them. More studies are necessary to elucidate the controversial role of bacteria in the pathogenesis of HS.

Synaptic function of nicastrin in hippocampal neurons

Synaptic dysfunction is widely thought to play a key role in the pathogenesis of Alzheimer's disease (AD). Presenilins, the major gene products involved in familial AD, are essential for short- and long-term synaptic plasticity in mature neurons as well as for the survival of cortical neurons during aging. Presenilin and nicastrin are both indispensable components of the γ-secretase complex, but it remains unknown whether presenilin regulates synaptic function in a γ-secretase-dependent or γ-secretase-independent manner and whether nicastrin plays similar roles in central synapses. In the current study, we address these questions using an electrophysiological approach to analyze nicastrin conditional knockout (cKO) mice in the hippocampal Schaffer collateral pathway. In these mice, we found that, even at 2 mo of age, deletion of nicastrin in excitatory neurons of the postnatal forebrain using Cre recombinase expressed under the control of the αCaMKII promoter led to deficits in presynaptic short-term plasticity including paired-pulse facilitation and frequency facilitation. Depletion of Ca(2+) in the endoplasmic reticulum mimics and occludes the presynaptic facilitation deficits in nicastrin cKO mice, suggesting that disrupted intracellular Ca(2+) homeostasis underlies the presynaptic deficits. In addition, NMDA receptor-mediated responses and long-term potentiation induced by theta-burst stimulation were decreased in nicastrin cKO mice at 3 mo but not at 2 mo of age. Together, these findings show that, similar to presenilins, nicastrin plays essential roles in the regulation of short- and long-term synaptic plasticity, highlighting the importance of γ-secretase in the function of mature synapses.

A fast growing spectrum of biological functions of γ-secretase in development and disease

γ-secretase, which assembles as a tetrameric complex, is an aspartyl protease that proteolytically cleaves substrate proteins within their membrane-spanning domain; a process also known as regulated intramembrane proteolysis (RIP). RIP regulates signaling pathways by abrogating or releasing signaling molecules. Since the discovery, already >15 years ago, of its catalytic component, presenilin, and even much earlier with the identification of amyloid precursor protein as its first substrate, γ-secretase has been commonly associated with Alzheimer's disease. However, starting with Notch and thereafter a continuously increasing number of novel substrates, γ-secretase is becoming linked to an equally broader range of biological processes. This review presents an updated overview of the current knowledge on the diverse molecular mechanisms and signaling pathways controlled by γ-secretase, with a focus on organ development, homeostasis and dysfunction. This article is part of a Special Issue entitled: Intramembrane Proteases.

Hidradenitis suppurrativa (acne inversa) as a systemic disease

Hidradenitis suppurativa (HS), also known as acne inversa, is a chronic follicular occlusive skin disorder characterized by recurrent abscesses, draining sinuses, and scarring, with a multifactorial pathogenesis. The answer to the question whether HS may be considered a systemic disease relies on the presence of accompanying systemic manifestations, on the proof of association with other diseases or conditions, and on the occurrence of systemic implications. We address these questions based on a systemic review of the existing literature. There are several reports in the literature of the coexistence of HS with other diseases, including pyoderma gangrenosum, PASH syndrome, Adamantiades-Behcet's disease, spondylarthropathy, Crohn's disease, SAPHO, pachyonychia congenita, Dowling-Degos disease, and the keratitis-ichthyosis-deafness (KID) syndrome. Case series exist only for Crohn's disease, while most other reports are anecdotal, thus, not providing high-quality scientific evidence. Based on well-designed studies, HS has been associated with the metabolic syndrome and with excess body weight or obesity. The link between HS and systemic associations may be attributed to common genetic or environmental factors or shared inflammatory pathways.

Impaired Notch-MKP-1 signalling in hidradenitis suppurativa: an approach to pathogenesis by evidence from translational biology

Recent findings in familial hidradenitis suppurativa (HS) demonstrated loss-of-function mutations of components of the γ-secretase (GS) complex leading to decreased protease cleaving activity, which may compromise canonical Notch signalling. Appropriate Notch signalling is of pivotal importance for maintaining the inner and outer root sheath of the hair follicle and skin appendages. This viewpoint on the pathogenesis of HS is primarily supported by circumstantial evidence derived from translational biology. Impaired Notch signalling is proposed to be the major pathogenic mechanism of HS. Deficient Notch signalling switches the fate of outer root sheath cells, resulting in conversion of hair follicles to keratin-enriched epidermal cysts. Impaired Notch signalling may compromise apocrine gland homoeostasis as well. Damage-associated molecular pattern molecules released by either ruptured epidermal cysts exposing keratin fibres or altered structural components of less maintained apocrine glands may both stimulate TLR-mediated innate immunity. All aggravating factors of HS, that is, smoking, obesity, skin occlusion, androgens and progesterone, may further promote inflammation by release of proinflammatory cytokines derived from activated monocyte/macrophages. Inappropriate Notch signalling may not only initiate inflammation in HS but may lead to insufficient feedback inhibition of overstimulated innate immunity. Regular Notch signalling via induction of MAPK phosphatase-1 (MKP-1) terminates TLR-MAPK-signalling in macrophages and IL-23 secreting DCs, the key players for Th17 cell polarization. Thus, impaired Notch signalling links HS to other Th17-driven comorbidities. All major therapeutic interventions in HS appear to attenuate increased MAPK activation of innate immune cells due to impaired Notch-mediated feedback regulation of innate immunity.

A novel splice site mutation in NCSTN underlies a Japanese family with hidradenitis suppurativa

BACKGROUND

 Hidradenitis suppurativa (HS) is a chronic follicular occlusive disease with characteristic recurrent draining sinuses, skin abscesses and disfiguring scars, mainly involving the axilla, groin, perianal and perineal regions. While most HS cases are nonfamilial, familial cases showing autosomal dominant inheritance have been reported. Recently, loss-of-function mutations in the genes encoding γ-secretase have been identified as a cause of familial HS in the Chinese and British populations.

OBJECTIVES

 To identify mutations in the genes encoding γ-secretase in Japanese patients with familial and nonfamilial HS.

METHODS

 Two affected and three unaffected individuals from a Japanese family with familial HS and nine patients with nonfamilial HS were recruited. We conducted mutation analysis of the γ-secretase genes in Japanese patients with familial and nonfamilial HS.

RESULTS

  A novel splice site mutation in the nicastrin gene NCSTN, one of the six key component genes encoding γ-secretase, was identified in the patients with familial HS. Neither unaffected individuals in the family nor 100 ethnically matched control alleles carry this mutation. None of the nine patients with nonfamilial HS carry nonsense, frameshift or splice site mutations in this gene.

CONCLUSIONS

  A novel splice site mutation, c.582+1delG, in NCSTN was identified in the familial patients with HS. We also reveal for the first time that a γ-secretase gene mutation is not linked to the development of nonfamilial HS. These results would further pave the way to a better understanding of the contribution of γ-secretase and other genes to the pathogenesis of HS and to the development of a new therapeutic strategy for HS.

Major carboxyl terminal fragments generated by γ-secretase processing of the Alzheimer amyloid precursor are 50 and 51 amino acids long

OBJECTIVES

To understand the cleavage of the amyloid β protein (Aβ) precursor (APP) by γ-secretase and to determine its changes in a representative familial Alzheimer disease (FAD) mutation.

METHODS

Transfected cells expressing wild-type and FAD mutant APP were analyzed for changes in the levels of the major secreted Aβ species and of the corresponding intracellular C-terminal APP fragments (APP intracellular domain, AICD) generated by γ-secretase, whereas radio-sequencing was used to precisely identify the resulting cleavage site(s).

RESULTS

The AICD fragment(s) generated by γ-secretase cleavage comigrated in gels with a 50-residue synthetic peptide used as control, which is smaller than the 59 and 57 residues predicted from Aβ ending at positions 40 (Aβ40) and 42 (Aβ42), respectively. In agreement with previous findings, an FAD mutant form of presenilin 1 (PS1-M139V) significantly increased the longer Aβ42 while showing trends toward reducing Aβ40. AICD levels were reduced by the mutation, suggesting that γ-secretase activity may be actually impaired by the mutation. Radiosequence analysis in cells expressing wild-type PS1 detected γ-secretase cleavage sites at the Aβ peptide bond L(49)-V(50) to generate a 50-amino acid (aa) AICD fragment (AICD50) and the Aβ peptide bond T(48)-L(49), generating an AICD of 51 aa (AICD51). No other cleavage sites were reliably detected.

CONCLUSIONS

Based on findings that the FAD mutation that increases Aβ42 also reduces AICD, we propose that γ-secretase activity is impaired by FAD mutations and predict that physiologic and environmental agents that inhibit γ-secretase will actually induce AD pathogenesis rather that prevent it. Furthermore, we propose that the cleavage site to generate AICD is naturally ragged and occurs predominantly at two sites 48 and 49 aa from the start of the Aβ sequence. Thus, end specific antibodies to these two sites will need to be generated to study the quantitative relationships between these two cleavages in sporadic AD and FAD.

Acne inversa (Hidradenitis suppurativa): A review with a focus on pathogenesis and treatment

Acne inversa (AI) is a disabilitating chronic inflammatory disease with major negative impact on quality of life and significant co-morbidities. This is an important link to insights into immune dysfunction, which stimulated therapeutic approaches like tumor necrosis-α inhibitor therapy. This new off-label drug treatment is particularly beneficial when used in combination with wide excision of inflamed skin and subcutaneous tissue. Retinoids have been reported to be helpful in secondary prevention. The standard of therapy in advanced cases is surgery with wide excisions and healing by secondary intention. This treatment results in significant reduction of complaints and achieves satisfactory body contouring.

Loss of RBPj in postnatal excitatory neurons does not cause neurodegeneration or memory impairments in aged mice

Previous studies suggest that loss of γ-secretase activity in postnatal mouse brains causes age-dependent memory impairment and neurodegeneration. Due to the diverse array of γ-secretase substrates, it remains to be demonstrated whether loss of cleavage of any specific substrate(s) is responsible for these defects. The bulk of the phenotypes observed in mammals deficient for γ-secretase or exposed to γ-secretase inhibitors are caused by the loss of Notch receptor proteolysis. Accordingly, inhibition of Notch signaling is the main cause for untoward effects for γ-secretase inhibitors as therapeutics for Alzheimer's disease. Therefore, we wished to determine if loss of canonical Notch signaling is responsible for the age-dependent neurodegeneration observed upon γ-secrectase deficiency in the mouse brain. We generated postnatal forebrain-specific RBPj conditional knockout (cKO) mice using the CamKII-Cre driver and examined behavior and brain pathology in 12-18 month old animals. Since all four mammalian Notch receptor homologues signal via this DNA binding protein, these mice lack canonical Notch signaling. We found that loss of RBPj in mature excitatory neurons was well tolerated, with no evidence for neurodegeneration or of learning and memory impairment in mice aged up to 18 months. The only phenotypic deficit we observed in the RBPj-deficient mice was a subtle abnormality in olfactory preferences, particularly in females. We conclude that the loss of canonical Notch signaling through the four receptors is not responsible for age-dependent neurodegeneration or learning and memory deficits seen in γ-secretase deficient mice.

Two novel mutations of the NCSTN gene in Chinese familial acne inverse

BACKGROUND

Acne inversa (AI; MIM 142690), or hidradenitis suppurativa (HS), is a type of autosomal-dominant genodermatosis caused by mutations in γ-secretase. The complex of γ-secretase is a transmembrane protease that catalyses the cleavage of a set of membrane proteins and is comprised of four subunits encoded by four genes, including PSEN1, PSENEN, NCSTN and APH1. However, mutations associated with AI vary significantly, and it is important to define the specific mutation with a particular AI patient.

OBJECTIVE

To determine specific mutations in the γ-secretase gene associated with two Chinese AI families.

METHODS

Two families of three generations with apparent AI symptoms were examined through proband analysis. Genomic DNAs of the family members and a cohort of 100 healthy individuals were isolated and subjected to polymerase chain reaction (PCR) and direct DNA sequencing.

RESULTS

Two heterozygous missense mutations, c.647A>C (p.Q216P) in the exon 6, and c.223G>A (p.V75I) in the exon 3 of the NCSTN gene, were identified in the two families respectively. No mutations were found in 100 healthy individuals.

CONCLUSIONS

We have identified two novel mutations within the NCSTN gene associated with AI.

Familial frontotemporal dementia-associated presenilin-1 c.548G>T mutation causes decreased mRNA expression and reduced presenilin function in knock-in mice

Mutations in the presenilin-1 (PSEN1) gene are associated with familial Alzheimer's disease and frontotemporal dementia (FTD). Interestingly, neuropathological analysis of a Belgian FTD family carrying a PSEN1 c.548G>T mutation confirmed neurodegeneration in the absence of amyloid plaques. To investigate the impact of the c.548G>T mutation on presenilin-1 (PS1) function in vivo, we introduced this mutation into the genomic Psen1 locus. The resulting c.548G>T knock-in (KI) mice are viable but express markedly lower levels of Psen1 mRNA and protein in the brain. This reduction is due to production of aberrantly spliced transcripts lacking either exon 6 or exons 6 and 7 and their subsequent degradation via non-sense-mediated decay (NMD); inhibition of NMD by cycloheximide treatment stabilized these transcripts and restored the level of Psen1 mRNA in KI/KI brains. Interestingly, the reduction of Psen1 mRNA expression and the degradation of aberrant Psen1 splice products occur exclusively in the brain but not in other tissues. Consistent with decreased Psen1 expression, γ-secretase activity was strongly reduced in the cerebral cortex of KI mice, as measured by de novo γ-secretase-mediated cleavage of APP and Notch. Moreover, PS1 expressed from Psen1 cDNA carrying the c.548G>T mutation displayed normal γ-secretase activity in cultured cells, indicating that the corresponding p.183G>V amino acid substitution does not affect γ-secretase activity. Finally, Psen1 c.548G>T(KI/KI);Psen2(-/-) mice exhibited mild spatial memory deficits in the Morris water maze task. Together, our findings demonstrate that the c.548G>T mutation results in a brain-specific loss of presenilin function due to decreased Psen1 mRNA expression.