The role of the ATP2C1 gene in Hailey–Hailey disease

Abstract

Hailey–Hailey disease (HHD) is a rare autosomal dominant acantholytic dermatosis, characterized by a chronic course of repeated and exacerbated skin lesions in friction regions. The pathogenic gene of HHD was reported to be the ATPase calcium-transporting type 2C member 1 gene (ATP2C1) located on chromosome 3q21–q24. Its function is to maintain normal intracellular concentrations of Ca²⁺/Mn²⁺ by transporting Ca²⁺/Mn²⁺ into the Golgi apparatus. ATP2C1 gene mutations are reportedly responsible for abnormal cytosolic Ca²⁺/Mn²⁺ levels and the clinical manifestations of HHD. Environmental factors and genetic modifiers may also affect the clinical variability of HHD. This article aims to critically discuss the clinical and pathological features of HHD, differential diagnoses, and genetic and functional studies of the ATP2C1 gene in HHD. Further understanding the role of the ATP2C1 gene in the pathogenesis of HHD by genetic, molecular, and animal studies may contribute to a better clinical diagnosis and provide new strategies for the treatment and prevention of HHD.

Introduction

Hailey–Hailey disease (HHD, OMIM 169600), also known as familial benign chronic pemphigus, is an autosomal dominant acantholytic genodermatosis with complete penetrance [1–3]. It was first reported by the Hailey brothers in 1939 [4]. The disease usually develops in the third or fourth decade with an incidence estimated to be 1:50,000–1:40,000 [2, 5, 6]. Two-thirds of all HHD patients have family histories [1], and both genders are affected equally [7]. Genetic studies on HHD have been comparatively productive over the past two decades. The ATPase calcium-transporting type 2C member 1 gene (ATP2C1, OMIM 604384) was discovered to be the disease-causing gene responsible for HHD [8, 9]. To date, at least 177 mutations in the ATP2C1 gene have been identified as the causes of HHD (Human Gene Mutation Database and Leiden Open Variation Database v.3.0) [10]. In vitro experiments and animal models promote the understanding of the relationship between the ATP2C1 gene and HHD [11–16]. This review provides an overview of HHD clinical and pathological features, differential diagnoses, and genetic and functional studies of the ATP2C1 gene in HHD, including causative mutations and genetically modified animal models.

Clinical and pathological features and differential diagnoses of Hailey–Hailey disease

The typical skin lesions of HHD usually occur in friction or intertriginous regions, including the neck, axillae, groin, perineum, and the submammary area. It may form blisters, erosions or scales with a foul-smelling exudate [2, 17]. Approximately 71% of HHD patients have longitudinal white bands in their fingernails [2]. The typical distribution of this genodermatosis usually occurs in a comparatively symmetrical pattern, exhibiting a generalized and bilateral involvement (Fig. 1a) [18]. In a few unusual cases, two distinct segmental patterns of mosaic manifestations can be distinguished from typical HHD (Table 1) [18–21]. A case report of type 1 segmental manifestation of HHD described skin lesions that always occurred in a segmental pattern on the left side of the body, but nowhere on the right. No family history of skin disorders was observed, and the patient’s daughters had no skin diseases (Fig. 1b) [19]. A case of type 2 segmental manifestation of HHD reported that the unilateral linear lesions occurred initially at the age of three months and persisted into adulthood with frequent aggravations. The eruption then evolved and nonsegmental bilateral skin lesions of HHD developed in body folds (Fig. 1c). Affected family members had nonsegmental HHD [22, 23]. The distinction between type 1 and type 2 segmental mosaics is of the utmost importance in genetic counseling. Type 1 segmental HHD patients have a slightly increased risk of having children with nonsegmental HHD. For type 2, this risk is 50% [24]. Type 2 segmental skin lesions are notoriously difficult to treat [25]. The three types of HHD have a chronic course of repeated remissions and aggravations [17]. Lesions are often triggered or aggravated by minor trauma, friction, heating, humidity, secondary infections, etc. [2, 17]. Squamous cell carcinomas have been observed in several HHD cases [26–29].

Fig. 1.

Schematic representation of Hailey–Hailey disease. a The distribution of typical HHD usually occurs in a comparatively symmetrical pattern, exhibiting a generalized and bilateral involvement. b The type 1 segmental HHD reflects a localized postzygotic mutation that occurs at an early stage of embryogenesis, resulting in heterozygosity and a segmental cutaneous disease. c The type 2 segmental HHD reflects a germline mutation in combination with somatic loss of heterozygosity in embryos, which develops a nonsegmental, diffuse distribution of skin lesions with a homozygous or hemizygous state of the underlying mutation in more severely affected segmental areas. d Histopathology section of HHD, showing acantholysis resulted from disruption of cell–cell contacts, which appears like a dilapidated brick wall (H&E stain; ×100; Olympus BX53, Japan)

Table 1.

Two types of mosaic manifestations of Hailey–Hailey disease

	Clinical manifestation	Family history	Mutation type	References
Type 1 segmental HHD	Segmental distribution of skin lesions	No	Postzygotic de novo mutation	[19]
Type 2 segmental HHD	Rather severely affected segmental lesions being superimposed on typical nonsegmental skin lesions	Yes	A germline mutation in combination with somatic loss of heterozygosity	[18, 21]

Histopathology of skin lesions in HHD shows an intercellular split of the epidermal suprabasal layers (acantholysis) resulting from disruption of cell–cell contacts, with an appearance like a dilapidated brick wall (Fig. 1d) [3, 17, 30]. Ultrastructural studies show perinuclear aggregation of keratin filaments, and keratinocytes remain loosely linked by adhesion structures [3, 31]. Desmosomal proteins (i.e., desmoplakin I and II) lose their peripheral, dotted patterns and stain diffusely in the cytoplasm of most acantholytic cells [31, 32].

The diagnoses of HHD are usually not difficult being based on family histories, clinical manifestations, histologic and immunofluorescent examinations, and genetic analyses. The differential diagnosis of HHD often includes conditions such as Darier disease (DD, OMIM 124200), pemphigus vulgaris (PV, OMIM 169610), and relapsing linear acantholytic dermatosis (Table 2). DD is an autosomal dominant genodermatosis caused by the ATP2A2 gene (OMIM 108740), which encodes sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPases (SERCA2) [33]. Typical clinical features of DD include warty papules and plaques in seborrheic and flexural regions, notches at the distal end of the nails, and palmoplantar pits [34, 35]. DD may also present as type 1 segmental disease caused by a localized postzygotic mutation, or type 2 segmental disease caused by a germline mutation in combination with postzygotic loss of heterozygosity [36, 37]. The symptoms of DD usually present in childhood and peak at puberty [38]. It may be accompanied by neuropsychiatric abnormalities, such as depression, mental retardation, bipolar disorder, and epilepsy [39, 40]. Sweating, heating, stress, and ultraviolent radiation may exacerbate DD [35]. Histologically, it is characterized by hyperkeratosis, suprabasal acantholysis, and eosinophilic dyskeratotic cells, known as corps ronds and grains [38, 39]. In PV, initial lesions usually present on the oral mucosa [41]. Histologically, it is characterized by suprabasal acantholysis and intraepithelial blisters. Direct immunofluorescent examination often shows IgG deposits in the epithelium [41]. Relapsing linear acantholytic dermatosis is clinically and histopathologically similar to HHD, and the skin lesions are along the lines of Blaschko [42, 43].

Table 2.

Differential diagnoses of Hailey–Hailey disease

	Responsible or related gene(s)	Age of onset	Skin lesions	Accompanied by neuropsychiatric disorders	Histologic examination	References
HHD	ATP2C1	The third or fourth decade	Blisters, erosions, or scale in friction or intertriginous regions	Occasionally reported	Dilapidated brick wall	[8, 90]
DD	ATP2A2	Childhood and peak at puberty	Warty papules in seborrheic and flexural regions, notches at the distal end of the nails, and palmoplantar pits	More prevalent	Hyperkeratosis, suprabasal cleavage, and eosinophilic dyskeratotic cells	[38, 40]
PV	HLA, ST18	The fifth or sixth decade	Flaccid mucosal bullae leading to erosions	High prevalence in a small sample study from India	Suprabasal acantholysis and intraepithelial blisters	[91–93]
Relapsing linear acantholytic dermatosis	N/A	4–81 years old	Similar to HHD and along the lines of Blaschko	N/A	Similar to HHD	[42, 43]

HLA Human leukocyte antigen, ST18 Suppression of tumorigenicity 18, N/A Not available

The ATP2C1 gene and protein

In 1994, the disease gene for HHD was first mapped to a 14-cM region on chromosome 3q21–q24 by linkage analysis [44]. Gene mapping was subsequently narrowed to a 5-cM region flanked by D3S1589 and D3S1290 [45]. In 2000, mutations in the ATP2C1 gene encoding secretory pathway Ca²⁺/Mn²⁺-ATPase (SPCA1) were reported to be responsible for HHD [8, 9].

The ATP2C1 gene contains 27 exons [46]. In addition to the four distinct splice isoforms (SPCA1 1a–d, corresponding to ATP2C1 1a–d) produced by alternative processing of the ATP2C1 pre-mRNA reported in most literature [11, 40], five other splice isoforms (SPCA1 1e–f and SPCA1 2a–c, corresponding to ATP2C1 1e–f and ATP2C1 2a–c) are also recorded in NCBI (https://www.ncbi.nlm.nih.gov/gene/27032; gene ID 27032) and UniProt (http://www.uniprot.org/uniprot/P98194; UniProt ID P98194). Of these nine splice isoforms, SPCA1 1a (NP_055197.2) is the “canonical” sequence containing 919 amino acids. SPCA1 2a (NP_001186109.1) is the longest isoform containing 973 amino acids. SPCA1 1c is an aberrant Ca²⁺ pump with limited functional capability due to the absence of exon 27, which results in the disruption of transmembrane 10 (M10) [11, 46]. SPCA1 is one of three type II phosphorylation (P)-type Ca²⁺ transport ATPases, consisting of ten hydrophobic transmembrane segments (M1–10) and three cytosolic domains, which are the actuator domain (A), the phosphorylation domain (P), and the nucleotide-binding domain (N) [47].

SPCAs include isoforms SPCA1 and SPCA2 [48]. SPCA1 is ubiquitously expressed in mammalian cells [49], and localized to the Golgi apparatus in keratinocytes with a high expression level and at variable levels in other human tissues [9, 14]. The precise location of SPCA1 is in tubular noncompact zones that interconnect Golgi stacks and within tubular parts of the trans–Golgi network [50]. SPCA1 is critical for maintenance of the Golgi ribbon and regulation of Ca²⁺ in the Golgi [50]. It accounts for 67% of Ca²⁺ transportation in the Golgi of keratinocytes [51].

SPCA2 is mainly located in the brain and testis [48]. SPCA2 transports Ca²⁺ with much poorer affinity compared with SPCA1. It is believed to preferentially transport Mn²⁺ and confers robust tolerance to Mn²⁺ toxicity [48].

Mutations of the ATP2C1 gene

HHD was reported to be caused by haploinsufficiency of the ATP2C1 mutants [52]. There are at least 177 identified mutations distributed throughout the ATP2C1 gene with no apparent clustering (Fig. 2) [40, 53]. The numbers and types of the ATP2C1 gene mutations are as follows: 51 missense mutations (28.8%), 44 small deletion mutations (24.9%), 36 splicing mutations (20.3%), 25 nonsense mutations (14.1%), 12 duplication mutations (6.8%), four complex rearrangements (2.3%), three gross deletions (1.7%), and two small indel mutations (1.1%). About 37.2% (66/177) of the ATP2C1 mutations localize to only six exons (exons 12, 13, 21, 23, 24, and 25). The amino acids located at exons 12 and 23 are critical for Ca²⁺ binding, at exon 13 for phosphorylation, and at exon 21 for Mn²⁺ binding (UniProt ID P98194). M7 is encoded by sequence of exon 24 and M8 is encoded by exon 25. It is reasonable to conclude that mutations occurring in these regions may severely affect the functionality of SPCA1 [53]. Point mutations (base substitutions) account for 62.1% (110/177) of all mutations. However, no ATP2C1 mutations have been identified in a few patients with classical HHD, which may be explained by the limitations of current detection methods [54, 55].

Fig. 2.

Schematic representation of the reported ATP2C1 gene mutations in Hailey–Hailey disease. SPCA1 consists of ten transmembrane domains, a phosphorylation domain, a calcium binding site, and a magnesium binding site. All ATP2C1 mutations are described according to the ATP2C1 isoform 1a (NM_014382.3, NP_055197.2). The exon 27 of the ATP2C1 gene is not drawn to scale (as indicated by dashed lines)

More than 50% of all ATP2C1 mutations may generate premature termination codons (PTCs), leading to mRNA degradation via nonsense-mediated mRNA decay pathway or the synthesis of a truncated SPCA1 [56, 57]. Truncated mutations result in functional hemizygosity and produce a disease phenotype [7]. The high proportion of PTCs supports the argument that haploinsufficiency is the main pathogenetic mechanism for HHD [7, 58].

Hu et al. firstly identified 21 disease-causing mutations in the ATP2C1 gene in 61 unrelated kindreds, all with classical clinical and histological results [9]. Of these, the c.457C>T (p.R153*) and c.2375_2378del (p.Phe792Serfs*10) mutations were also identified in other four and five investigations, respectively [56, 59–64]. Shortly afterwards, two studies from the Welcome Trust Center for Human Genetics described 22 mutations in 20 HHD families and three sporadic cases with HHD [8, 56]. Haplotype analysis revealed a founder effect for a c.76delins24 bp mutation in two families [56]. In vitro studies showed that HHD mutant proteins L341P, C344Y, C411R, A528P, T570I, and G789R caused low expression levels of SPCA1. Normal mRNA levels and correct targeting to the Golgi suggest abnormal folding or instability of the mutated SPCA1 polypeptides, making the mutated SPCA1 sensitive to endoplasmic reticulum-mediated degradation [11, 56]. The P201L mutant had little functional effect on the enzymatic cycle of SPCA1, while I580V impeded the high-energy E1~P state to the lower energy E2-P state conformational transition [11, 65]. In the E1~P state, the enzyme binds Ca²⁺ with high affinity from the cytoplasmic side. In the E2-P state, the enzyme releases Ca²⁺ at the opposite side of the membrane as its affinity for Ca²⁺ decreases [65, 66]. SPCA1 possesses a single high-affinity ion binding and transport site, which is formed by E³⁰⁸ in M4 and N⁷³⁸ and D⁷⁴² in M6 [58]. This site in SPCA1 corresponds to site II residues in SERCA2 [11, 47, 67]. Work on Pmr1, a yeast homolog of the ATP2C1 gene, revealed that N⁷⁷⁴ and D⁷⁷⁸ in M6 play vital roles in related ion pumps [67]. Yeast D51A and D53A mutations in an N-terminal EF hand-like Ca²⁺ binding motif caused a marked decreased affinity for Ca²⁺ transport, even though the pump activity was retained [12]. D742Y and G309C mutations blocked Ca²⁺- and Mn²⁺-dependent phosphoenzyme formation from ATP, highlighting the vital role of D⁷⁴² (equivalent to D⁷⁷⁸ in Pmr1) in the structure of SPCA1 ion binding site. It also revealed a role for G³⁰⁹ in Mn²⁺ transport selectivity [11]. Mutations in the Ca²⁺ binding and translocation sites, formed by the precise juxtaposition of Ca²⁺ binding residues located in M4, M5, and M6, may block Ca²⁺ pump activity, in a manner similar to SERCAs [68]. In the type 1 segmental HHD cases, the segmental skin lesions were similarly severe to that of nonsegmental phenotype [19]. No molecular study was performed, and it was hypothesized that this type may be caused by a localized postzygotic mutation that occurs at an early stage of embryogenesis, resulting in heterozygosity and a segmental cutaneous disease [19]. In the type 2 segmental HHD cases, pronounced segmental skin lesions were superimposed on the typical nonsegmental phenotype [18]. Molecular and genetic studies revealed that the reported type 2 manifestation of HHD was caused by a germline mutation in combination with somatic loss of heterozygosity in embryos. This developed into a nonsegmental, diffuse distribution of skin lesions with a homozygous or hemizygous state of the underlying mutation in more severely affected segmental areas [18]. In addition to the identification of a splice site mutation (c.2146+1G>A) in the ATP2C1 gene within the heterozygous skin areas, the paternal wild-type allele harboring the ATP2C1 gene was consistently lost in more severely affected segmental skin areas [18].

Mutations in the ATP2C1 gene may lead to the Golgi apparatus fragmentation [50, 69], dysfunction of intra-Golgi transport [50], inefficient protein sorting [69], defective cell proliferation [50], impaired lipid handling [15], defects in post-translational processing of thyroglobulin, and endoplasmic reticulum-associated degradation of mutant thyroglobulin [70].

No obvious genotype–phenotype relationships in HHD were reported. Patients with the same mutation type in the ATP2C1 gene had various clinical features, indicating that the appearance of HHD may also be affected by intrinsic and/or extrinsic factors, such as modifier genes, high temperature, minor trauma, and pathogens [61, 71–75]. Exome sequencing of a Greek family suggested that other ATPase gene variants may affect the expressivity of HHD via increasing Golgi stress and ionic imbalance, making certain individuals more susceptible to environmental triggers [76]. A few studies reported that the more severely the SPCA1 structure or production was impaired, the more severe the clinical phenotypes were [16, 58].

The ATP2C1 gene dysfunction and its mechanism in HHD

No differences in ATP2C1 mRNA levels were found between HHD patient skin taken from the axilla and the buttock (regions prone versus resistant to blistering). The Ca²⁺ gradient in epidermis of HHD was significantly decreased compared with normal skin [9, 14]. Ca²⁺ gradient impairment in the HHD upper epidermis correlates well with acantholysis in the suprabasal layer [14]. SPCA1 dysfunction may result in an inefficient increase of external Ca²⁺ in the granular layer, failing to stabilize desmosome integrity and activate Ca²⁺-sensing receptors. Ca²⁺-sensing receptors are responsible for triggering cell-to-cell adhesion, cell differentiation in the granular layer, and reconstituting the Ca²⁺ gradient [47].

In the skin of HHD patients, Golgi Ca²⁺ uptake rate of keratinocytes slowed, and the maximum Ca²⁺ level in the Golgi was notably lower [14]. Reduced Ca²⁺ concentration in the Golgi may impair the glycosylation of desmosomes [46]. Desmosome formation was delayed in SPCA1-deficient keratinocytes, but its assembly may be re-established by being cultured in an elevated Ca²⁺ concentration solution [5, 77]. Defective desmosomes may lead to an inability to form the intact structure of desmosomes and the split of epidermal cells [46]. The resting free cytoplasmic Ca²⁺ concentration in HHD skin was normal or higher [78, 79]. Keratinocytes responded less to increased extracellular Ca²⁺ concentrations [9]. High cytoplasmic Ca²⁺ concentrations could lead to decreased adenosine triphosphate (ATP) production by overloading mitochondria [80], followed by a failure of Ca²⁺-induced actin reorganization in keratinocytes of HHD [79]. Aging may reduce mitochondrial Ca²⁺ handling ability and ATP production capacity and increase mitochondrial vulnerability to Ca²⁺ concentration overload. This is in accordance with the relatively late-onset age of HHD [46, 80]. Other intrinsic and extrinsic factors, such as genetic modifiers, gene background, heating, and minor trauma, may also influence the onset age of HHD [40, 60].

Intriguingly, the main affected organ of HHD is skin, but no other organs. One reasonable explanation may be that a partial functional overlap of SPCA1 and other Ca²⁺ pumps may compensate for decreased pump function in these unaffected organs, which maintains a subtle, but steady state [57, 61]. The comparatively large percentage (67%) of Ca²⁺ uptake in the Golgi apparatus contributed by SPCA1 in keratinocytes may partially explain why skin is the main affected organ of HHD [51, 81]. However, a case report of a fatal liver injury was speculated to have been caused by HHD [17]. The predominant involvement of skin folds may be caused by exposure to external stimulations, such as friction, sweating, and secondary infections, and be unable to be compensated by other Ca²⁺ regulatory mechanisms [46].

In a gene-targeted mouse model for SPCA1, squamous cell tumors were observed in Spca1 ^+/− mouse models, while Spca1 ^−/− embryos showed growth retardation and neural tube closure failure and did not survive beyond gestation day 10.5 [15]. The mouse model phenotypes were different from the main characteristic of HHD in humans, which is acantholysis, although squamous cell tumors were occasionally observed in HHD patients [26–29]. A Caenorhabditis elegans (C. elegans) pmr-1 mutant showed wrong migration directions as well as shorter migration distances of ectodermal cells [16]. The defective migration of certain ectodermal cells in mouse and C. elegans models is superficially similar to HHD, suggesting that ATP2C1 gene mutations may lead to inefficient migration of epidermal cells in HHD [16]. The different responses in HHD patients and Spca1-deficient mouse models may result from Ca²⁺ pump dysfunction, causing chronic Golgi stress and/or ER stress, with the predominance of pro-apoptotic pathways in HHD patients and the predominance of pro-survival ones in mice, respectively [82]. In other words, mutations in the same gene may favor development of tumors in mice and acantholytic skin diseases in humans [15]. Another study suggested that apoptosis may not be the initial factor related to the pathogenetic mechanisms of HHD, but be the result of HHD acantholysis [83].

The splicing and expression of sense genes may be regulated by antisense transcription via certain mechanisms, for instance, RNA masking, increased RNA polymerase II occupancy, transcriptional interference, and DNA replication interference [84, 85]. The ATP2C1 gene transcription site is partially overlapped with the ASTE1 gene (asteroid homolog 1) on the opposing strand in the human but not mouse genome. Therefore, the different SPCA1 deficiency responses between humans and mouse models may be caused by a negative regulatory effect of the ASTE1 gene on splicing and expression of the ATP2C1 gene through antisense transcription [72].

The SPCA1 pump is crucial for Mn²⁺ detoxification by transporting Mn²⁺ from the cytoplasm into the Golgi [47]. Cytosolic overload and depletion of Golgi-hosted Mn²⁺ may lead to the loss of cell cycle control, genetic instability, and multinucleation [86]. Deficient Mn²⁺ homeostasis caused by ATP2C1 mutations may be the initial event in tumorigenesis in some HHD patients and Spca1 ^+/− mouse models [15, 86].

Conclusion

Acknowledging the three different classifications of HHD may help in understanding the clinical presentations and prognosis of HHD [24]. A close study of family history, clinical manifestations, and histologic and immunofluorescent examinations simplifies attempts to distinguish HHD from DD, PV, and other skin diseases. Mutation analysis provides valuable information to confirm clinical diagnoses [40]. Genetic studies, animal models, and functional analyses have supported the view that the ATP2C1 gene plays a vital role in the pathogenesis of HHD by regulating Ca²⁺/Mn²⁺ homeostasis [8, 9, 14–16]. Further investigations into the roles of Mn²⁺ in HHD with tumors [86], possible compensatory Ca²⁺ pumps for SPCA1 in nonlesional skin and noncutaneous tissues [46, 87], ATP2C1 mRNA expression levels in peripheral blood [71], genetic modifiers and environmental factors [2, 17, 76] may also help shed light on the pathogenetic mechanisms of HHD. Patients with atypical presentations and prenatal examinations may benefit from discovering novel ATP2C1 gene mutations and revealing the pathogenetic mechanisms of HHD [88, 89]. Therapies for this chronic and stubborn disease would include not only routine treatments such as corticosteroids, antibiotics, and surgical treatments [1], but also gene-targeted therapy.

Abbreviations

A

Actuator domain

ASTE1

Asteroid homolog 1

ATP

Adenosine triphosphate

ATP2C1

ATPase calcium–transporting type 2C member 1

C. elegans

Caenorhabditis elegans

DD

Darier disease

HHD

Hailey–Hailey disease

M

Transmembrane

N

Nucleotide–binding domain

P

Phosphorylation domain

PTCs

Premature termination codons

PV

Pemphigus vulgaris

SERCA

Sarcoplasmic/endoplasmic reticulum Ca²⁺–ATPase

SPCA

Secretory pathway Ca²⁺/Mn²⁺–ATPase

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.