Expression of the keratinocyte lipid transporter ABCA12 in developing and reconstituted human epidermis

Case Report: Novel rare mutation c.6353C > G in the ABCA12 gene causing harlequin ichthyosis identified by whole exome sequencing

BACKGROUND

Harlequin ichthyosis (HI) is a severe rare genetic disease that mainly affects the skin. Neonates with this disease are born with thick skin and large diamond-shaped plates covering most of their bodies. Affected neonates lose the ability to control dehydration and regulate temperature and are more susceptible to infections. They also face respiratory failure and feeding problems. These clinical symptoms are factors associated with high mortality rates of neonates with HI. Until now, there are still no effective treatments for HI patients and most patients die in the newborn period. Mutation in the ABCA12 gene, which encodes an adenosine triphosphate-binding cassette (ABC) transporter, has been demonstrated as the major cause of HI.

CASE PRESENTATION

In this study, we report the case who is one infant that was born prematurely at 32 gestational weeks with the whole body covered with thick plate-like scales of skin. The infant was severely infected with mild edema, multiple cracked skins full of the body, yellow discharge, and necrosis of fingers and toes. The infant was suspected to be affected by HI. Whole exome sequencing (WES) was performed as a tool for detecting the novel mutation in one prematurely born Vietnam infant with HI phenotype. And after that, the mutation was confirmed by the Sanger sequencing method in the patient and the members of his family. In this case, one novel mutation c.6353C > G (p.S2118X, Hom) in the ABCA12 gene, was detected in the patient. The mutation has not been reported in any HI patients previously. This mutation was also found in a heterozygous state in the members of the patient's family, including his parents, an older brother, and an older sister who are no symptoms.

CONCLUSIONS

In this study, we identified a novel mutation in a Vietnamese patient with HI by whole exome sequencing. The results for the patient and the members of his family will be helpful in understanding the etiology of the disease, diagnosing carriers, assisting in genetic counseling, and emphasizing the need for DNA-based prenatal screening for families with a history of the disease.

RSPO1-mutated keratinocytes from palmoplantar keratoderma display impaired differentiation, alteration of cell-cell adhesion, EMT-like phenotype and invasiveness properties: implications for squamous cell carcinoma susceptibility in patients with 46XX disorder of sexual development

Background

Secreted R-spondin (RSPO) proteins play a key role in reproductive organ development, epithelial stem cell renewal and cancer induction by reinforcing canonical Wnt signaling. We have previously reported that palmoplantar keratoderma (PPK), predisposition to cutaneous squamous cell carcinoma (SCC) development and sex reversal segregate as autosomal recessive trait in patients carrying RSPO1-mutations.

Although our previous findings suggested that RSPO1 secreted from fibroblasts regulates keratinocyte growth or differentiation, the role of this protein in the epidermis remains largely unexplored. Our study was aimed at expanding the phenotypic, molecular and functional characterization of RSPO1-mutated skin and keratinocytes.

Results

Cultured primary keratinocytes from PPK skin of a RSPO1-mutated XX-sex reversed patient displayed highly impaired differentiation and epithelial-mesenchymal transition (EMT)-like phenotype. Interestingly, RSPO1-mutated PPK skin expressed markers of increased proliferation, dedifferentiation and altered cell–cell adhesion. Furthermore, all these signs were more evident in SCC specimens of the patient. Cultured PPK patient’s keratinocytes exhibited increased expression of cell‒matrix adhesion proteins and extracellular matrix remodeling enzymes. Moreover, they showed invasiveness properties in an organotypic skin model in presence of PPK fibroblasts, which behave like cancer-associated fibroblasts. However, the co-culture with normal fibroblasts or treatment with the recombinant RSPO1 protein did not revert or reduce the EMT-like phenotype and invasion capability of PPK keratinocytes. Notably, RSPO1-mutated PPK fibroblasts induced a hyperproliferative and dedifferentiated phenotype of age-matched normal control plantar keratinocytes. Wnt signaling has a key role in both PPK promotion and SCC development. Accordingly, Wnt mediators were differentially expressed in both PPK keratinocytes and skin specimens of RSPO1-mutated patient compared to control.

Conclusions

Altogether our data indicate that the absence of RSPO1 in patients with 46XX disorder of sexual development affects the skin microenvironment and epidermal integrity, thus contributing to the risk of SCC tumorigenesis in palmoplantar regions exposed to major frictional stresses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02434-2.

ABCA12 Promotes Proliferation and Migration and Inhibits Apoptosis of Pancreatic Cancer Cells Through the AKT Signaling Pathway

Background: As a malignant tumor, pancreatic cancer is difficult to detect in its early stage. Pancreatic cancer progresses rapidly and has a short survival time. Most cases have metastasized to distant organs before diagnosis. The mechanism of induction of pancreatic cancer is not fully understood.

Methods: In this study, bioinformatics predicted ATP binding cassette subfamily A member 12 (ABCA12) expression in pancreatic tissues and performed survival analysis, risk assessment, and enrichment analysis. The expression of ABCA12 in 30 pairs of clinical samples was detected by immunohistochemistry and we analyzed its correlation with clinical information. Both reverse transcription polymerase chain reaction (RT–PCR) and western blot analysis were used to detect mRNA and protein expression in cell lines. Two different siRNAs and SW1990 cell line were used to construct pancreatic cancer cell models with ABCA12 knockdown. Cell viability was evaluated by cell counting kit-8 (CCK-8) and EdU proliferation assays. Wound healing assays and Transwell assays were used to measure the ability of cell migration and invasion. Flow cytometry was used to investigate the effect of ABCA12 on the proliferation cycle and apoptosis of pancreatic cancer. Western blot analysis detected changes in apoptosis, migration, and other pathway proteins in SW1990 cells after transfection.

Results:ABCA12 is highly expressed in pancreatic cancer tissues and cells. After ABCA12 was knocked down, the proliferation, invasion, and migration of SW1990 cells were significantly reduced, and apoptosis was increased. The changes in pathway proteins suggested that ABCA12 may regulate the progression of pancreatic cancer through the AKT pathway.

Conclusion: We found that ABCA12 is differentially expressed in pancreatic tissues and cells. ABCA12 can also affect the biological behavior of pancreatic cancer cells effectively, which may serve as a new target for pancreatic cancer diagnosis and treatment.

Acute skin barrier disruption alters the secretion of lamellar bodies via the multilayered expression of ABCA12

BACKGROUND

The skin barrier consists of multiple lipid-enriched layers, which are characterized by lamellar repeated structures within the intercellular space. Sodium lauryl sulfate is a well-known substance that can disrupt the skin barrier. The mechanisms underlying the barrier repair process, especially the influence of topical sodium lauryl sulfate treatment on lipid transport in the barrier recovery phase, remain unresolved.

OBJECTIVE

To understand the process of reconstruction of the intercellular lipid layer of the skin after acute barrier disruption by sodium lauryl sulfate treatment in vivo.

METHODS

Female hairless mice were treated with 3 % sodium lauryl sulfate. Transepidermal water loss measurement, histopathological analysis, and gene expression analysis were performed from 1 to 288 h after the topical application of sodium lauryl sulfate. Western blot analysis, immunofluorescence staining, and transmission electron microscopy analysis were performed to examine the expression level of ATP-binding cassette, sub-family A, member 12 (ABCA12), and the secretion level of lamellar bodies.

RESULTS

We observed rapid hyper-keratinization at the stratum corneum and the subsequent concurrent secretion of lamellar bodies into the intercellular space of the stratum corneum during the process of skin barrier recovery. ABCA12 expression associated with lipid transportation into lamellar bodies was transiently upregulated and observed in multiple layers in the upper epidermis, especially in the stratum granulosum.

CONCLUSION

The skin reacts appropriately to maintain its barrier function by first initiating hyper-keratinization and then increasing lamellar body secretion. Activation of ABCA12 is an essential factor for the recovery of skin barrier function.

The KEAP1/NRF2 Signaling Pathway in Keratinization

Keratinization is a tissue adaptation, but aberrant keratinization is associated with skin disorders such as ichthyoses, atopic dermatitis, psoriasis, and acne. The disease phenotype stems from the interaction between genes and the environment; therefore, an understanding of the adaptation machinery may lead to a new appreciation of pathomechanisms. The KEAP1/NRF2 signaling pathway mediates the environmental responses of squamous epithelial tissue. The unpredicted outcome of the Keap1-null mutation in mice allowed us to revisit the basic principle of the biological process of keratinization: sulfur metabolism establishes unparalleled cytoprotection in the body wall of terrestrial mammals. We summarize the recent understanding of the KEAP1/NRF2 signaling pathway, which is a thiol-based sensor-effector apparatus, with particular focuses on epidermal differentiation in the context of the gene-environment interaction, the structure/function principles involved in KEAP1/NRF2 signaling, lessons from mouse models, and their pathological implications. This synthesis may provide insights into keratinization, which provides physical insulation and constitutes an essential innate integumentary defense system.

Loricrin: Past, Present, and Future

The terminal differentiation of the epidermis is a complex physiological process. During the past few decades, medical genetics has shown that defects in the stratum corneum (SC) permeability barrier cause a myriad of pathological conditions, ranging from common dry skin to lethal ichthyoses. Contrarily, molecular phylogenetics has revealed that amniotes have acquired a specialized form of cytoprotection cornification that provides mechanical resilience to the SC. This superior biochemical property, along with desiccation tolerance, is attributable to the proper formation of the macromolecular protein-lipid complex termed cornified cell envelopes (CE). Cornification largely depends on the peculiar biochemical and biophysical properties of loricrin, which is a major CE component. Despite its quantitative significance, loricrin knockout (LKO) mice have revealed it to be dispensable for the SC permeability barrier. Nevertheless, LKO mice have brought us valuable lessons. It is also becoming evident that absent loricrin affects skin homeostasis more profoundly in many more aspects than previously expected. Through an extensive review of aggregate evidence, we discuss herein the functional significance of the thiol-rich protein loricrin from a biochemical, genetic, pathological, metabolic, or immunological aspect with some theoretical and speculative perspectives.

Marked Changes in Lamellar Granule and Trans-Golgi Network Structure Occur during Epidermal Keratinocyte Differentiation

Epidermal lamellar granules transport various lipids, proteins, and protein inhibitors from the trans-Golgi network to the extracellular space, and play an important role in skin barrier formation. We elucidated the 3-dimensional structure of lamellar granules and the trans-Golgi network in normal human skin by focused ion beam scanning electron microscopy. Reconstructed focused ion beam scanning electron microscopy 3-dimensional images revealed that the overall lamellar granule structure changed from vesicular to reticular within the second layer of the stratum granulosum. Furthermore, the trans-Golgi network was well developed within this layer and spread through the cytoplasm with branched, tubular structures that connected to lamellar granules. Our study reveals the unique overall 3-dimensional structure of lamellar granules and the trans-Golgi network within the cells of the epidermis, and provides the basis for an understanding of the skin barrier formation.

Mutation and expression of ABCA12 in keratosis pilaris and nevus comedonicus

Keratosis pilaris (KP) and nevus comedonicus (NC) are congenital keratinized dermatoses; however, the exact etiology of these two diseases is unclear. The objective of the present study was to identify the disease-causing genes and their association with functional alterations in the development of KP and NC. Peripheral blood samples of one KP family, two NC families and 100 unrelated healthy controls were collected. The genomic sequences of 147 genes associated with 143 genetic skin diseases were initially analyzed from the KP proband using a custom-designed GeneChip. A novel heterozygous missense mutation in the ATP-binding cassette sub-family A member 12 (ABCA12) gene, designated c.6694G>T (p.Asp2232Tyr), was identified in the KP proband and confirmed by Sanger sequencing. The same mutation was also present in the affected family members but not in the healthy family members, the two patients with NC or population-matched controls. The predictions provided by PolyPhen-2 and SIFT analyses suggested that the mutation may produce a damaged protein. The region surrounding the mutation is the extra-membrane domain, which is conserved among particular species, as suggested by ClustalX; however, no ABCA12 mutations were reported in the patients with NC. As observed by immunofluorescence, ABCA12 expression was upregulated in the sebaceous glands of the patients with NC compared with that of normal controls. In summary, ABCA12-associated mutations or alterations in expression may exhibit causative or contributive effects to the development of keratinized dermatoses, including KP and NC.

The roles of ABCA12 in epidermal lipid barrier formation and keratinocyte differentiation

ATP-binding cassette (ABC) transporters form a large superfamily of transporters that bind and hydrolyze ATP to transport various molecules across limiting membranes or into vesicles. The ABCA subfamily members are thought to transport lipid materials. ABCA12 is a keratinocyte transmembrane lipid transporter protein associated with the transport of lipids via lamellar granules. ABCA12 is considered to transport lipids including ceramides to form extracellular lipid layers in the stratum corneum of the epidermis, which is essential for skin barrier function. ABCA12 mutations are known to underlie the three major types of autosomal recessive congenital ichthyoses: harlequin ichthyosis, lamellar ichthyosis and congenital ichthyosiform erythroderma. ABCA12 mutations result in defective lipid transport via lamellar granules in the keratinocytes, leading to ichthyosis phenotypes from malformation of the stratum corneum lipid barrier. Studies on ABCA12-deficient bioengineered models have revealed that lipid transport by ABCA12 is required for keratinocyte differentiation and epidermal morphogenesis. Defective lipid transport due to loss of ABCA12 function leads to the accumulation of intracellular lipids, including glucosylceramides and gangliosides, in the epidermal keratinocytes. The accumulation of gangliosides seems to result in the apoptosis of Abca12(-/-) keratinocytes. It was reported that AKT activation occurs in Abca12(-/-) granular-layer keratinocytes, which suggests that AKT activation serves to prevent the cell death of Abca12(-/-) keratinocytes. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Airway gene expression in COPD is dynamic with inhaled corticosteroid treatment and reflects biological pathways associated with disease activity

Background

A core feature of chronic obstructive pulmonary disease (COPD) is the accelerated decline in forced expiratory volume in one second (FEV₁). The recent Groningen and Leiden Universities study of Corticosteroids in Obstructive Lung Disease (GLUCOLD) study suggested that particular phenotypes of COPD benefit from fluticasone±salmeterol by reducing the rate of FEV₁ decline, yet the underlying mechanisms are unknown.

Methods

Whole-genome gene expression profiling using the Affymetrix Gene ST array (V.1.0) was performed on 221 bronchial biopsies available from 89 COPD patients at baseline and after 6 and 30 months of fluticasone±salmeterol and placebo treatment in GLUCOLD.

Results

Linear mixed effects modelling revealed that the expression of 138 genes decreased, whereas the expression of 140 genes significantly upregulated after both 6 and 30 months of treatment with fluticasone±salmeterol versus placebo. A more pronounced treatment-induced change in the expression of 50 and 55 of these 278 genes was associated with a lower rate of decline in FEV₁ and Saint George Respiratory Questionnaire, respectively. Genes decreasing with treatment were involved in pathways related to cell cycle, oxidative phosphorylation, epithelial cell signalling, p53 signalling and T cell signalling. Genes increasing with treatment were involved in pathways related to focal adhesion, gap junction and extracellular matrix deposition. Finally, the fluticasone-induced gene expression changes were enriched among genes that change in the airway epithelium in smokers with versus without COPD in an independent data set.

Conclusions

The present study suggests that gene expression in biological pathways of COPD is dynamic with treatment and reflects disease activity. This study opens the gate to targeted and molecular phenotype-driven therapy of COPD.

The retinoid-related orphan receptor RORα promotes keratinocyte differentiation via FOXN1

RORα is a retinoid-related orphan nuclear receptor that regulates inflammation, lipid metabolism, and cellular differentiation of several non-epithelial tissues. In spite of its high expression in skin epithelium, its functions in this tissue remain unclear. Using gain- and loss-of-function approaches to alter RORα gene expression in human keratinocytes (HKCs), we have found that this transcription factor functions as a regulator of epidermal differentiation. Among the 4 RORα isoforms, RORα4 is prominently expressed by keratinocytes in a manner that increases with differentiation. In contrast, RORα levels are significantly lower in skin squamous cell carcinoma tumors (SCCs) and cell lines. Increasing the levels of RORα4 in HKCs enhanced the expression of structural proteins associated with early and late differentiation, as well as genes involved in lipid barrier formation. Gene silencing of RORα impaired the ability of keratinocytes to differentiate in an in vivo epidermal cyst model. The pro-differentiation function of RORα is mediated at least in part by FOXN1, a well-known pro-differentiation transcription factor that we establish as a novel direct target of RORα in keratinocytes. Our results point to RORα as a novel node in the keratinocyte differentiation network and further suggest that the identification of RORα ligands may prove useful for treating skin disorders that are associated with abnormal keratinocyte differentiation, including cancer.

Updated molecular genetics and pathogenesis of ichthiyoses

Research into the molecular genetics and pathomechanisms of ichthyoses have advanced considerably, resulting in the identification of several causative genes and molecules underlying the disease. In 2009, the First Ichthyosis Consensus Conference was held to establish a consensus for the nomenclature and classification of inherited ichthyoses, by which an international consensus for the classification of inherited ichthyosis was achieved. In this review, the pathogeneses of various ichthyoses are summarized based on their revised classification and terminology. Skin barrier defects are involved in the pathogenesis of various types of ichthyosis. The known causative molecules underlying ichthyosis include ABCA12, lipoxygenase-3, 12R-lipoxygenase, CYP4F22, ichthyin and steroid sulfatase, all of which are thought to be related to the intercellular lipid layers. ABCA12 is a known keratinocyte lipid transporter associated with lipid transport in lamellar granules and a loss of ABCA12 function leads to defective lipid transport in the keratinocytes, resulting in the most severe, harlequin ichthyosis phenotype. Other causative molecules for ichthyoses are transglutaminase 1, keratins and filaggrin. Transglutaminase 1 plays a role in cornified cell envelope formation. Keratins 1, 10 and 2 are involved in the keratin network of suprabasal keratinocytes and filaggrin is essential for the formation of keratohyalin granules. It is important to obtain information concerning genetic defects and to elucidate ichthyotic disease pathomechanisms for the establishment of an effective therapy and beneficial genetic counseling, including a prenatal diagnosis for families affected by ichthyotic disease.

The roles of ABCA12 in keratinocyte differentiation and lipid barrier formation in the epidermis

ABCA12 is a member of the large superfamily of ATP-binding cassette (ABC) transporters, which bind and hydrolyze ATP to transport various molecules across limiting membranes or into vesicles. The ABCA subfamily members are thought to be lipid transporters. ABCA12 is a keratinocyte transmembrane lipid transporter protein associated with the transport of lipids in lamellar granules to the apical surface of granular layer keratinocytes. Extracellular lipids, including ceramide, are thought to be essential for skin barrier function. ABCA12 mutations are known to underlie the three main types of autosomal recessive congenital ichthyoses: harlequin ichthyosis, lamellar ichthyosis and congenital ichthyosiform erythroderma. ABCA12 mutations lead to defective lipid transport via lamellar granules in the keratinocytes, resulting in malformation of the epidermal lipid barrier and ichthyosis phenotypes. Studies of ABCA12-deficient model mice indicate that lipid transport by ABCA12 is also indispensable for intact differentiation of keratinocytes.

Ichthyoses - Part 1: Differential diagnosis of vulgar ichthyoses and therapeutic options

Ichthyoses are a group of genetic disorders with defective cornification, clinically characterized by scaling of the skin. Additionally, distinctive cutaneous inflammation can often be observed. For most of the patients these diseases lead to a significant restriction in quality of life. The diagnostic criteria include clinical and histological findings, often confirmed by specialized tests. Because many of the ichthyoses are extremely rare, their accurate diagnosis is often carried out in specialized centers. We summarize isolated vulgar and congenital ichthyoses both with and without associated symptoms and focus on the common genetic changes and their clinical phenotype. Specific therapies are still not available for most of these genetic disorders. The use of different topical agents (e. g. urea, retinoids and salicylic acid) and baths followed by mechanical keratolysis (sometimes in combination with systemic retinoids) reduce symptoms. Patients with uncommon congenital ichthyoses often benefit from interdisciplinary management which involves specialized dermatological centers. In this first part of the paper the vulgar ichthyoses as well as the diagnostic and therapeutic options are discussed. The second part focuses on the congenital ichthyoses and their differential diagnosis.

Ceramide biosynthesis in keratinocyte and its role in skin function

The enucleate layer of the epidermis, i.e. the stratum corneum, is responsible for certain critical protective functions, such as epidermal permeability barrier function. Within the epidermal membrane lamella component, ceramides are the dominant lipid class by weight (over 50%) and exhibit the greatest molecular heterogeneity in terms of sphingoid base and fatty acid composition. It is now evermore important to understand how ceramide production and functions are controlled in the epidermis, since decreased epidermal ceramide content has been linked to water loss and barrier dysfunction. During the past several years, critical enzymes in ceramide biosynthesis have been identified, including ceramide synthases (CerS) and ceramide hydroxylase/desaturase. In this review, we describe the molecular heterogeneity of ceramides synthesized in the epidermis and their possible roles in epidermal permeability barrier functions. We also describe recent studies that identified the family of CerS (CerS1-CerS6) in mammals. We further focus on the roles of specific isoforms of these enzymes in synthesizing the epidermal ceramides, especially in relation to chain-length specificity. In addition, we provide experimental information, including our recent findings, as to how applying ceramide or ceramide-containing substances to skin, orally or directly, can benefit skin health.

Premature terminal differentiation and a reduction in specific proteases associated with loss of ABCA12 in Harlequin ichthyosis

One of the primary functions of skin is to form a defensive barrier against external infections and water loss. Disrupted barrier function underlies the most severe and often lethal form of recessive congenital ichthyosis, harlequin ichthyosis (HI). HI is associated with mutations in the gene that encodes the ABC transporter protein, ABCA12. We have investigated the morphological and biochemical alterations associated with abnormal epidermal differentiation and barrier formation in HI epidermis. An in vitro model of HI skin using human keratinocytes retrovirally transduced with shRNA targeting ABCA12 in a three-dimensional, organotypic co-culture (OTCC) system has also been developed. A robust reduction in ABCA12 expression had a dramatic effect on keratinocyte differentiation and morphology comparable with that observed in HI skin, including a thicker epidermis and abnormal lipid content with a reduction in nonpolar lipids. As seen in HI epidermis, proteins that are normally expressed in late differentiation were highly dysregulated in the ABCA12-ablated OTCC system. These proteins were expressed in the stratum basale and also in the stratum spinosum, indicative of a premature terminal differentiation phenotype. Expression of the proteases kallikrein 5 and cathepsin D was dramatically reduced in both HI epidermis and the OTCC model. These data suggest that ABCA12 is a key molecule in regulating keratinocyte differentiation and transporting specific proteases associated with desquamation.

CGI-58 is an alpha/beta-hydrolase within lipid transporting lamellar granules of differentiated keratinocytes

CGI-58 is the causative molecule underlying Dorfman-Chanarin syndrome, a neutral lipid storage disease exhibiting apparent clinical features of ichthyosis. CGI-58, associated with triacylglycerol hydrolysis, has an alpha/beta-hydrolase fold and is also known as the alpha/beta-hydrolase domain-containing protein 5. The purpose of this study was to elucidate the function of CGI-58 and the pathogenic mechanisms of ichthyosis in Dorfman-Chanarin syndrome. Using an anti-CGI-58 antibody, we found CGI-58 to be expressed in the upper epidermis, predominantly in the granular layer cells, as well as in neurons and hepatocytes. Immunoelectron microscopy revealed that CGI-58 was also localized to the lamellar granules (LGs), which are lipid transport and secretion granules found in keratinocytes. CGI-58 expression was markedly reduced in the epidermis of patients with harlequin ichthyosis, demonstrating defective LG formation. In cultured keratinocytes, CGI-58 expression was mildly up-regulated under high Ca(2+) conditions and markedly up-regulated in three-dimensional, organotypic cultures. In the developing human epidermis, CGI-58 immunostaining was observed at an estimated gestational age of 49 days, and CGI-58 mRNA expression was up-regulated concomitantly with both epidermal stratification and keratinocyte differentiation. CGI-58 knockdown reduced expression of keratinocyte differentiation/keratinization markers in cultured human keratinocytes. Our results indicate that CGI-58 is expressed and packaged into LGs during keratinization and likely plays crucial role(s) in keratinocyte differentiation and LG lipid metabolism, contributing to skin lipid barrier formation.

A mouse model of harlequin ichthyosis delineates a key role for Abca12 in lipid homeostasis

Harlequin Ichthyosis (HI) is a severe and often lethal hyperkeratotic skin disease caused by mutations in the ABCA12 transport protein. In keratinocytes, ABCA12 is thought to regulate the transfer of lipids into small intracellular trafficking vesicles known as lamellar bodies. However, the nature and scope of this regulation remains unclear. As part of an original recessive mouse ENU mutagenesis screen, we have identified and characterised an animal model of HI and showed that it displays many of the hallmarks of the disease including hyperkeratosis, loss of barrier function, and defects in lipid homeostasis. We have used this model to follow disease progression in utero and present evidence that loss of Abca12 function leads to premature differentiation of basal keratinocytes. A comprehensive analysis of lipid levels in mutant epidermis demonstrated profound defects in lipid homeostasis, illustrating for the first time the extent to which Abca12 plays a pivotal role in maintaining lipid balance in the skin. To further investigate the scope of Abca12's activity, we have utilised cells from the mutant mouse to ascribe direct transport functions to the protein and, in doing so, we demonstrate activities independent of its role in lamellar body function. These cells have severely impaired lipid efflux leading to intracellular accumulation of neutral lipids. Furthermore, we identify Abca12 as a mediator of Abca1-regulated cellular cholesterol efflux, a finding that may have significant implications for other diseases of lipid metabolism and homeostasis, including atherosclerosis.

Harlequin ichthyosis model mouse reveals alveolar collapse and severe fetal skin barrier defects

Harlequin ichthyosis (HI), which is the most severe genodermatosis, is caused by loss-of-function mutations in ABCA12, a member of the ATP-binding cassette transporter family. To investigate the pathomechanism of HI and the function of the ABCA12 protein, we generated ABCA12-deficient mice (Abca12(-/-)) by targeting Abca12. Abca12(-/-) mice closely reproduce the human HI phenotype, showing marked hyperkeratosis with eclabium and skin fissure. Lamellar granule abnormalities and defective ceramide distribution were remarkable in the epidermis. Skin permeability assay of Abca12(-/-) fetuses revealed severe skin barrier dysfunction after the initiation of keratinization. Surprisingly, the Abca12(-/-) mice also demonstrated lung alveolar collapse immediately after birth. Lamellar bodies in alveolar type II cells of the Abca12(-/-) mice lacked normal lamellar structures. The level of surfactant protein B, an essential component of alveolar surfactant, was reduced in the Abca12(-/-) mice. Fetal therapeutic trials with systemic administration of retinoid or dexamethasone, which are effective for HI and respiratory distress, respectively, to the pregnant mother mice neither improved the skin phenotype nor extended the survival period. Our HI model mice reproduce the human HI skin phenotype soon after the initiation of fetal skin keratinization and provide evidence that ABCA12 plays pivotal roles in lung and skin barrier functions.