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Pathomechanisms of Harlequin Ichthyosis and ABCA Transporters in Human Diseases
Masashi Akiyama, MD, PhD
Arch Dermatol. 2006;142:914-918.
ABSTRACT
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Objectives To review recent advances in our understanding of the genetic pathomechanisms of harlequin ichthyosis (HI) (the most devastating subtype of congenital ichthyoses) and its prenatal diagnosis and to discuss the possibility of future gene therapy.
Data Source PubMed search for articles about HI, its causative protein adenosine triphosphate–binding cassette A12 (ABCA12), and related molecules.
Study Selection English-language studies were selected if they provided useful information about the pathomechanisms of HI and ABCA lipid transporters.
Data Synthesis This article describes ABCA12 as a causative molecule involved in defects in HI, summarizes the known genetic disorders caused by genetic defects in ABCA lipid transporters, and highlights the prospects of prenatal diagnosis and gene therapy for HI.
Conclusions Harlequin ichthyosis is caused by a serious functional deficiency of ABCA12. ABCA12 and ABCA3 are essential lipid transporters for human adaptation to a dry terrestrial environment. In clinical practice, information regarding the genetic defects and pathomechanisms underlying HI is important for precise diagnosis, genetic counseling, and prenatal diagnosis.
INTRODUCTION
Harlequin ichthyosis (HI) manifests devastating features and is fatal in most affected newborns. For a long time, the pathomechanisms and underlying genetic defects were unknown. Significant progress was recently made in understanding the molecular basis of various congenital cornification diseases.
In 2003, ABCA12 was reported as a causative gene in type 2 lamellar ichthyosis (LI) mapped to chromosome 2q33-q35 (Online Mendelian Inheritance in Man 601277).1 Recently, ABCA12 mutations were found to underlie HI,2-3 and their function and pathomechanisms in HI were clarified.2 Adenosine triphosphate–binding cassette A12 (ABCA12) is a keratinocyte lipid transporter associated with lamellar granule (LG) formation and lipid transport via LGs on the surface of keratinocytes.2 The ABCA12 lipid transporter and LG lipid transport system have attracted great attention for their key roles in keratinization and cornification disorders. This article focuses on recent advances in research into the pathomechanisms of HI. In addition, the feasibility of DNA-based prenatal diagnosis and future strategies for the treatment for HI are discussed. These data are based on the results of a PubMed search for English-language articles about HI, its causative protein ABCA12, and related molecules.
ABCA12 DEFICIENCY IN HI
Harlequin ichthyosis is an extreme congenital ichthyosis, and its clinical features at birth include severe ectropion, eclabium, flattened ears, and large thick platelike scales over the entire body (Figure 1). Affected newborns rarely survive beyond the first few weeks of life. The observation that certain long-term survivors of HI progressed to a more LI-like disease4 stimulated controversy over the differences between HI and LI.
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Figure 1. Newborn with harlequin ichthyosis harboring adenosine triphosphate–binding cassette A12 (ABCA12) mutations. A and C, Thick scales and fissures are seen over the entire body. B, Ectropion and eclabium are severe.
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Ultrastructurally, LG abnormalities are apparent in the epidermis of patients with HI.5-8 However, the cornified cell envelope appears to be normal in HI and is associated with the normal distribution of major cornified cell envelope precursor proteins (involucrin, small proline-rich proteins 1 and 2, and loricrin), and the enzyme transglutaminase 1 has been detected in HI epidermis,9-10 distinct from the abnormal cornified cell envelope seen in LI with transglutaminase 1 gene mutations. Several morphologic abnormalities (eg, the abnormal LGs in the granular layer keratinocytes and a lack of extracellular lipid lamellae) reflect the defective lipid transport via LGs and the malformation of intercellular stratum corneum lipid layers in HI.5-9
These histologic findings support the hypothesis that HI is a distinct clinical entity governed by a pathogenetic mechanism that differs from the malformation of the cornified cell envelope in LI. However, the possibility that HI and the LI subtype without transglutaminase 1 gene mutations share similar pathogenic mechanisms cannot be excluded.
In this context, missense mutations in ABCA12 were reported to cause type 2 LI, a milder form of ichthyosis.1 ABCA3 (a member of the same protein subfamily as ABCA12) is involved in pulmonary surfactant lipid secretion via LGs from alveolar type II cells.11 From these facts, it was hypothesized that HI might be caused by truncation or deletion mutations in ABCA12 that seriously affect ABCA12 function.2
It was subsequently confirmed that mutations on both ABCA12 alleles underlie HI.2 ABCA12 belongs to the large superfamily of the ABC transporters, which bind adenosine triphosphate while aiding the transport of various molecules across the cell membrane.12 The members of the ABCA subfamily are all involved in lipid transport.13 Most mutations in HI are nonsense, deletion, or splice site mutations affecting the conserved domains of the ABCA12 protein and are predicted to cause serious defects in ABCA12 function.2 Ultrastructural and immunofluorescent examination of skin and cultured epidermal keratinocytes of patients carrying ABCA12 mutations have revealed defective lipid content transport via LGs.2 In addition, using ABCA12 corrective gene transfer, cultured keratinocytes from patients have recovered the ability to transport lipid normally.2 These findings indicate that ABCA12 mutations seriously affect ABCA12 protein function and lead to HI by disrupting LG lipid transport in the upper keratinizing epidermal cells (Figure 2).2 Using linkage analysis, other investigators independently reported that ABCA12 mutations underlie HI.3
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Figure 2. Pathogenesis of harlequin ichthyosis (HI) caused by an adenosine triphosphate–binding cassette A12 (ABCA12) deficiency. A, Model of formation of the normal intercellular lipid layers and cornified cell envelope in the stratum corneum. Formation of intercellular lipid layers in the stratum corneum is essential for correct epidermal barrier function. ABCA12 works in lipid transport via lamellar granules (LGs) to form an intercellular lipid coat. B, Model of malformation of the stratum corneum barrier in HI. Loss-of-function mutations in ABCA12 lead to defective lipid transport via LGs and malformation of intercellular lipid layers, resulting in loss of epidermal barrier function and abnormal hyperkeratosis.
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Malformed LGs, including empty LGs, are observed in most cases of HI, although LGs are absent in some cases.5-8 Therefore, ABCA12 may be located in the limiting membranes of LGs and may be included in the transport of lipid from the cytoplasm into the LGs. Immunoelectron microscopic observations2 support the localization of ABCA12 in the limiting membrane of LGs.
ABC TRANSPORTERS IN HUMAN GENETIC DISEASES
Mutations in ABCC6 have been shown to underlie pseudoxanthoma elasticum (Online Mendelian Inheritance in Man 264800 and 177850), an autosomal recessive disease with cutaneous, ophthalmologic, and cardiovascular manifestations.14-15 Several genetic diseases are caused by mutations in ABCA subfamily genes. The ABCA subfamily, of which the ABCA12 gene is a member, comprises 12 full transporters and 1 pseudogene (ABCA11), which are essential for lipid transport and secretion.13 Three ABCA genes of the same subfamily as ABCA12 are implicated in the development of genetic diseases affecting cellular lipid transport, as summarized in the Table.13 ABCA3, which is a close molecule in a phylogenetic tree of ABCA subfamily proteins,16 aids lipid secretion from alveolar type II cells via LGs.11 An ABCA3 deficiency underlies fatal surfactant deficiency in newborns.17
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Table. ABCA Subfamily Members, Their Functions, and Associated Disorders*
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NECESSITY OF ABCA12 and ABCA3 FOR HUMAN LIFE IN A DRY ENVIRONMENT
During the evolutionary process, when ancestral vertebrates left the safety of the oceans, they developed an air-breathing respiratory system and a robust protective mechanism (keratinization of the skin) to help their bodies adjust to a dryer environment. Pulmonary surfactant forms a lipid-rich monolayer that coats the airways of the lung and is essential for proper inflation and function of the lung. Serious deficiency of alveolar surfactant leads to fatal surfactant deficiency of the newborn.17 The other adaptation for life in a dry environment, cutaneous keratinization (which helps form a water-resistant skin), is indispensable for terrestrial life. Intercellular lipid layers in the stratum corneum are important for the barrier function of the skin. Congenital defects in this barrier cause ichthyosis, from the Greek ichthys, meaning fish. As already described, severe malformation of the intercellular lipid layers in the stratum corneum of the skin leads to this severe and frequently lethal type of named HI.
Lipid transport systems in the alveolar cells and the epidermal keratinocytes share similar secretory granules, termed lamellar granules (bodies), with important lipid cargoes. Alveolar surfactant is produced by alveolar type II cells, stored in the intracellular compartment within lamellar bodies, and secreted by exocytosis. ABCA3 lipid transporter is localized in lamellar bodies in alveolar type II cells,11 which play a key role in surfactant lipid transport. Mutations in the ABCA3 gene are known to be the cause of fatal surfactant deficiency in newborns.17 ABCA12 acts as a lipid transporter associated with LGs in the epidermal keratinocytes and is crucial for the correct formation of intercellular lipid layers in the stratum corneum of the skin.2 ABCA12 mutations, which seriously affect its function, cause a loss of the skin lipid barrier, leading to HI.2
Two distinct epidermal and alveolar lipid transporters, ABCA12 and ABCA3, respectively, are essential molecules for human terrestrial life. Characterization of ABCA genes in several simple eukaryotes demonstrated that the ABCA subfamily is an ancient group of transporters.13 As vertebrates left the aquatic environment and began terrestrial lives, the ABCA12 and ABCA3 transporters probably aided the adaptation of the respiratory and integumentary systems to the dry environment.
GENOTYPE-PHENOTYPE CORRELATION IN ABCA12 MUTATIONS
In a series of patients with HI,2 none of the ABCA12 mutations was a missense mutation, and most of them led to severe truncations and loss of function of ABCA12 peptide, affecting the important nucleotide-binding fold domains or transmembrane domains. Other nontruncation HI mutations were deletion mutations involving highly conserved ABCA12 sequences. Therefore, ABCA12 mutations in HI are thought to seriously affect the protein's function. This was supported by the finding of ABCA12 mutations in a series of families with HI in which all the mutations were truncation or deletion mutations except for 1 missense mutation.3 In contrast, in type 2 LI, 5 ABCA12 mutations were reported in 9 families, and all 5 mutations were missense mutations, resulting in only 1 amino acid alteration.1 These facts suggest that in HI at least 1 mutation on each allele must be a truncation or deletion mutation in the conserved region, which seriously affects the ABCA12 function.2 Diseases caused by mutations in other members of the ABCA subfamily include ABCA4 mutations causing Stargardt disease; 80% of these mutations are missense, many of which occur in conserved ABCA4 domains. ABCA1 mutations result in Tangier disease; 60% of the mutations are missense, also in conserved ABCA1 domains. Missense mutations and truncation mutations were found among ABCA3 mutations in newborns with fatal surfactant deficiency.17 In this context, ABCA12 mutations causing HI and LI are unique in that the type and severity of mutations help to define the clinical phenotype.
Clinically, patients with LI with ABCA12 mutations do not show typical HI features at birth. On the other hand, patients with HI show a typical HI phenotype at birth, even if they survive and later develop clinical features similar to those of LI or nonbullous congenital ichthyosiform erythroderma.2, 4 Therefore, HI survivors can be distinguished from LI survivors with ABCA12 mutations not only by the type and severity of ABCA12 mutations but also by their clinical features at birth.
Among newborns with HI, some die within a few weeks of life, and others survive. The nature and location of ABCA12 mutations and the level of function loss of the ABCA12 transporter in the patients have some relevance to this difference in prognosis. Indeed, in a series of patients with HI,2 1 patient was homozygous for the splice acceptor site mutation IVS23-2A G, and a certain amount of mutated ABCA12 protein was expressed within the granular cells of the patient's epidermis. This might have contributed to the survival of this patient beyond infancy. However, high-dose systemic retinoid therapy and major corrective treatment in the neonatal intensive care unit are major factors that affect the prognosis of newborns. Further data from patients with HI harboring ABCA12 mutations are needed to confirm the genotype-phenotype correlation in patients with HI.
FEASIBILITY OF DNA-BASED PRENATAL DIAGNOSIS OF HI
Because HI is often fatal, parental requests for prenatal diagnosis are important. For more than 20 years, prenatal diagnosis was performed by fetal skin biopsy and electron microscopy during the late stage of pregnancies at 19 to 23 weeks' estimated gestational age.7, 18-19 The 2005 discovery of the underlying gene causing HI2 enabled DNA-based prenatal diagnosis of HI by chorionic villus or amniotic fluid sampling in the earlier stages of pregnancy. This lower-risk procedure excludes severe keratinization disorders.20 In the future, earlier prenatal diagnosis by noninvasive analysis of DNA from fetal cells in maternal circulation21 and preimplantation genetic diagnosis22 may be available.
FUTURE STRATEGIES FOR GENE THERAPY TREATMENT OF HI
The skin is the most easily accessible organ for direct topical and injectable gene transfer in a minimally invasive way. Using ex vivo gene transfer, normal gene expression of transglutaminase 1 has been restored in LI, and the phenotypic correction of skin was observed in the engrafted skin in vivo on the backs of immunodeficient mice.23 However, further development for trial in humans has not yet been achieved. Recently, phenotypic recovery by corrective gene transfer was demonstrated in HI-derived keratinocytes harboring ABCA12 mutations.2 This advance will stimulate research regarding in vivo gene therapy for patients with HI.
One of the serious potential problems in gene therapy is inadvertent immune reaction against transgene products, especially in the therapy for recessive diseases such as HI. The unique immunological features of the cutaneous microenvironment containing antigen-presenting cells and the secretion of inflammatory cytokines from keratinocytes and dendritic cells are thought to be disadvantageous to long-term expression of a transgene.24 The immunogenicity of neoantigens in hosts with a null mutation of certain proteins was supported by desmoglein 3 complementary DNA findings in which 50% of injected animals developed anti–desmoglein 3 IgG.25 During gene therapy for HI, immunosuppressive therapy is necessary to avoid the immune response against a transgene product.
AUTHOR INFORMATION
Correspondence: Masashi Akiyama, MD, PhD, Department of Dermatology, Hokkaido University Graduate School of Medicine, North 15 West 7, Sapporo 060-8638, Japan (akiyama{at}med.hokudai.ac.jp).
Accepted for Publication: November 20, 2005.
Financial Disclosure: None reported.
Funding/Support: This study was supported in part by grant-in-aid Kiban B 16390312 from the Ministry of Education, Science, Sports, and Culture of Japan.
Acknowledgment: I thank Hiroshi Shimizu, MD, PhD, for his critical reading of the manuscript and James R. McMillan, PhD, for his proofreading and comments during the preparation of the manuscript.
Author Affiliation: Department of Dermatology, Hokkaido University Graduate School of Medicine, Sapporo, Japan.
REFERENCES
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1. Lefèvre C, Audebert S, Jobard F, et al. Mutations in the transporter ABCA12 are associated with lamellar ichthyosis type 2. Hum Mol Genet. 2003;12:2369-2378.
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2. Akiyama M, Sugiyama-Nakagiri Y, Sakai K, et al. Mutations in ABCA12 in harlequin ichthyosis and functional rescue by corrective gene transfer. J Clin Invest. 2005;115:1777-1784.
FULL TEXT
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ISI
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3. Kelsell DP, Norgett EE, Unsworth H, et al. Mutations in ABCA12 underlie the severe congenital skin disease harlequin ichthyosis. Am J Hum Genet. 2005;76:794-803.
FULL TEXT
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ISI
| PUBMED
4. Haftek M, Cambazard F, Dhouailly D, et al. A longitudinal study of a harlequin infant presenting clinically as non-bullous congenital ichthyosiform erythroderma. Br J Dermatol. 1996;135:448-453.
FULL TEXT
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ISI
| PUBMED
5. Dale BA, Holbrook KA, Fleckman P, Kimball JR, Brumbaugh S, Sybert VP. Heterogeneity in harlequin ichthyosis, an inborn error of epidermal keratinization: variable morphology and structural protein expression and a defect in lamellar granules. J Invest Dermatol. 1990;94:6-18.
FULL TEXT
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ISI
| PUBMED
6. Milner ME, O’Guin WM, Holbrook KA, Dale BA. Abnormal lamellar granules in harlequin ichthyosis. J Invest Dermatol. 1992;99:824-829.
FULL TEXT
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ISI
| PUBMED
7. Akiyama M, Kim DK, Main DM, Otto CE, Holbrook KA. Characteristic morphologic abnormality of harlequin ichthyosis detected in amniotic fluid cells. J Invest Dermatol. 1994;102:210-213.
FULL TEXT
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ISI
| PUBMED
8. Akiyama M, Dale BA, Smith LT, Shimizu H, Holbrook KA. Regional difference in expression of characteristic abnormality of harlequin ichthyosis in affected fetuses. Prenat Diagn. 1998;18:425-436.
FULL TEXT
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ISI
| PUBMED
9. Akiyama M, Yoneda K, Kim SY, Koyama H, Shimizu H. Cornified cell envelope proteins and keratins are normally distributed in harlequin ichthyosis. J Cutan Pathol. 1996;23:571-575.
FULL TEXT
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ISI
| PUBMED
10. Akiyama M, Kim SY, Yoneda K, Shimizu H. Expression of transglutaminase 1 (transglutaminase K) in harlequin ichthyosis. Arch Dermatol Res. 1997;289:116-119.
FULL TEXT
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ISI
| PUBMED
11. Yamano G, Funahashi H, Kawanami O, et al. ABCA3 is a lamellar body membrane protein in human lung alveolar type II cells. FEBS Lett. 2001;508:221-225.
FULL TEXT
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ISI
| PUBMED
12. Borst P, Elferink RO. Mammalian ABC transporters in health and disease. Annu Rev Biochem. 2002;71:537-592.
FULL TEXT
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ISI
| PUBMED
13. Peelman F, Labeur C, Vanloo B, et al. Characterization of the ABCA transporter subfamily: identification of prokaryotic and eukaryotic members, phylogeny and topology. J Mol Biol. 2003;325:259-274.
FULL TEXT
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ISI
| PUBMED
14. Uitto J, Pulkkinen L, Ringpfeil F. Molecular genetics of pseudoxanthoma elasticum: a metabolic disorder at the environment-genome interface? Trends Mol Med. 2001;7:13-17.
FULL TEXT
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ISI
| PUBMED
15. Uitto J. The gene family of ABC transporters: novel mutations, new phenotypes. Trends Mol Med. 2005;11:341-343.
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ISI
| PUBMED
16. Annilo T, Shulenin S, Chen ZQ, et al. Identification and characterization of a novel ABCA subfamily member, ABCA12, located in the lamellar ichthyosis region on 2q34. Cytogenet Genome Res. 2002;98:169-176.
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ISI
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17. Shulenin S, Nogee LM, Annilo T, Wert SE, Whitsett JA, Dean M. ABCA3 gene mutations in newborns with fatal surfactant deficiency. N Engl J Med. 2004;350:1296-1303.
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18. Akiyama M, Suzumori K, Shimizu H. Prenatal diagnosis of harlequin ichthyosis by the examinations of keratinized hair canals and amniotic fluid cells at 19 weeks' estimated gestational age. Prenat Diagn. 1999;19:167-171.
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ISI
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19. Shimizu A, Akiyama M, Ishiko A, Yoshiike T, Suzumori K, Shimizu H. Prenatal exclusion of harlequin ichthyosis: potential pitfalls in the timing of the fetal skin biopsy. Br J Dermatol. 2005;153:811-814.
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20. Tsuji-Abe Y, Akiyama M, Nakamura H, et al. DNA-based prenatal exclusion of bullous congenital ichthyosiform erythroderma at the early stage, 10 to 11 weeks' of pregnancy, in two consequent siblings. J Am Acad Dermatol. 2004;51:1008-1011.
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21. Uitto J, Pfendner E, Jackson LG. Probing the fetal genome: progress in non-invasive prenatal diagnosis. Trends Mol Med. 2003;9:339-343.
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| PUBMED
22. Wells D, Delhantry JD. Preimplantation genetic diagnosis: applications of molecular medicine. Trends Mol Med. 2001;7:23-30.
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23. Choate KA, Medalie DA, Morgan JR, Khavari PA. Corrective gene transfer in the human skin disorder lamellar ichthyosis. Nat Med. 1996;2:1263-1267.
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24. Hengge UR, Bardenheuer W. Gene therapy and the skin. Am J Med Genet C Semin Med Genet. 2004;131C:93-100.25. Ohyama M, Ota T, Aoki M, et al. Suppression of the immune response against exogenous desmoglein 3 in desmoglein 3 knockout mice: an implication for gene therapy. J Invest Dermatol. 2003;120:610-615.
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