Cornified cell envelope proteins and keratins are normally distributed in harlequin ichthyosis

Epidemiology, medical genetics, diagnosis and treatment of harlequin ichthyosis in Japan

Ichthyoses are a group of disorders marked by whitish, brown or dark-brown scales on the skin of almost the whole body. Harlequin ichthyosis (HI) is the most severe form. Neonatal death from HI was once common. Due to intensive neonatal care and, probably, to the early introduction of oral retinoids, HI outcome has improved. For definitive diagnosis and the exclusion of other disorders, such as lamellar ichthyosis, which also shows a collodion baby phenotype, it is helpful to refer to electron microscopy of abnormal or absent lamellar granules and a heavy accumulation of lipid droplets in the keratinocytes. ATP-binding cassette transporter A12 (ABCA12) is known as the causative gene of HI. Severe ABCA12 deficiency results in malformation of intercellular lipid layers in the cornified layers and leads to epidermal lipid barrier disruption. In HI patients, at least one mutation on each allele must be a truncation or deletion mutation to cause serious loss of ABCA12 function. Identification of the gene underlying HI has enabled DNA-based prenatal diagnosis for HI at the earlier stages of pregnancy with low risk. There are no curative treatments for HI. Abca12-deficient mice were created as a model of HI. Treatment of the model mice with retinoid or steroid has not been successful.

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

Serious defects in the epidermal keratinocyte lipid transporter ABCA12 are known to result in a deficient skin lipid barrier, leading to harlequin ichthyosis (HI). HI is the most severe inherited keratinizing disorder and is frequently fatal in the perinatal period. To clarify the role of ABCA12, ABCA12 expression was studied in developing human skin and HI lesions artificially reconstituted in immunodeficient mice. By immunofluorescent study, ABCA12 was expressed in the periderm of the early stage two-layered human fetal epidermis. After formation of a three-layered epidermis, ABCA12 staining was seen throughout the entire epidermis. ABCA12 mRNA expression significantly increased during human skin development and reached 62% of the expression in normal adult skin, whereas the expression rate of transglutaminase 1, loricrin, and kallikrein 7 remained low. We transplanted keratinocytes from patients with HI and succeeded in reconstituting HI skin lesions in immunodeficient mice. The reconstituted lesions showed similar changes to those of patients with HI. Our findings demonstrate that ABCA12 is highly expressed in fetal skin and suggest that ABCA12 may play an essential role under both the wet and dry conditions, including the dramatic turning point from a wet environment of the amniotic fluid to a dry environment after birth.

Harlequin ichthyosis and other autosomal recessive congenital ichthyoses: the underlying genetic defects and pathomechanisms

Autosomal recessive congenital ichthyoses (ARCI) include several severe subtypes including harlequin ichthyosis (HI), lamellar ichthyosis and non-bullous congenital ichthyosiform erythroderma. Patients with these severe types of ichthyoses frequently show severe hyperkeratosis and scales over a large part of the body surface form birth and their quality of life is often severely affected. Recently, research into the pathomechanisms of these severe congenital ichthyoses have advanced dramatically and led to the identification of several causative genes and molecules underlying the genetic defects. To date, seven loci have been identified that are associated with ARCI and, among them, five causative genes and molecules have been detected. The five genes are transglutaminase 1 gene (TGM1), ABCA12, two lipoxygenase genes, ALOXE3 and ALOX12B and ichthyin. One of these components, ABCA12, has recently been shown to be a keratinocyte lipid transporter associated with lipid transport in lamellar granules and loss of ABCA12 function leads to a defective lipid barrier in the stratum corneum, resulting in the HI phenotype. Transglutaminse 1 deficiency was reported to cause a malformed cornified cell envelope leading to a defect in the intercellular lipid layers in the stratum corneum and defective stratum corneum barrier function resulting in an ichthyosis phenotype. Thus, defective intercellular lipid layers are major findings in autosomal recessive congenital ichthyoses. Information concerning ARCI genetic defects and disease pathomechanisms are beneficial for providing better treatments and genetic counseling including prenatal diagnosis for families affect by ichthyoses.

Prenatal exclusion of harlequin ichthyosis; potential pitfalls in the timing of the fetal skin biopsy

BACKGROUND

Harlequin ichthyosis (HI) is a severe and usually fatal congenital skin disorder with autosomal recessive inheritance. Several cases of HI prenatal diagnosis have been performed using fetal skin biopsy, mainly at around 23 weeks estimated gestational age (EGA), and reported in the literature. However, prenatal testing must be done earlier than 21 weeks EGA in several countries including Japan where the present HI families live, because termination is legally allowed only until 22 weeks EGA.

OBJECTIVES

We report the successful prenatal exclusion of HI in two fetuses from two independent families and discuss the technical difficulties and potential pitfalls in the prenatal exclusion of HI at early gestation stages.

METHODS

Fetal skin biopsy specimens and amniotic fluid samples at 19 and 20 weeks EGA from two fetuses at risk of HI were examined by light and electron microscopy.

RESULTS

For the prenatal diagnosis in case 1, the fetal skin biopsy samples were obtained at 20 weeks EGA and showed normal keratinization in the hair canals; no abnormalities were observed in the keratinized cells. In case 2, the interfollicular epidermis and the hair follicles in the samples obtained at 19 weeks EGA had not differentiated enough to show proper keratinization. However, lamellar granules were normally formed in the inner root sheath cells of the late bulbous hair pegs. From these ultrastructural findings, the case 1 fetus was diagnosed as unaffected with HI, and the case 2 fetus was diagnosed as unlikely to be affected. Subsequently, both were born as healthy, unaffected babies.

CONCLUSIONS

The timing of biopsies at 19 weeks EGA is not ideal for fetal skin biopsy because the samples are not always sufficiently differentiated for the prenatal diagnosis of HI. However, morphological observations of lamellar granules gives us important additional information useful for HI prenatal diagnosis.

Mutations in ABCA12 underlie the severe congenital skin disease harlequin ichthyosis

Harlequin ichthyosis (HI) is the most severe and frequently lethal form of recessive congenital ichthyosis. Although defects in lipid transport, protein phosphatase activity, and differentiation have been described, the genetic basis underlying the clinical and cellular phenotypes of HI has yet to be determined. By use of single-nucleotide-polymorphism chip technology and homozygosity mapping, a common region of homozygosity was observed in five patients with HI in the chromosomal region 2q35. Sequencing of the ABCA12 gene, which maps within the minimal region defined by homozygosity mapping, revealed disease-associated mutations, including large intragenic deletions and frameshift deletions in 11 of the 12 screened individuals with HI. Since HI epidermis displays abnormal lamellar granule formation, ABCA12 may play a critical role in the formation of lamellar granules and the discharge of lipids into the intercellular spaces, which would explain the epidermal barrier defect seen in this disorder. This finding paves the way for early prenatal diagnosis. In addition, functional studies of ABCA12 will lead to a better understanding of epidermal differentiation and barrier formation.

Harlequin ichthyosis in two greater kudu (Tragelaphus strepsiceros)

Two greater kudu calves (Tragelaphus strepsiceros) born 7 years apart were found with fissures and thickened, scaly, cutaneous plates covering over 80% of their bodies. One was dead at presentation, and the other was euthanized shortly after birth. Both animals shared a common sire. On necropsy, chemosis, ectropion, eclabium, and bilateral valgus deformities of the tarsal joints were observed in one calf, presumed to be secondary to the plates restricting normal fetal development. The principal microscopic lesion was severe lamellar orthokeratosis, with focal mild parakeratosis. Ultrastructural epidermal lesions included the absence of normal lamellar granules, large dilated endoplasmic reticulum, and abnormal retention of organelles and vesicles. Gross, histopathologic, and electron microscopic findings in both kudu calves were consistent with those of harlequin ichthyosis, a rare dermatosis of humans believed to have an autosomal recessive inheritance pattern. The underlying genetic and molecular abnormality and heritability of this condition in this greater kudu herd were not determined.

Expression of transglutaminase 1 in human hair follicles, sebaceous glands and sweat glands

To explore the function of the enzyme transglutaminase 1 (TGase 1), its distribution was analysed by immunofluorescence microscopy, postembedding immunoelectron microscopy and in situ hybridization. TGase 1 was expressed in the outer root sheath (ORS) cells in the distal portion of the isthmus and infundibulum of human hair follicles. In the level of the proximal and middle portion of the isthmus, TGase 1 was observed in the keratinized area of the inner root sheath (IRS) and ORS cells. In the bulbar and suprabulbar portions, TGase 1 was present in the ORS and IRS cells. The cortex and medulla cells in these regions also contained TGase 1. A high level of fluorescence was observed at the cuticle of the cortex. Sebaceous and sweat gland cells contained abundant TGase 1. Possible functions of TGase 1 in these epidermal appendages are discussed.