Clinical, light and electron microscopic features of recessive ichthyosis congenita type III

Autosomal recessive congenital ichthyosis

The term autosomal recessive congenital ichthyosis (ARCI) refers to a group of rare disorders of keratinization classified as nonsyndromic forms of ichthyosis. This group was traditionally divided into lamellar ichthyosis (LI) and congenital ichthyosiform erythroderma (CIE) but today it also includes harlequin ichthyosis, self-healing collodion baby, acral self-healing collodion baby, and bathing suit ichthyosis. The combined prevalence of LI and CIE has been estimated at 1 case per 138 000 to 300 000 population. In some countries or regions, such as Norway and the coast of Galicia, the prevalence may be higher due to founder effects. ARCI is genetically highly heterogeneous and has been associated with 6 genes to date: TGM1, ALOXE3, ALOX12B, NIPAL4, CYP4F22, and ABCA12. In this article, we review the current knowledge on ARCI, with a focus on clinical, histological, ultrastructural, genetic, molecular, and treatment-related aspects.

Congenital ichthyosis: mutations in ichthyin are associated with specific structural abnormalities in the granular layer of epidermis

BACKGROUND

Autosomal recessive congenital ichthyosis (ARCI) is a heterogeneous group of skin disorders. Several mutant genes have been identified in ARCI, but the association between genotype and phenotype is poorly understood.

METHODS

To investigate genotype-phenotype correlations in ARCI, we selected 27 patients from 18 families with specific ultrastructural features of the epidermis. The characteristic findings using electron microscopy (EM) were abnormal lamellar bodies and elongated membranes in the stratum granulosum, classified as ARCI EM type III. DNA samples from a subset of affected individuals were screened for homozygous genomic regions, and a candidate gene region was identified on chromosome 5q33. The region coincides with the ichthyin gene, previously reported as mutated in ARCI.

RESULTS

Mutation screening of ichthyin revealed missense or splice-site mutations in affected members from 16 of 18 (89%) families with characteristics of ARCI EM type III. In a control group of 18 patients with ARCI without EM findings consistent with type III, we identified one patient homozygous for a missense mutation in ichthyin.

DISCUSSION

Our findings indicate a strong association between ultrastructural abnormalities in the granular layer of epidermis and ichthyin mutations. The results also suggest that EM provides a tool for specific diagnosis in a genetically homogenous subgroup of patients with ARCI.

Genetic and pharmacological evidence that a retinoic acid cannot be the RXR-activating ligand in mouse epidermis keratinocytes

Using genetic and pharmacological approaches, we demonstrate that both RARgamma/RXRalpha heterodimers involved in repression events, as well as PPARbeta(delta)/RXRalpha heterodimers involved in activation events, are cell-autonomously required in suprabasal keratinocytes for the generation of lamellar granules (LG), the organelles instrumental to the formation of the skin permeability barrier. In activating PPARbeta(delta)/RXRalpha heterodimers, RXRalpha is transcriptionally active as its AF-2 activation function is required and can be inhibited by an RXR-selective antagonist. Within repressing RARgamma/RXRalpha heterodimers, induction of the transcriptional activity of RXRalpha is subordinated to the addition of an agonistic ligand for RARgamma. Thus, the ligand that possibly binds and activates RXRalpha heterodimerized with PPARbeta(delta) cannot be a retinoic acid, as it would also bind RARgamma and relieve the RARgamma-mediated repression, thereby yielding abnormal LGs. Our data also demonstrate for the first time that subordination of RXR transcriptional activity to that of its RAR partner plays a crucial role in vivo, because it allows RXRs to act concomitantly, within the same cell, as heterodimerization partners for repression, as well as for activation events in which they are transcriptionally active.

Ultrastructural features resembling those of harlequin ichthyosis in patients with severe congenital ichthyosiform erythroderma

Congenital ichthyoses are a group of heterogeneous disorders of cornification. Autosomal recessive congenital ichthyosis (ARCI) can be clinically subdivided into congenital ichthyosiform erythroderma and lamellar ichthyosis. Ultrastructurally, ARCI is classified into four groups: ichthyosis congenita (IC) types I-IV. The genetic background of the ARCI disorders is heterogeneous, but only one disease gene, transglutaminase 1, has been detected so far. We describe six patients with severe congenital ichthyosis from six different Scandinavian families. They could not be classified ultrastructurally into the four IC groups because of atypical findings of electron microscopy. These included abnormal lamellar bodies, alterations in keratohyalin, remnant organelles and lipid inclusions in the upper epidermal cells, which resembled the ultrastructural findings of harlequin ichthyosis (HI), although the HI phenotype was not present at birth. Some clinical features, such as thick scales, erythroderma, alopecia and ectropion were common to all patients. Ichthyosis was usually accentuated in the scalp and four patients had clumped fingers and toes. None of the patients carried the transglutaminase 1 mutation. We conclude that ultrastructural findings resembling those detected in previous HI cases (type 1 and 2) can also be found in patients who do not have classic clinical features of that rare ichthyosis. This may be due to lack of specificity of ultrastructural markers for HI or to its clinical heterogeneity.

Collodion baby and lamellar ichthyosis

It is important to differentiate the collodion baby from harlequin ichthyosis as the latter rarely survives past the first few days of life. Occasionally, babies share features of both disorders and defy a clinical diagnosis. We recently encountered such a baby who initially presented with harlequin-like features, but evolved into lamellar ichthyosis once the keratin cast was shed. Since the routine histology of all these ichthyoses is similar, we used electron microscopy to study serial biopsy specimens from the affected infant on days 7, 14, and 150, and compared them to our own other cases of harlequin ichthyosis and lamellar ichthyosis. Electron microscopic studies of our case revealed that the marginal band of cornified cells of the stratum corneum was absent when the baby exhibited collodion/harlequin ichthyosis features. Another biopsy taken when the clinical picture evolved into lamellar-like ichthyosis, showed a well-formed marginal band in the cornified cells. In harlequin ichthyosis, the marginal band is present at birth. It is suggested that electron microscopy can differentiate severe collodion baby from harlequin ichthyosis at birth using the absence of the marginal band. Previously reported features of harlequin ichthyosis, such as the presence of giant mitochondria and an abnormal formation of the marginal band in luminal villi of acrosyringeal eccrine duct, were absent in our case.

Epidermal barrier in disorders of the skin

The water permeability barrier of the stratum corneum (SC) seems primarily to be regulated by the lamellarly arranged lipid bilayers between the corneocytes, which originate largely from polar lipid precursors provided by the cells of stratum granulosum via exocytosis of the lamellar body (LB) content. In particular, the structural organization of these intercellular lipid lamellae seems to be responsible for the very low water permeability of the intact skin, and these lipid-rich structures might also influence the desquamation process in the SC. The aim of this study was to obtain further insight into the distribution and organization of the epidermal lipids (EL) and the mechanism involved in desquamation and barrier function in normal human skin and scaling skin disorders. Biopsies of healthy human skin (n = 12), of inflammatory skin diseases (atopic dry skin (n = 9), psoriatic skin lesions [n = 2]), and of hereditary keratinization disorders (autosomal recessive ichthyoses congenita (n = 3), X-chromosomal ichthyosis (XCI) [n = 3]) were analyzed utilizing a special fixation protocol with ruthenium tetroxide (RuO4) postfixation. While the atopic dry skin revealed normal barrier structures, the psoriasis lesions were characterized by severe alteration of the lipid structures leading to an abnormal interaction with the desmosomal unit. While the intercellular domains in some of the studied keratinization disorders showed an impaired distribution of the EL (autosomal recessive ichthyoses), X-chromosomal ichthyosis showed normal lipid architecture. Dry and scaly skin disorders are therefore not always accompanied by an impairment of the water permeability barrier.

Clinical, light and electron microscopic features of recessive congenital ichthyosis type I

Based on electron microscopic features, recessive congenital ichthyoses have recently been divided into four subgroups designated ichthyosis congenita (IC) types I, II, III and IV. Type II is characterized by cholesterol clefts in the horny cells, type III by perinuclear elongated membranes in the granular and horny cells, and type IV by masses of lipid membranes in granular and horny cells. Clear electron microscopic criteria for type I are lacking, although the presence of lipid droplets in the horny cells has been suggested as a criterion. In the present study we included ichthyosis patients with (i) recessive inheritance, (ii) erythrodermic fine scaling, (iii) lack of fine structural markers of IC types II-IV. Patients with ichthyotic syndromes were excluded. The case material consisted of 21 patients from 14 families. Eight were collodion babies at birth, but three were normal. Nine had ectropion, the flexures were affected in 12, and the palms and soles were thickened in all but one patient. On electron microscopy lipid vacuoles in the horny cells were common, but were absent in four patients. Changes in other lipid-related structures, including keratinosomes, were common. We conclude that currently type I can be diagnosed only by excluding the other types of ichthyosis. Clinically, IC type I corresponds to classical non-bullous congenital ichthyosiform erythroderma, but there is marked heterogeneity among affected individuals.

Histological and ultrastructural study of a family with erythrokeratodermia progressiva symmetrica

We have examined a family with 4 members in three succeeding generations suffering from a severe keratinization disorder. The clinical phenotype, with symmetric plaques on the extremities, corresponded to erythrokeratodermia progressiva symmetrica. It was manifested at birth, however, and in addition to the hyperkeratotic plaques, follicular hyperkeratosis was also observed. Electron microscopy revealed multiple morphological changes such as myelinated membrane structures, or needles, which were similar to those occurring in ichthyotic disorders and tyrosinemia, as well as in harlequin fetuses, all of which were excluded clinically or biochemically in our patients.