Two new loci for autosomal recessive ichthyosis on chromosomes 3p21 and 19p12-q12 and evidence for further genetic heterogeneity

Autosomal recessive ALOX12B gene and consecutive collodion baby

Autosomal recessive congenital ichthyosis is a type of inherited ichthyosis which is a rare cluster of genetic disorders leading to defective keratinisation. The combined prevalence for lamellar ichthyosis and congenital ichthyosiform erythroderma is almost 1 per 200 000-300 000 people. Among all the mutations in this gene, missense and frameshift mutations are most common which account for 80% of the cases. Our patient had a mutation in R-type arachidonate 12-lipoxygenase gene (ALOX12B, OMIM*603741).

Structural and functional insights into ABHD5, a ligand-regulated lipase co-activator

Alpha/beta hydrolase domain-containing protein 5 (ABHD5) is a highly conserved protein that regulates various lipid metabolic pathways via interactions with members of the perilipin (PLIN) and Patatin-like phospholipase domain-containing protein (PNPLA) protein families. Loss of function mutations in ABHD5 result in Chanarin-Dorfman Syndrome (CDS), characterized by ectopic lipid accumulation in numerous cell types and severe ichthyosis. Recent data demonstrates that ABHD5 is the target of synthetic and endogenous ligands that might be therapeutic beneficial for treating metabolic diseases and cancers. However, the structural basis of ABHD5 functional activities, such as protein-protein interactions and ligand binding is presently unknown. To address this gap, we constructed theoretical structural models of ABHD5 by comparative modeling and topological shape analysis to assess the spatial patterns of ABHD5 conformations computed in protein dynamics. We identified functionally important residues on ABHD5 surface for lipolysis activation by PNPLA2, lipid droplet targeting and PLIN-binding. We validated the computational model by examining the effects of mutating key residues in ABHD5 on an array of functional assays. Our integrated computational and experimental findings provide new insights into the structural basis of the diverse functions of ABHD5 as well as pathological mutations that result in CDS.

Identification of a novel missense mutation in NIPAL4 gene: First 3D model construction predicted its pathogenicity

Background

The NIPAL4 gene is described to be implicated of Congenital Ichthyosiform Erythroderma (CIE). It encodes a magnesium transporter membrane‐associated protein, hypothetically involved in epidermal lipid processing and in lamellar body formation. The aim of this work is to investigate the causative mutation in a consanguineous Tunisian family with a clinical feature of CIE with a yellowish severe palmoplantar keratoderma.

Methods

Four patients were dignosed with CIE. The blood samples were collected from patients and all members of their nuclear family for mutation analysis. The novel mutation of NIPAL4 gene was analysed with several software tools to predict its pathogenicity. Then, the secondary structure and the 3D model of ichthyn was generated in silico.

Results

The sequencing analysis of the NIPAL4 gene in patients revealed a novel homozygous missense mutation c.534A>C (p.E178D) in the exon 4. Bioinformatic tools predicted its pathogenicity. The secondary structure prediction and the 3D model construction expected the presence of 9 transmembrane helices and revealed that mutation p.E178D was located in the middle of the second transmembrane helices. Besides, the 3D model construction revealed that the p.E178D mutation is inducing a shrinking in the transport channel containing the mutated NIPA4 protein.

Conclusion

We found a homozygous mutation in exon 4 of NIPAL4 c.534A>C (p.E178D), which was identified for the first time in our study. Bioinformatic investigations supported its involvement in the phenotype of patients with CIE. Interestingly, this mutation was located in the hypothetical transport channel cavity and leads to changes in the channel architecture, which would probably affect its transport function.

Identification and association of recurrent ALOXE3 mutation with non-bullous congenital ichthyosiform erythroderma in two ethnically distinct Pakistani families

Non-bullous congenital ichthyosiform erythroderma (NCIE) is characterized by skin scaling with erythema. In this study, two Pakistani families with NCIE are genetically characterized through Whole Exome and Sanger sequencing to identify molecular basis of the disease. We identified a nonsense homozygous c.2026C>T mutation of ALOXE3, causing premature termination of the eLOX3 protein (p.Q676X). In silico studies predicted impaired enzymatic activity of the premature truncated eLOX3, leading to abnormal synthesis of specific hepoxilin derivatives, essential for epidermal barrier formation. It is the first ever study reporting homozygotes of p.Q676X mutation in ethnically distinct two Pakistani families; otherwise, heterozygotes of the said mutation have been reported in South Asian population only. Hence, mutation seems to be region-specific and may be useful for molecular diagnosis of NCIE. Moreover, our findings should help in genetic counseling and career screening.

Transglutaminase-1 mutations in Omani families with lamellar ichthyosis

OBJECTIVE

To determine the molecular basis of familial ichthyosis in three Omani families.

SUBJECTS AND METHODS

Nine patients from three consanguineous families, A, B, and C, were born with typical features of lamellar ichthyosis subtype including collodion membrane and maintained ectropion, and epidermal scaling through their childhood. The 4 patients from family B had more severe symptoms requiring neonatal critical care and subsequent regular treatment with emollients, eye lubricants, and low-dose acitretin. DNA was extracted from peripheral blood by standard methods. The samples were initially genotyped to screen known loci linked to recessive ichthyosis on chromosomes 2q33-32 (ABCA12), 14q11 (TGM1), and 19p12-q12 using commercially supplied polymorphic fluorescent microsatellite markers. TGM1 was analyzed by direct sequencing for disease-associated mutations.

RESULTS

Two known pathogenic mutations in TGM1 were detected: p.Gly278Arg in families A and B and p.Arg396His in family C. These two mutations were segregating in an autosomal recessive mode of inheritance.

CONCLUSION

Two known pathogenic TGM1 mutations were detected in three large consanguineous Omani families with lamellar ichthyosis. This study confirmed the geographic distribution of known mutations to an apparently unrelated population.

Molecular analysis of Chanarin-Dorfman syndrome (CDS) patients: Identification of novel mutations in the ABHD5 gene

Chanarin-Dorfman syndrome (CDS) is an autosomal recessive metabolic disorder associated with congenital ichthyosis and a multisystemic accumulation of neutral lipids in various types of cells. Recently, mutations of the ABHD5 gene were identified as the cause of CDS. In this work, we carried out molecular analysis of the ABHD5 gene in 6 unrelated patients. We identified one previously reported mutation, N209X and two novel genetic alterations; a nonsense mutation (p.Y50X) and missense mutation (p.S73A).

Neutral lipid storage disease: genetic disorders caused by mutations in adipose triglyceride lipase/PNPLA2 or CGI-58/ABHD5

Neutral lipid storage disease (NLSD) is a group of autosomal recessive disorders characterized by the excessive accumulation of neutral lipids in multiple tissues. Recently, two genes, adipose triglyceride lipase (ATGL/PNPLA2) and comparative gene identification-58 (CGI-58/ABHD5), have been shown to cause NLSD. ATGL specifically hydrolyzes the first fatty acid from triacylglycerols (TG) and CGI-58/ABHD5 stimulates ATGL activity by a currently unknown mechanism. Mutations in both the ATGL and the CGI-58 genes are associated with systemic TG accumulation, yet the resulting clinical manifestations are not identical. Patients with defective ATGL function suffer from more severe myopathy (NLSDM) than patients with defective CGI-58 function. On the other hand, CGI-58 mutations are always associated with ichthyosis (NLSDI), which was not observed in patients with defective ATGL function. These observations indicate an ATGL-independent function of CGI-58. This review summarizes recent findings with the goal of relating structural variants of ATGL and CGI-58 to functional consequences in lipid metabolism.

Mutations in the fatty acid transport protein 4 gene cause the ichthyosis prematurity syndrome

Ichthyosis prematurity syndrome (IPS) is an autosomal-recessive disorder characterized by premature birth and neonatal asphyxia, followed by a lifelong nonscaly ichthyosis with atopic manifestations. Here we show that the gene encoding the fatty acid transport protein 4 (FATP4) is mutated in individuals with IPS. Fibroblasts derived from a patient with IPS show reduced activity of very long-chain fatty acids (VLCFA)-CoA synthetase and a specific reduction in the incorporation of VLCFA into cellular lipids. The human phenotype is consistent with Fatp4 deficiency in mice that is characterized by a severe skin phenotype, a defective permeability barrier function, and perturbed VLCFA metabolism. Our results further emphasize the importance of fatty acid metabolism for normal epidermal barrier function illustrated by deficiency of a member in the FATP family of proteins.

New mutations in the transglutaminase 1 gene in three families with lamellar ichthyosis

BACKGROUND

Autosomal recessive lamellar ichthyosis (LI) is a severe skin disorder characterized by generalized hyperkeratosis. Gene mutation in transglutaminase 1 (TGM1), which mediates cross-links in the formation of the cell envelope during terminal differentiation of epidermis, has been identified as a cause of LI.

OBJECTIVES

To determine mutations of TGM1 gene in three Chinese families with LI.

METHODS

The TGM1 gene was sequenced to identify disease-causing mutations in the three families with LI. One of the results was confirmed by using reverse transcriptase PCR and in situ hybridization. An in situ transglutaminase (TGase) 1 assay was performed to estimate TGase 1 activity in the patients' skin.

RESULTS

Four novel mutations of keratinocyte TGase1 (Q203X, D254N, R687H and IVS4 + 1G-->T) were found in the three families. No TGase 1 mRNA was detected in patient skin using RT-PCR and in situ hybridization, and the in situ TGase assay showed that there was no or decreased TGase 1 activity in patient skin.

CONCLUSIONS

Our findings suggest that four novel mutations in TGM1 gene result in decrease or absence of TGase activity in the skin and, as a consequence, cause the phenotype of LI.

Transglutaminase-1 gene mutations in autosomal recessive congenital ichthyosis: summary of mutations (including 23 novel) and modeling of TGase-1

Autosomal recessive congenital ichthyosis (ARCI) is a heterogeneous group of rare cornification diseases. Germline mutations in TGM1 are the most common cause of ARCI in the United States. TGM1 encodes for the TGase-1 enzyme that functions in the formation of the cornified cell envelope. Structurally defective or attenuated cornified cell envelop have been shown in epidermal scales and appendages of ARCI patients with TGM1 mutations. We review the clinical manifestations as well as the molecular genetics of ARCI. In addition, we characterized 115 TGM1 mutations reported in 234 patients from diverse racial and ethnic backgrounds (Caucasion Americans, Norwegians, Swedish, Finnish, German, Swiss, French, Italian, Dutch, Portuguese, Hispanics, Iranian, Tunisian, Moroccan, Egyptian, Afghani, Hungarian, African Americans, Korean, Japanese and South African). We report 23 novel mutations: 71 (62%) missense; 20 (17%) nonsense; 9 (8%) deletion; 8 (7%) splice-site, and 7 (6%) insertion. The c.877-2A>G was the most commonly reported TGM1 mutation accounting for 34% (147 of 435) of all TGM1 mutant alleles reported to date. It had been shown that this mutation is common among North American and Norwegian patients due to a founder effect. Thirty-one percent (36 of 115) of all mutations and 41% (29 of 71) of missense mutations occurred in arginine residues in TGase-1. Forty-nine percent (35 of 71) of missense mutations were within CpG dinucleotides, and 74% (26/35) of these mutations were C>T or G>A transitions. We constructed a model of human TGase-1 and showed that all mutated arginines that reside in the two beta-barrel domains and two (R142 and R143) in the beta-sandwich are located at domain interfaces. In conclusion, this study expands the TGM1 mutation spectrum and summarizes the current knowledge of TGM1 mutations. The high frequency of mutated arginine codons in TGM1 may be due to the deamination of 5' methylated CpG dinucleotides.

Pathogenesis of permeability barrier abnormalities in the ichthyoses: inherited disorders of lipid metabolism

Many of the ichthyoses are associated with inherited disorders of lipid metabolism. These disorders have provided unique models to dissect physiologic processes in normal epidermis and the pathophysiology of more common scaling conditions. In most of these disorders, a permeability barrier abnormality "drives" pathophysiology through stimulation of epidermal hyperplasia. Among primary abnormalities of nonpolar lipid metabolism, triglyceride accumulation in neutral lipid storage disease as a result of a lipase mutation provokes a barrier abnormality via lamellar/nonlamellar phase separation within the extracellular matrix of the stratum corneum (SC). Similar mechanisms account for the barrier abnormalities (and subsequent ichthyosis) in inherited disorders of polar lipid metabolism. For example, in recessive X-linked ichthyosis (RXLI), cholesterol sulfate (CSO(4)) accumulation also produces a permeability barrier defect through lamellar/nonlamellar phase separation. However, in RXLI, the desquamation abnormality is in part attributable to the plurifunctional roles of CSO(4) as a regulator of both epidermal differentiation and corneodesmosome degradation. Phase separation also occurs in type II Gaucher disease (GD; from accumulation of glucosylceramides as a result of to beta-glucocerebrosidase deficiency). Finally, failure to assemble both lipids and desquamatory enzymes into nascent epidermal lamellar bodies (LBs) accounts for both the permeability barrier and desquamation abnormalities in Harlequin ichthyosis (HI). The barrier abnormality provokes the clinical phenotype in these disorders not only by stimulating epidermal proliferation, but also by inducing inflammation.

Severe bilateral rachitic genu valgum in patients with nonbullous congenital ichthyosiform erythroderma: a report of two cases and review of literature

We present two cases of ichthyosiform erythroderma associated with severe bilateral genu valgum. Other musculoskeletal features associated with this condition are described. The details and outcome of operative intervention for the correction of the deformities are discussed. The disturbances of the metabolism of vitamin D and medical management are discussed. A review of literature is presented.

Novel mutations in ALOX12B in patients with autosomal recessive congenital ichthyosis and evidence for genetic heterogeneity on chromosome 17p13

We report clinical and molecular findings in 20 patients from 11 families with autosomal recessive congenital ichthyosis (ARCI) linked to chromosome 17p13, and attributed to mutations in the ALOX gene cluster, which includes three lipoxygenase genes, ALOXE3, ALOX12B, and ALOX15B. We identified six novel missense mutations and one novel deletion leading to a premature stop codon in ALOX12B in only six out of the 11 families which led us to investigate a possible implication of ALOX15B. Mutation analysis of this gene, as well as ALOXE3, which is known to be mutated in some cases of ARCI, failed to reveal causative mutations in the five remaining ARCI families, indicating that other genes on chromosome 17p13 may be involved in this disease. However, by adding new variants to the repertoire of ALOX12B mutations in non-bullous congenital ichthyosiform erythroderma, our data contribute to an enlargement of the spectrum of mutations for the development of efficient molecular genetic tests for analysis of at risk individuals whose carrier status is unknown.

Bathing suit ichthyosis is caused by transglutaminase-1 deficiency: evidence for a temperature-sensitive phenotype

Bathing suit ichthyosis (BSI) is a striking and unique clinical form of autosomal recessive congenital ichthyosis characterized by pronounced scaling on the bathing suit areas but sparing of the extremities and the central face. Here we report on a series of 10 BSI patients. Our genetic, ultrastructural and biochemical investigations show that BSI is caused by transglutaminase-1 (TGase-1) deficiency. Altogether, we identified 13 mutations in TGM1-among them seven novel missense mutations and one novel nonsense mutation. Structural modeling for the Tyr276Asn mutation reveals that the residue is buried in the hydrophobic interior of the enzyme and that the hydroxyl side chain of Tyr276 is exposed to solvent in a cavity of the enzyme. Cryosections of healthy skin areas demonstrated an almost normal TGase activity, in contrast to the affected BSI skin, which only showed a cytoplasmic and clearly reduced TGase-1 activity. The distribution of TGase-1 substrates in the epidermis of affected skin corresponded to the situation in TGase-1 deficiency. Interestingly, the expression of TGase-3 and cathepsin D was reduced. Digital thermography validated a striking correlation between warmer body areas and presence of scaling in patients suggesting a decisive influence of the skin temperature. In situ TGase testing in skin of BSI patients demonstrated a marked decrease of enzyme activity when the temperature was increased from 25 to 37 degrees C. We conclude that BSI is caused by TGase-1 deficiency and suggest that it is a temperature-sensitive phenotype.

Homozygosity mapping as a screening tool for the molecular diagnosis of hereditary skin diseases in consanguineous populations

BACKGROUND

The routine diagnosis of genodermatoses is significantly complicated by the fact that in this group of disorders, clinical manifestations may result from mutations in unrelated genes (genetic heterogeneity) and mutations in the same gene often lead to dissimilar clinical signs (phenotypic heterogeneity).

METHODS

In this study, we applied the principles of homozygosity mapping as a screening method before formal mutational analysis in an attempt to facilitate the molecular diagnosis of genodermatoses in consanguineous families. The method was evaluated in a retrospective fashion in 4 families previously assessed with junctional epidermolysis bullosa and in a prospective manner in 11 families with congenital recessive ichthyosis.

RESULTS

The method was found to be efficient in directing the molecular analysis to one of the 4 genes commonly involved in the pathogenesis of junctional epidermolysis bullosa or in identifying cases of congenital recessive ichthyosis caused by mutations in TGM1. We found that this diagnostic strategy results in a 5-fold decrease in the cost of mutation analysis.

LIMITATIONS

The proposed diagnostic strategy is applicable to consanguineous families only and, therefore, cannot be used in outbred populations.

CONCLUSION

Our results indicate that homozygosity mapping may serve as a useful adjunct in the molecular diagnosis of junctional epidermolysis bullosa or congenital recessive ichthyosis in inbred populations. This study emphasizes the usefulness in human genetics of diagnostic strategies tailored to the demographic features of target populations.

A founder mutation for ichthyosis prematurity syndrome restricted to 76 kb by haplotype association

Autosomal recessive congenital ichthyosis (ARCI) is a group of keratinisation disorders that includes the ichthyosis prematurity syndrome (IPS). IPS is rare and almost exclusively present in a restricted region in the middle of Norway and Sweden, which indicates a founder effect for the disorder. We recently reported linkage of IPS to chromosome 9q34, and we present here the subsequent fine-mapping of this region with known and novel microsatellite markers as well as single nucleotide polymorphisms (SNPs). Allelic association, evaluated with Fisher's exact test and P (excess), was used to refine the IPS haplotype to approximately 1.6 Mb. On the basis of the average length of the haplotype in IPS patients, we calculated the age of a founder mutation to approximately 1,900 years. The IPS haplotype contains a core region of 76 kb consisting of four marker alleles shared by 97.7% of the chromosomes associated with IPS. This region spans four known genes, all of which are expressed in mature epidermal cells. We present the results from the analysis of these four genes and their corresponding transcripts in normal and patient-derived samples.

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.

Mutations in a new cytochrome P450 gene in lamellar ichthyosis type 3

We report the identification of mutations in a non-syndromic autosomal recessive congenital ichthyosis (ARCI) in a new gene mapping within a previously identified locus on chromosome 19p12-q12, which has been defined as LI3 in the OMIM database (MIM 604777). The phenotype usually presents as lamellar ichthyosis and hyperlinearity of palms and soles. Seven homozygous mutations including five missense mutations and two deletions were identified in a new gene, FLJ39501, on chromosome 19p12 in 21 patients from 12 consanguineous families from Algeria, France, Italy and Lebanon. FLJ39501 encodes a protein which was found to be a cytochrome P450, family 4, subfamily F, polypeptide 2 homolog of the leukotriene B4-omega-hydroxylase (CYP4F2) and could catalyze the 20-hydroxylation of trioxilin A3 from the 12(R)-lipoxygenase pathway. Further oxidation of this substrate by the fatty alcohol:nicotinamide-adenine dinucleotide oxidoreductase (FAO) enzyme complex, in which one component, ALDH3A2, is known to be mutated in Sjögren-Larsson syndrome (characterized by ichthyosis and spastic paraplegia), would lead to 20-carboxy-(R)-trioxilin A3. This compound could be involved in skin hydration and would be the essential missing product in most forms of ARCI. Its chiral homolog, 20-carboxy-(S)-trioxilin A3, could be implicated in spastic paraplegia and in the maintenance of neuronal integrity.

Identification of a novel locus associated with congenital recessive ichthyosis on 12p11.2-q13

Congenital recessive ichthyoses represent a vast and markedly heterogeneous group of diseases that have been mapped to at least seven distinct chromosomal loci. In this study, we ascertained two consanguineous families presenting with congenital ichthyosis. Using homozygosity mapping, we identified a 6.5 cM homozygous region on 12p11.2-q13 shared by all affected individuals. Multipoint logarithm of odds ratio (LOD) score analysis placed the new locus between markers D12S345 and D12S390 with a maximum LOD score of 4.79 at marker CH12SSR13. This region harbors PPHLN1, encoding periphilin 1, a protein involved in the cornification process. No deleterious mutations were identified within the coding region of this gene, suggesting the existence of another gene associated with epidermal differentiation on 12p11.2-q13.

Erythrokeratoderma variabilis-like ichthyosis in Chanarin-Dorfman syndrome

Neutral lipid storage disease (Chanarin-Dorfman syndrome) is an autosomal recessive metabolic disorder associated with congenital ichthyosis and a multisystemic accumulation of neutral lipids (lipid droplets) in various types of cells. The clinical presentation has been reported to correspond to that of nonbullous congenital ichthyosiform erythroderma. We report a 4-year-old boy presenting a generalized ichthyosiform disorder manifested by migrating scaly plaques alternating with areas of normal-looking skin, showing erythematous borders with sharp margins, clinically suggestive of erythrokeratoderma variabilis (EKV). A peripheral blood smear revealed cytoplasmic vacuoles in most granulocytes. Genetic studies from the patient and his parents revealed that the patient carried two different and novel mutations of the ABHD5 gene: a nonsense mutation in exon 6 (transmitted by the father) and an insertion/deletion in exon 4 (transmitted by the mother). Our observation demonstrates the clinical heterogeneity of the ichthyosiform dermatoses observed in Chanarin-Dorfman syndrome and widens the clinical range of conditions presenting migrating scaly plaques mimicking EKV.

The expression of the 27-kd heat shock protein in keratinization disorders: an immunohistological study

In human skin, the 27-kd heat shock protein (hsp27), a member of the small hsp family, is expressed mainly in the upper epidermal layers. Hsp27 functions as a molecular chaperone and is involved in the regulation of cell growth and differentiation. According to experimental evidence, epidermal hsp27, through its chaperone function, might play a role in the assembly of keratin filaments and the cornified cell envelope. This study was conducted to assess the expression pattern of hsp27 in a panel of different ichthyoses. Twelve hereditary and acquired skin diseases associated with an ichthyotic phenotype and 2 corresponding mouse models were investigated by immunohistochemistry on formalin-fixed paraffin-embedded tissue, using a monoclonal antibody specific for hsp27. In ichthyosis vulgaris, lamellar ichthyosis, Sjögren-Larsson syndrome, Netherton syndrome, and acquired ichthyosiform skin condition, the pattern of hsp27 expression resembles healthy human skin. Hsp27 expression was reduced in bullous ichthyosiform erythroderma and annular epidermolytic ichthyosis, and absent in X-linked recessive ichthyosis (1/3 patients) and congenital hemidysplasia with ichthyosiform nevus and limb defects syndrome (1/1). In X-linked dominant chondrodysplasia, 3 small samples are completely negative and 2 larger samples show a pattern resembling random X inactivation. In the mouse models, tattered and bare patches, representing the murine analogues to X-linked dominant chondrodysplasia and congenital hemidysplasia with ichthyosiform nevus and limb defects syndrome, expression of hsp25 (the murine homologue of hsp27) also showed lyonization, demonstrating a clear-cut link between hsp27 expression and underlying molecular pathology. Our results show that loss of hsp27 is a rare event in human epidermis that is associated with specific genetic defects. Among the cases described here, these defects are either in suprabasal keratins or in enzymes involved in cholesterol biosynthesis. The expression and chaperone function of hsp27 might be modified by low/absent epidermal cholesterol and aberrant substrates (ie, keratins) resulting in protein misfolding, dyskeratosis, and thus contribute to the ichthyotic phenotype.

The cornified envelope: a model of cell death in the skin

The epidermis functions as a barrier against the environment by means of several layers of terminally differentiated, dead keratinocytes - the cornified layer, which forms the endpoint of epidermal differentiation and death. The cornified envelope replaces the plasma membrane of differentiating keratinocytes and consists of keratins that are enclosed within an insoluble amalgam of proteins, which are crosslinked by transglutaminases and surrounded by a lipid envelope. New insights into the molecular mechanisms and the physiological endpoints of cornification are increasing our understanding of the pathological defects of this unique form of programmed cell death, which is associated with barrier malfunctions and ichthyosis.

Avances biomoleculares en los trastornos epidérmicos hereditarios

In recent years, the genes responsible for many hereditary skin diseases have been discovered. These genes encode different proteins that participate in the terminal differentiation of the epidermis, so their alteration or absence causes a keratinization disorder and/or an increase in skin fragility. Thanks to genetic analyses, we have been able to understand the physiopathology of numerous genodermatoses and we have become closer to diagnosing many others. In the not-too-distant future, biomolecular techniques may foreseeably help us prevent and treat these processes, which include skin diseases as serious as epidermolysis bullosa or epidermolytic hyperkeratosis. In this article, we will study the most recent biomolecular findings referring to keratinization and epidermal disorders, mentioning the altered genes and/ or the defective proteins that cause them.

Molecular genetics of the ichthyoses

The ichthyoses are a large, clinically, genetically, and etiologically heterogeneous group of disorders of cornification due to abnormal differentiation and desquamation of the epidermis. Although they differ in clinical features, inheritance, and structural and biochemical abnormalities of the epidermis, they often pose a diagnostic challenge. For each of the 12 ichthyoses and related disorders described here, the major disease genes have been identified and genotype-phenotype correlation have begun to emerge. The molecular findings reveal the functional importance and interactions of many different epidermal proteins and metabolic pathways, including major structural proteins (keratins, loricrin), enzymes involved in lipid metabolism (transglutaminase 1, lipoxygenases, fatty aldehyde dehydrogenase, steroid sulfatase, glucocerebrosidase, Delta8-Delta7 sterol isomerase, 3beta-hydroxysteroid dehydrogenase), and protein catabolism (LEKTI), peroxisomal transport and processing (Peroxin 7 receptor, Phytanoyl-CoA hydroxylase) and DNA repair (proteins of the transcription repair complex). This review highlights the spectacular advances in the molecular genetics and biology of heritable ichthyoses over the past decade. It illustrates the power of molecular diagnostics for refining disease classification, providing prenatal diagnosis, improving genetic counseling, and clinical management.

L-2-Hydroxyglutaric aciduria: identification of a mutant gene C14orf160, localized on chromosome 14q22.1

l-2-Hydroxyglutaric aciduria (l-2-HGA) is characterized by progressive deterioration of central nervous system function including epilepsy and macrocephaly in 50% of cases, and elevated levels of l-2-hydroxyglutaric acid in urine, blood and cerebrospinal fluid (CSF). Nuclear magnetic resonance imaging shows distinct abnormalities. We report the identification of a gene for l-2-HGA aciduria (MIM 236792) using homozygosity mapping. Nine homozygous mutations including three missense mutations, two nonsense mutations, two splice site mutations and two deletions were identified in the gene C14orf160, localized on chromosome 14q22.1, in 21 patients from one non-consanguineous and 14 consanguineous Turkish families. We propose to name the gene duranin. Duranin encodes a putative mitochondrial protein with homology to FAD-dependent oxidoreductases. The functional role of this enzyme in intermediary metabolism in humans remains to be established.

Mutations in ichthyin a new gene on chromosome 5q33 in a new form of autosomal recessive congenital ichthyosis

We report the genomic localization by homozygosity mapping and the identification of a gene for a new form of non-syndromic autosomal recessive congenital ichthyosis. The phenotype usually presents as non-bullous congenital ichthyosiform erythroderma with fine whitish scaling on an erythrodermal background; larger brownish scales are present on the buttocks, neck and legs. A few patients presented a more generalized lamellar ichthyosis. Palmoplantar keratoderma was present in all cases, whereas only 60% of the patients were born as collodion babies. Six homozygous mutations including one nonsense and five missense mutations were identified in a new gene, ichthyin, on chromosome 5q33 in 23 patients from 14 consanguineous families from Algeria, Colombia, Syria and Turkey. Ichthyin encodes a protein with several transmembrane domains which belongs to a new family of proteins of unknown function localized in the plasma membrane (PFAM: DUF803), with homologies to both transporters and G-protein coupled receptors. This family includes NIPA1, in which a mutation was recently described in a dominant form of spastic paraplegia (SPG6). We propose that ichthyin and NIPA1 are membrane receptors for ligands (trioxilins A3 and B3) from the hepoxilin pathway.

The clinical spectrum of nonbullous congenital ichthyosiform erythroderma and lamellar ichthyosis

Until about 20 years ago, the term lamellar ichthyosis (LI) represented all nonbullous autosomal recessive ichthyoses except for harlequin ichthyosis and ichthyosis syndromes. Since the 1980s, nonbullous autosomal recessive ichthyoses have been divided into two major clinical entities, nonbullous congenital ichthyosiform erythroderma (NBCIE) and LI. The nature of scaling and intensity of erythroderma are important clinical features that distinguish between NBCIE and LI. However, a considerable number of cases show an intermediate phenotype between the two classic clinical features. Histologically, parakeratosis and inflammatory cell infiltration are seen more frequently in NBCIE than in LI and the stratum corneum is usually thicker in LI than in NBCIE. However, neither histopathological findings nor ultrastructural features seem to help clearly distinguish between NBCIE and LI. Mutations in any of the three known causative genes, TGM1, ALOXE3 or ALOX12B, can lead either to NBCIE or LI. Candidate genes specific to either NBCIE or LI alone have not been identified. Based on these facts, it might be better to consider NBCIE and LI as variations of a single keratinization disorder, although the classification of these autosomal recessive congenital ichthyosis patients into NBCIE or LI depending on their clinical features is still useful for practical patient management.

A novel homozygous mutation 371delA in TGM1 leads to a classic lamellar ichthyosis phenotype

Malformation of the cornified cell envelope (CCE) arising from mutations of the transglutaminase (TGase) 1 gene (TGM1) is the cause of some cases of lamellar ichthyosis (LI). However, genotype/phenotype correlation in TGM1 mutations has not yet been fully clarified. We report a typical case of LI caused by a novel mutation in TGM1. The patient, a 33-year-old woman, showed thick, lamellar scales on the entire body surface. Immunofluorescence labelling with anti-TGase 1 antibodies was negative in the patient's epidermis. In situ TGase activity assay detected markedly reduced TGase activity in granular layers of the patient's epidermis. Electron microscopy revealed incomplete thickening of the CCE during keratinization in the epidermis. Sequencing of the entire exons and exon-intron borders of TGM1 revealed that the patient was a homozygote for a novel deletion mutation 371delA in exon 3. This mutation leads to a frameshift resulting in a premature termination codon 43 bp downstream from the mutation site. According to the protein modelling of TGase 1, the truncated protein from this mutated allele loses the entire catalytic core domain of TGase 1. Thus, the present homozygous mutation is expected to cause total loss of TGase 1 activity, resulting in large, dark, lamellar scales on the entire body, the classic phenotype of LI, in this patient.

Collodion baby: a follow-up study of 17 cases

Seventeen cases of collodion baby are reported. Clinical aspects, complications, treatment, final outcome and family history were studied. We did not observe any clinical features in the collodion baby that could serve as a clue in predicting the final diagnosis. Infections were observed in nine, hypothermia in five and hypernatraemic dehydration in four cases. Skin infection mainly occurred in babies treated with emollients (petrolatum, lanolin and cetomacrogolis cream were used). We therefore recommend treating the collodion baby in a humidified incubator, if necessary with intravenous rehydration, but not to use emollients. The final outcome of these study patients was erythrodermic autosomal recessive lamellar ichthyosis in seven cases (41%), non-erythrodermic autosomal recessive lamellar ichthyosis in three cases (18%), Sjögren-Larsson in one case (6%), epidermolytic hyperkeratosis in one case (6%), acute neonatal variant of Gaucher disease in one case (6%) and normal skin in four cases (24%).

Homozygosity mapping of a locus for a novel syndromic ichthyosis to chromosome 3q27-q28

Ichthyosis is a heterogeneous group of skin disorders characterized by abnormal epidermal scaling. Occasionally, extracutaneous features are associated. A novel autosomal recessive ichthyosis syndrome is described here with scalp hypotrichosis, scarring alopecia, sclerosing cholangitis, and leukocyte vacuolization in two inbred kindreds of Moroccan origin. We also report the mapping of the diseased gene to a 21.2 cM interval of chromosome 3q27-q28. Homo zygosity for polymorphic markers has enabled us to reduce the genetic interval to a 16.2 cM region. Furthermore, comparison of mutant chromosomes in the two families has suggested a common ancestral mutant haplotype. This linkage disequilibrium has reduced the genetic interval encompassing the diseased gene to less than 9.5 cM maximum. Further study of additional families from the same geographic area will hopefully reduce the genetic interval as well as help in the cloning of the gene involved in this rare disorder.

Characterization and in silico mapping of a novel murine zinc finger transcription factor

Transcription factors play important roles in development and homeostasis. We have completed an embryonic stem cell-based neural differentiation screen, which was carried out with a view to isolating early regulators of neurogenesis. Fifty eight of the expressed sequence tags isolated from this screen represent known transcription factors or sequences containing transcription factor motifs. We have determined the full-length sequence of a novel mouse zinc finger-containing gene (ZFEND; also known as Mus musculus zinc finger protein 358 (Zfp358)) that was identified from this screen. ZFEND has 87% nucleotide and 86% amino acid identity to a previously identified human cDNA, FLJ10390, which is moderately similar to zinc finger protein 135. Northern blotting and RPAs demonstrate highest expression of ZFEND during mid-late mouse embryogenesis. Expression is also observed in several adult tissues with highest expression in heart, brain, and liver. Whole-mount in situ hybridization studies reveal apparent ubiquitous expression of ZFEND during mid-gestation stages (embryonic days 11.5, 12.5), while sections of whole-mount embryos reveal much higher expression levels in the neural folds during neural tube closure and at the boundary between the forelimb buds and the body wall. Bioinformatic analysis maps ZFEND to mouse chromosome 8pter, while FLJ10390 resides on 19p13.3-p13.2, a gene-rich region to which a number of disorders have been mapped. More precise mapping indicates that the involvement of FLJ10390 in atherogenic lipoprotein phenotype, familial febrile convulsions 2, and psoriasis susceptibility cannot be ruled out.

Development of ichthyosiform skin compensates for defective permeability barrier function in mice lacking transglutaminase 1

Transglutaminase 1 (TGase 1) is one of the genes implicated in autosomal recessive congenital ichthyosis. Skin from TGase 1(-/-) mice, which die as neonates, lacks the normal insoluble cornified envelope and has impaired barrier function. Characterization of in situ dye permeability and transepidermal water loss revealed defects in the development of the skin permeability barrier in TGase 1(-/-) mice. In the stratum corneum of the skin, tongue, and forestomach, intercellular lipid lamellae were disorganized, and the corneocyte lipid envelope and cornified envelope were lacking. Neonatal TGase 1(-/-) mouse skin was taut and erythrodermic, but transplanted TGase 1(-/-) mouse skin resembled that seen in severe ichthyosis, with epidermal hyperplasia and marked hyperkeratosis. Abnormalities in those barrier structures remained, but transepidermal water loss was improved to control levels in the ichthyosiform skin. From these results, we conclude that TGase 1 is essential to the assembly and organization of the barrier structures in stratified squamous epithelia. We suggest that the ichthyosiform skin phenotype in TGase 1 deficiency develops the massive hyperkeratosis as a physical compensation for the defective cutaneous permeability barrier required for survival in a terrestrial environment.

Transglutaminases in disease

Transglutaminases (TGases) are enzymes that are widely used in many biological systems for generic tissue stabilization purposes. Mutations resulting in lost activity underlie several serious disorders. In addition, new evidence documents that they may also be aberrantly activated in tissues and cells and contribute to a variety of diseases, including neurodegenerative diseases such as Alzheimer's and Huntington's diseases. In these cases, the TGases appear to be a factor in the formation of inappropriate proteinaceous aggregates that may be cytotoxic. In other cases such as celiac disease, however, TGases are involved in the generation of autoantibodies. Further, in diseases such as progressive supranuclear palsy, Huntington's, Alzheimer's and Parkinson's diseases, the aberrant activation of TGases may be caused by oxidative stress and inflammation. This review will examine the role and activation of TGases in a variety of diseases.

Mutations in CGI-58, the gene encoding a new protein of the esterase/lipase/thioesterase subfamily, in Chanarin-Dorfman syndrome

Chanarin-Dorfman syndrome (CDS) is a rare autosomal recessive form of nonbullous congenital ichthyosiform erythroderma (NCIE) that is characterized by the presence of intracellular lipid droplets in most tissues. We previously localized a gene for a subset of NCIE to chromosome 3 (designated "the NCIE2 locus"), in six families. Lipid droplets were found in five of these six families, suggesting a diagnosis of CDS. Four additional families selected on the basis of a confirmed diagnosis of CDS also showed linkage to the NCIE2 locus. Linkage-disequilibrium analysis of these families, all from the Mediterranean basin, allowed us to refine the NCIE2 locus to an approximately 1.3-Mb region. Candidate genes from the interval were screened, and eight distinct mutations in the recently identified CGI-58 gene were found in 13 patients from these nine families. The spectrum of gene variants included insertion, deletion, splice-site, and point mutations. The CGI-58 protein belongs to a large family of proteins characterized by an alpha/beta hydrolase fold. CGI-58 contains three sequence motifs that correspond to a catalytic triad found in the esterase/lipase/thioesterase subfamily. Interestingly, CGI-58 differs from other members of the esterase/lipase/thioesterase subfamily in that its putative catalytic triad contains an asparagine in place of the usual serine residue.

Bullous and non-bullous ichthyosiform erythroderma associated with generalized pustular psoriasis of von Zumbusch type

Bullous ichthyosiform erythroderma (BIE) and non-bullous ichthyosiform erythroderma (NBIE) are rare congenital ichthyoses. Generalized pustular psoriasis (GPP) of von Zumbusch type is a rare and severe form of psoriasis marked by desquamative and pustular erythroderma associated with fever and altered general conditions. We report two adults with an ichthyosis typical of BIE in the first case and NBIE in the second, without any previous history of psoriasis, who presented with a severe and relapsing GPP of von Zumbusch type. Using current knowledge of the genetic relationship between psoriasis and congenital ichthyoses, we discuss the possibility of a common physiopathological link between congenital ichthyoses and GPP, and examine the possible therapeutic problems resulting from this pathological association, especially in BIE.

Loricrin keratoderma: a cause of congenital ichthyosiform erythroderma and collodion baby

A group of hereditary palmoplantar keratodermas due to heterozygous mutation in the loricrin gene has recently been identified. Of five reported pedigrees, four presented as mutilating keratoderma with ichthyosis (variant Vohwinkel syndrome), and one as progressive symmetric erythrokeratoderma. We report a new Japanese pedigree of loricrin keratoderma. A 14-year-old male and his 11-year-old female sibling had both been born as collodion babies and were initially diagnosed as having non-bullous congenital ichthyosiform erythroderma, but later developed palmoplantar keratoderma with pseudoainhum. Their father was similarly affected. Direct sequencing of genomic DNA revealed a G residue insertion at codon 230-231 of the loricrin gene. Antibody studies confirmed the presence of mutant loricrin in the retained nuclei. We conclude that loricrin gene mutation may present as congenital ichthyosiform erythroderma, and should be included in the differential diagnosis of collodion baby.

Aberrant lipid organization in stratum corneum of patients with atopic dermatitis and lamellar ichthyosis

There are several skin diseases in which the lipid composition in the intercellular matrix of the stratum corneum is different from that of healthy human skin. It has been shown that patients suffering from atopic dermatitis have a reduced ceramide content in the stratum corneum, whereas in the stratum corneum of lamellar ichthyosis patients, the amount of free fatty acids is decreased and the ceramide profile is altered. Both patient groups also show elevated levels of transepidermal water loss indicative of an impaired barrier function. As ceramides and free fatty acids are essential for a proper barrier function, we hypothesized that changes in the composition of these lipids would be reflected in the lipid organization in stratum corneum of atopic dermatitis and lamellar ichthyosis patients. We investigated the lateral lipid packing using electron diffraction and the lamellar organization using freeze fracture electron microscopy. In atopic dermatitis stratum corneum, we found that, in comparison with healthy stratum corneum, the presence of the hexagonal lattice (gel phase) is increased with respect to the orthorhombic packing (crystalline phase). In lamellar ichthyosis stratum corneum, the hexagonal packing was predominantly present, whereas the orthorhombic packing was observed only occasionally. This is in good agreement with studies on stratum corneum lipid models that show that the presence of long-chain free fatty acids is involved in the formation of the orthorhombic packing. The results of this study also suggest that the ceramide composition is important for the lateral lipid packing. Finally, using freeze fracture electron microscopy, changes in the lamellar organization in stratum corneum of both patient groups could be observed.

Transglutaminase 1 gene mutations in Italian patients with autosomal recessive lamellar ichthyosis

We analyzed the transglutaminase 1 gene locus in patients from six unrelated Italian families affected by autosomal recessive lamellar ichthyosis. In two families we identified a novel mutation (E520G) in the gene coding region, a previously reported splicing mutation (A3447G), and the mis-sense mutations S272P and V518M. The latter mutation, hitherto considered disease causing, was found to be a simple polymorphism. Linkage to transglutaminase 1 gene was excluded in two of the other four families examined. Single strand conformational polymorphism analysis of the transglutaminase 1 gene in the remaining two families did not reveal any alteration in the coding region. This finding confirms the genetic heterogeneity of the disease.

Identification, by homozygosity mapping, of a novel locus for autosomal recessive congenital ichthyosis on chromosome 17p, and evidence for further genetic heterogeneity

Autosomal recessive congenital ichthyosis (ARCI) comprises a group of severe disorders of keratinization, characterized by variable erythema and skin scaling. It is known for its high degree of genetic and clinical heterogeneity. Mutations in the gene for keratinocyte transglutaminase (TGM1) on chromosome 14q11 were shown in patients with ARCI, and a second locus was described, on chromosome 2q, in families from northern Africa. Three other loci for ARCI, on chromosomes 3p and 19p, were identified recently. We have embarked on a whole-genome scan for further loci for ARCI in four families from Germany, Turkey, and the United Arab Emirates. A novel ARCI locus was identified on chromosome 17p, between the markers at D17S938 and D17S1856, with a maximum LOD score of 3.38, at maximum recombination fraction 0.00, at D17S945, under heterogeneity. This locus is linked to the disease in the Turkish family and in the German family. Extensive genealogical studies revealed that the parents of the German patients with ARCI were eighth cousins. By homozygosity mapping, the localization of the gene could then be refined to the 8.4-cM interval between D17S938 and D17S1879. It could be shown, however, that ARCI in the two Arab families is linked neither to the new locus on chromosome 17p nor to one of the five loci known previously. Our findings give evidence of further genetic heterogeneity that is not linked to distinctive phenotypes.

Diagnosis of autosomal recessive lamellar ichthyosis with mutations in the TGM1 gene

BACKGROUND

Autosomal recessive lamellar ichthyosis (ARLI) is a clinically and genetically heterogeneous disorder. In many cases, mutations in the transglutaminase 1 gene (TGM1) have been identified, however, other clinically indistinguishable cases have been linked to chromosomes 2, 3 and 19. Previous studies have failed to establish any correlation between clinical characteristics and genetic mutations.

OBJECTIVES

To investigate the molecular basis of ARLI in 10 patients with the typical clinical presentation of the disorder.

METHODS

We performed polymerase chain reaction and direct sequencing-based mutation screening in all of these patients, and TGM1 immunofluorescence microscopy and in vitro enzyme activity assays in selected patients.

RESULTS

Mutation screening revealed 14 mutations, four of which have been previously described. While immunofluorescence microscopy was negative in patients with non-sense mutations or out-of-frame insertions or deletions, the results were variable in cases with mis-sense mutations and in cases with no mutations in the TGM1 gene. In vitro enzyme activity assays gave results consistent with the mutation data.

CONCLUSIONS

Our findings support the importance of mutation screening in the evaluation of ARLI.