Genetic and immunohistochemical detection of mutations inactivating the keratinocyte transglutaminase in patients with lamellar ichthyosis

Autosomal recessive lamellar ichthyosis is a clinically heterogeneous group of severe congenital keratinization disorders that is characterized by generalized hyperkeratosis and variable erythema. About half of the patients have mutations in the TGM1 gene, which encodes the keratinocyte transglutaminase. Linkage studies have shown that at least two further loci for autosomal recessive lamellar ichthyosis must exist. We present here two patients with lamellar ichthyosis caused by mutations in the TGM1 gene. The first patient is compound heterozygous for the novel missense mutation C53S and the splice mutation A3447G. The second patient, a child of consanguineous parents from Tunisia, is homozygous for the unknown nonsense mutation W263X. This is the first report of a mutation, C53S, that affects the region of the keratinocyte transglutaminase that is essential for anchorage of the enzyme to the plasma membrane. A novel, rapid in situ transglutaminase activity assay revealed the absence of keratinocyte transglutaminase activity in both patients. The mutations described are hence causative for the ichthyosis phenotype. Received: 27 October 1997 / Accepted: 24 November 1997     

Novel transglutaminase 1 mutations in patients affected by lamellar ichthyosis

Lamellar Ichthyosis (LI) is a form of congenital ichthyosis that is caused by mutations in the TGM1 gene that encodes for the transglutaminase 1 (TG1) enzyme. Functional inactivation of TG1 could be due to mutations, deletion or insertions. In this study, we have screened 16 patients affected by LI and found six new mutations: two transition/transversion (R37G, V112A), two nonsense mutations and two putative splice site both leading to a premature stop codon. The mutations are localized in exons 2 (N-terminal domain), 5, 11 (central catalytic domain), and none is located in the two beta-barrel C-terminal domains. In conclusion, this study expands the current knowledge on TGM1 mutation spectrum, increasing the characterization of mutations would provide more accurate prenatal genetic counselling for parents at-risk individuals.     

Genotype/phenotype correlation in autosomal recessive lamellar ichthyosis.

Autosomal recessive lamellar ichthyosis is a severe congenital disorder of keratinization, characterized by variable erythema of the whole body surface and by different scaling patterns. Recently, mutations have been identified in patients with lamellar ichthyosis in the TGM1 gene coding for keratinocyte transglutaminase, and a second locus has been mapped to chromosome 2. We have now analyzed the genotype/phenotype correlation in a total of 14 families with lamellar ichthyosis. Linkage analyses using microsatellites in the region of the TGM1 gene confirmed genetic heterogeneity. In patients not linked to the TGM1 gene, the second region identified on chromosome 2 and a further candidate region on chromosome 20 were excluded, confirming as well the existence of at least three loci for lamellar ichthyosis. Sequence analyses of the TGM1 gene in families compatible with linkage to this locus revealed seven different missense mutations, five of these unpublished so far, and one splice mutation. No genotype/phenotype correlation for mutations in the TGM1 gene was found in this group of patients, which included two unrelated patients homozygous for the same mutation. Similarly, no clear difference in the clinical picture was seen between patients with TGM1 mutations and those unlinked to the TGM1 locus. Comparison of genetic and clinical classifications for patients with lamellar ichthyosis shows no consistency and thus indicates that clinical criteria currently in use cannot discriminate between the molecularly different forms of the disease.     

Splice-site mutation in TGM1 in congenital recessive ichthyosis in American families: molecular,genetic, genealogic,and clinical studies

Lamellar ichthyosis (LI, OMIM no. 242300) is a severe autosomal recessive genodermatosis with an estimated prevalence of 1:200,000. LI represents one end of the spectrum of congenital recessive ichthyosis (CRI). Mutations in the gene for transglutaminase-1 (TGM1) are responsible for many cases of LI and occur throughout the coding sequence of the gene. Our analyses of patients with CRI revealed a common TGM1 mutation involving loss of the intron 5 splice acceptor site leading to alternative splicing of the message. We found families in which the splice acceptor site mutation was homozygous, and families where the patients were compound heterozygotes for the splice acceptor site mutation and another TGM1 mutation. A mutation at this same site occurs in the majority of Norwegian patients as a founder effect. In our ethnically diverse patient population, none of whom have known Norwegian ancestry, haplotype analysis of the TGM1 chromosomal region also suggested the existence of a founder effect. Comparison of the common haplotype in our data with the Norwegian data showed that 2/7 of our splice acceptor site mutation chromosomes had the full reported Norwegian haplotype, and the remaining five chromosomes exhibited recombination at the most distal marker studied. History, family origins, and haplotype analysis suggested that the mutation originally arose on a German background and was introduced into Norway around 800-1000 AD. We also found a limited correlation between genotype and phenotype in our study, with the four homozygous patients having less severe disease than many of the heterozygotes, and no patient with a splice acceptor site mutation having erythroderma or a congenital ichthyosiform erythroderma phenotype.     

Characterization of bovine TGM1 and exclusion as candidate gene for ichthyosis in Chianina

Ichthyosis is a heterogeneous group of keratinization disorders reported both in human and animals. Two rare, inherited forms have been reported in cattle, both characterized by autosomal recessive transmission. Because mutations of transglutaminase 1 (TGM1) gene are associated with autosomal recessive ichthyosis in people, this gene was investigated as a candidate for the diseases in cattle. Three different polymorphisms were identified in 5' end region of cattle TGM1. Marker homozygosity was not found among affected calves. Linkage analysis excluded (logarithmic odds [LOD] score -2.0) TGM1 as the cause for ichthyosis phenotype in the analyzed Chianina cases.     

Assignment of a novel locus for autosomal recessive congenital ichthyosis to chromosome 19p13.1-p13.2

Autosomal recessive congenital ichthyosis (ARCI) is a rare, clinically and genetically heterogeneous genodermatosis. One gene (transglutaminase 1, on 14q11) and one additional locus (on 2q33-35, with an unidentified gene) have been shown to be associated with a lamellar, nonerythrodermic type of ARCI. We performed a genomewide scan, with 370 highly polymorphic microsatellite markers, on five affected individuals from one large Finnish family with nonerythrodermic, nonlamellar ARCI. The only evidence for linkage emerged from markers in a 6.0-cM region on chromosome 19p13.1-2. The maximum two-point LOD score of 7.33 was obtained with the locus D19S252, and multipoint likelihood calculations gave a maximum location score of 5.2. The affected individuals share two common core haplotypes, which makes compound heterozygosity possible. The novel disease locus is the third locus linked to ARCI, supporting previous evidence for genetic heterogeneity of ARCI. This is also the first locus for a nonlamellar, nonerythrodermic phenotype of ARCI.     

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

Autosomal recessive ichthyosis (ARI) includes a heterogeneous group of disorders of keratinization characterized by desquamation over the whole body. Two forms largely limited to the skin have been defined: lamellar ichthyosis (LI) and nonbullous congenital ichthyosiform erythroderma (NCIE). A first gene for LI, transglutaminase TGM1, has been identified on chromosome 14, and a second one has been localized on chromosome 2. In a genomewide scan of nine large consanguineous families, using homozygosity mapping, two new loci for ARI were found, one for a lamellar form in a 6-cM interval on chromosome 19 and a second for an erythrodermic form in a 7.7-cM interval on chromosome 3. Linkage to one of the four loci could be demonstrated in more than half of 51 consanguineous families, most of them from the Mediterranean basin. All four loci could be excluded in the others, implying further genetic heterogeneity in this disorder. Multipoint linkage analysis gave maximal LOD scores of 11.25 at locus D19S566 and 8.53 at locus D3S3564.     

Defects in the DNA repair and transcription gene ERCC2(XPD) in trichothiodystrophy.

Trichothiodystrophy (TTD) is a rare autosomal recessive disorder characterized by brittle hair with reduced sulfur content, ichthyosis, peculiar face, and mental and growth retardation. Clinical photosensitivity is present in approximately 50% of TTD patients but is not associated with an elevated frequency of cancers. Previous complementation studies show that the photosensitivity in nearly all of the studied patients is due to a defect in the same genetic locus that underlies the cancer-prone genetic disorder xeroderma pigmentosum group D (XP-D). Nucleotide-sequence analysis of the ERCC2 cDNA from three TTD cell strains (TTD1V1, TTD3VI, and TTD1RO) revealed mutations within the region from amino acid 713-730 and within previously identified helicase functional domains. The various clinical presentations and DNA repair characteristics of the cell strains can be correlated with the particular mutations found in the ERCC2 locus. Mutations of Arg658 to either His or Cys correlate with TTD cell strains with intermediate UV-sensitivity, mutation of Arg722 to Trp correlates with highly UV-sensitive TTD cell strains, and mutation of Arg683 to Trp correlates with XP-D. Alleles with mutation of Arg616 to Pro or with the combined mutation of Leu461 to Val and deletion of 716-730 are found in both XP-D and TTD cell strains.     

Linkage of autosomal recessive lamellar ichthyosis to chromosome 14q.

We have mapped the locus for lamellar ichthyosis (LI), an autosomal recessive skin disease characterized by abnormal cornification of the epidermis. Analysis using both inbred and outbred families manifesting severe LI showed complete linkage to several markers within a 9.3-cM region on chromosome 14q11. Affected individuals in inbred families were also found to have striking homozygosity for markers in this region. Linkage-based genetic counseling and prenatal diagnosis is now available for informative at-risk families. Several transcribed genes have been mapped to the chromosome 14 region containing the LI gene. The transglutaminase 1 gene (TGM1), which encodes one of the enzymes responsible for cross-linking epidermal proteins during formation of the stratum corneum, maps to this interval. The TGM1 locus was completely linked to LI (Z = 9.11), suggesting that TGM1 is a good candidate for further investigation of this disorder. The genes for four serine proteases also map to this region but are expressed only in hematopoietic or mast cells, making them less likely candidates.     

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.     

Distinguishing Ichthyoses by Protein Profiling

To explore the usefulness of protein profiling for characterization of ichthyoses, we here determined the profile of human epidermal stratum corneum by shotgun proteomics. Samples were analyzed after collection on tape circles from six anatomic sites (forearm, palm, lower leg, forehead, abdomen, upper back), demonstrating site-specific differences in profiles. Additional samples were collected from the forearms of subjects with ichthyosis vulgaris (filaggrin (FLG) deficiency), recessive X-linked ichthyosis (steroid sulfatase (STS) deficiency) and autosomal recessive congenital ichthyosis type lamellar ichthyosis (transglutaminase 1 (TGM1) deficiency). The ichthyosis protein expression patterns were readily distinguishable from each other and from phenotypically normal epidermis. In general, the degree of departure from normal was lower from ichthyosis vulgaris than from lamellar ichthyosis, parallel to the severity of the phenotype. Analysis of samples from families with ichthyosis vulgaris and concomitant modifying gene mutations (STS deficiency, GJB2 deficiency) permitted correlation of alterations in protein profile with more complex genetic constellations.     

Congenital lamellar ichthyosis in Tunisia is caused by a founder nonsense mutation in the TGM1 gene

Lamellar ichthyosis (LI, MIM# 242300) is a severe autosomal recessive genodermatosis present at birth in the form of collodion membrane covering the neonate. Mutations in the TGM1 gene encoding transglutaminase-1 are a major cause of LI. In this study molecular analysis of two LI Tunisian patients revealed a common nonsense c.788G>A mutation in TGM1 gene. The identification of a cluster of LI pedigrees carrying the c.788G>A mutation in a specific area raises the question of the origin of this mutation from a common ancestor. We carried out a haplotype-based analysis by way of genotyping 4 microsatellite markers and 8 SNPs flanking and within the TGM1 gene spanning a region of 6 Mb. Haplotype reconstruction from genotypes of all members of the affected pedigrees indicated that all carriers for the mutation c.788G>A harbored the same haplotype, indicating common ancestor. The finding of a founder effect in a rare disease is essential for the genetic diagnosis and the genetic counselling of affected LI pedigrees in Tunisia.     

Molekulare Charakterisierung der Ichthyosen

Ichthyoses comprise an etiologically heterogeneous group of Mendelian disorders of cornification (MEDOC) that affect the entire integument and are characterized by hyperkeratosis and/or visible scaling. In recent years basic research has elucidated the genetic basis of almost all ichthyosis forms and enormously improved diagnostic facilities. The Network for Ichthyoses and Related Keratinization Disorders (NIRK; www.netzwerk-ichthyose.de) and the patient support organization Selbsthilfe Ichthyose e. V. (www.ichthyose.de) provide helpful contact points for physicians and patients in Germany. The first worldwide Ichthyosis Consensus Classification was approved in August 2009. Its nosology is based on clinical presentation but also reflects pathogenic aspects. The major criterion is the distinction between syndromic and non-syndromic forms. Controversial disease names have been re-defined: Ichthyoses due to keratin mutations are referred to under the new umbrella term keratinopathic ichthyosis (KPI), which comprises epidermolytic ichthyosis (mutation in keratin 1 or 10) and superficial epidermolytic ichthyosis (keratin 2). The term autosomal recessive congenital ichthyosis (ARCI) represents the umbrella for harlequin ichthyosis and for the group of lamellar ichthyosis and congenital ichthyosiform erythroderma. The international classification should serve as a reference for future research into these diseases, including further genotype-phenotype correlation studies.     

Loss-of-function variants of the human melanocortin-1 receptor gene in melanoma cells define structural determinants of receptor function.

The alpha-melanocyte-stimulating hormone (alphaMSH) receptor (MC1R) is a major determinant of mammalian skin and hair pigmentation. Binding of alphaMSH to MC1R in human melanocytes stimulates cell proliferation and synthesis of photoprotective eumelanin pigments. Certain MC1R alleles have been associated with increased risk of melanoma. This can be theoretically considered on two grounds. First, gain-of-function mutations may stimulate proliferation, thus promoting dysplastic lesions. Second, and opposite, loss-of-function mutations may decrease eumelanin contents, and impair protection against the carcinogenic effects of UV light, thus predisposing to skin cancers. To test these possibilities, we sequenced the MC1R gene from seven human melanoma cell (HMC) lines and three giant congenital nevus cell (GCNC) cultures. Four HMC lines and two GCNC cultures contained MC1R allelic variants. These were the known loss-of-function Arg142His and Arg151Cys alleles and a new variant, Leu93Arg. Moreover, impaired response to a superpotent alphaMSH analog was demonstrated for the cell line carrying the Leu93Arg allele and for a HMC line homozygous for wild-type MC1R. Functional analysis in heterologous cells stably or transiently expressing this variant demonstrated that Leu93Arg is a loss-of-function mutation abolishing agonist binding. These results, together with site-directed mutagenesis of the vicinal Glu94, demonstrate that the MC1R second transmembrane fragment is critical for agonist binding and maintenance of a resting conformation, whereas the second intracellular loop is essential for coupling to the cAMP system. Therefore, loss-of-function, but not activating MC1R mutations are common in HMC. Their study provides important clues to understand MC1R structure-function relationships.     

Ceramide Analysis in Combination With Genetic Testing May Provide a Precise Diagnosis for Self-Healing Collodion Babies

Self-healing collodion baby (SHCB), also called “self-improving collodion baby”, is a rare mild variant of autosomal recessive congenital ichthyosis and is defined as a collodion baby who shows the nearly complete resolution of scaling within the first 3 months to 1 year of life. However, during the neonatal period, it is not easy to distinguish SHCB from other inflammatory forms of autosomal recessive congenital ichthyosis, such as congenital ichthyosiform erythroderma. Here, we report a case study of two Japanese SHCB patients with compound heterozygous mutations, c.235G>T (p.(Glu791))/ c.1189C>T (p.(Arg397Cys)) and c.1295A>G (p.(Tyr432Cys))/ c.1138delG (p.(Asp380Thrfs13)), in CYP4F22, which encodes cytochrome P450, family 4, subfamily F, polypeptide 22 (CYP4F22). Immunohistochemically, inflammation with the strong expression of IL-17C, IL-36γ, and TNF-α was seen in the skin at birth. CYP4F22 is an ultra-long-chain FA ω-hydroxylase responsible for ω-O-acylceramide (acylceramide) production. Among the epidermal ceramides, acylceramide is a key lipid in maintaining the epidermal permeability barrier function. We found that the levels of ceramides with ω-hydroxy FAs including acylceramides and the levels of protein-bound ceramides were much lower in stratum corneum samples obtained by tape stripping from SHCB patients than in those from their unaffected parents and individuals without SHCB. Additionally, our cell-based enzyme assay revealed that two mutants, p.(Glu791) and p.(Arg397Cys), had no enzyme activity. Our findings suggest that genetic testing coupled with noninvasive ceramide analyses using tape-stripped stratum corneum samples might be useful for the early and precise diagnosis of congenital ichthyoses, including SHCB.     

Functional effects of congenital myopathy-related mutations in gamma-tropomyosin gene

Missense mutations in human TPM3 gene encoding γ-tropomyosin expressed in slow muscle type 1 fibers, were associated with three types of congenital myopathies-nemaline myopathy, cap disease and congenital fiber type disproportion. Functional effects of the following substitutions: Leu100Met, Ala156Thr, Arg168His, Arg168Cys, Arg168Gly, Lys169Glu, and Arg245Gly, were examined in biochemical assays using recombinant tropomyosin mutants and native proteins isolated from skeletal muscle. Most, but not all, mutations decreased the affinity of tropomyosin for actin alone and in complex with troponin (±Ca(2+)). All studied tropomyosin mutants reduced Ca-induced activation but had no effect on the inhibition of actomyosin cross-bridges. Ca(2+)-sensitivity of the actomyosin interactions, as well as cooperativity of myosin-induced activation of the thin filament was affected by individual tropomyosin mutants with various degrees. Decreased motility of the reconstructed thin filaments was a result of combined functional defects caused by myopathy-related tropomyosin mutants. We conclude that muscle weakness and structural abnormalities observed in TPM3-related congenital myopathies result from reduced capability of the thin filament to fully activate actin-myosin cross-bridges.     

TGFBI gene mutations in the Ukrainian patients with inherited corneal stromal dystrophies

Mutations Arg124Cys, Thr538Arg, Arg555Thr, Arg555Gln, Leu558Pro, and His626Arg in TGFBI gene were analyzed by polymerase chain reaction and restriction in 84 patients with various forms of corneal stromal dystrophies from 49 unrelated families and 29 clinically healthy relatives of these patients. A new mutation in TGFBI gene, Leu558Pro, was identified in the patients with atypical lattice dystrophy. The haplotypes of four microsatellite markers surrounding TGFBI gene region were analyzed in 22 families. The data on association of genotype and phenotype suggest that the analysis of TGFBI gene mutations is important for differential diagnostics of corneal dystrophies.     

Congenital hypogonadotropic hypogonadism due to GnRH receptor mutations in three brothers reveal sites affecting conformation and coupling

Congenital hypogonadotropic hypogonadism (CHH) is characterized by low gonadotropins and failure to progress normally through puberty. Mutations in the gene encoding the GnRH receptor (GNRHR1) result in CHH when present as compound heterozygous or homozygous inactivating mutations. This study identifies and characterizes the properties of two novel GNRHR1 mutations in a family in which three brothers display normosmic CHH while their sister was unaffected. Molecular analysis in the proband and the affected brothers revealed two novel non-synonymous missense GNRHR1 mutations, present in a compound heterozygous state, whereas their unaffected parents possessed only one inactivating mutation, demonstrating the autosomal recessive transmission in this kindred and excluding X-linked inheritance equivocally suggested by the initial pedigree analysis. The first mutation at c.845 C>G introduces an Arg substitution for the conserved Pro 282 in transmembrane domain (TMD) 6. The Pro282Arg mutant is unable to bind radiolabeled GnRH analogue. As this conserved residue is important in receptor conformation, it is likely that the mutation perturbs the binding pocket and affects trafficking to the cell surface. The second mutation at c.968 A>G introduces a Cys substitution for Tyr 323 in the functionally crucial N/DPxxY motif in TMD 7. The Tyr323Cys mutant has an increased GnRH binding affinity but reduced receptor expression at the plasma membrane and impaired G protein-coupling. Inositol phosphate accumulation assays demonstrated absent and impaired Gα(q/11) signal transduction by Pro282Arg and Tyr323Cys mutants, respectively. Pretreatment with the membrane permeant GnRHR antagonist NBI-42902, which rescues cell surface expression of many GNRHR1 mutants, significantly increased the levels of radioligand binding and intracellular signaling of the Tyr323Cys mutant but not Pro282Arg. Immunocytochemistry confirmed that both mutants are present on the cell membrane albeit at low levels. Together these molecular deficiencies of the two novel GNRHR1 mutations lead to the CHH phenotype when present as a compound heterozygote.     

Mutations in CERS3 Cause Autosomal Recessive Congenital Ichthyosis in Humans

Autosomal recessive congenital ichthyosis (ARCI) is a rare genetic disorder of the skin characterized by abnormal desquamation over the whole body. In this study we report four patients from three consanguineous Tunisian families with skin, eye, heart, and skeletal anomalies, who harbor a homozygous contiguous gene deletion syndrome on chromosome 15q26.3. Genome-wide SNP-genotyping revealed a homozygous region in all affected individuals, including the same microdeletion that partially affects two coding genes (ADAMTS17, CERS3) and abolishes a sequence for a long non-coding RNA (FLJ42289). Whereas mutations in ADAMTS17 have recently been identified in autosomal recessive Weill-Marchesani-like syndrome in humans and dogs presenting with ophthalmologic, cardiac, and skeletal abnormalities, no disease associations have been described for CERS3 (ceramide synthase 3) and FLJ42289 so far. However, analysis of additional patients with non-syndromic ARCI revealed a splice site mutation in CERS3 indicating that a defect in ceramide synthesis is causative for the present skin phenotype of our patients. Functional analysis of patient skin and in vitro differentiated keratinocytes demonstrated that mutations in CERS3 lead to a disturbed sphingolipid profile with reduced levels of epidermis-specific very long-chain ceramides that interferes with epidermal differentiation. Taken together, these data present a novel pathway involved in ARCI development and, moreover, provide the first evidence that CERS3 plays an essential role in human sphingolipid metabolism for the maintenance of epidermal lipid homeostasis.     

Co-inheritance of two ABCC8 mutations causing an unresponsive congenital hyperinsulinism: Clinical and functional characterization of two novel ABCC8 mutations

Congenital hyperinsulinism (CHI) occurs as a consequence of unregulated insulin secretion from the pancreatic beta-cells. Severe recessive mutations and milder dominant mutations have been described in the ABCC8 and KCNJ11 genes encoding SUR1 and Kir6.2 subunits of the beta-cell ATP-sensitive K(+) channel. Here we report two patients with CHI unresponsive to medical therapy with diazoxide. Sequencing analysis identified a compound heterozygous mutation in ABCC8 in both patients. The first one is a carrier for the known mild dominant mutation p.Glu1506Lys jointly with the novel mutation p.Glu1323Lys. The second carries the p.Glu1323Lys mutation and a second novel mutation, p.Met1394Arg. Functional studies of both novel alleles showed reduced or null cell surface expression, typical of recessive mutations. Compound heterozygous mutations in congenital hyperinsulinism result in complex interactions. Studying these mechanisms can improve the knowledge of this disease and modify its therapy.