Abnormal epidermal keratinization in the repeated epilation mutant mouse

Juvenile-onset PSAT1-related neuropathy: A milder phenotype of serine deficiency disorder

Background: Primary serine deficiency disorders have a broad range of the phenotypic spectrum. As an inborn error of metabolism, individuals with severe phenotype may be easily recognized with Neu-Laxova syndrome. However, late-onset mild phenotypes may be underdiagnosed and will lead to disastrous consequences due to treatment delays.

Materials and Methods: Clinical features of patients with serine deficiency disorders were summarized in two unrelated patients. Skin and sural nerve biopsies were conducted on the patients. Whole exome sequencing (WES) was performed in the index patients. Sanger sequencing was used to analyze family cosegregation.

Results: Patient 1 was a 19-year-old male presenting with infancy-onset ichthyosis and juvenile-onset neuropathy. Patient 2 was a 17-year-old male manifesting childhood-onset ichthyosis and juvenile-onset neuropathy. Except for nystagmus, no other developmental or neurodegenerative disorders were found in the patients. Electrophysiological studies indicated a severe sensorimotor axonal neuropathy with a possible demyelinating component. High-dose oral L-serine and glycine completely alleviated skin lesions and only slightly improved neuropathy symptoms. Skin biopsies showed typical features consistent with ichthyosis and severe loss of unmyelinated axons. Sural biopsies revealed a severe loss of axons and a few thinly myelinated fibers. WES found the same homozygous variant c.43G > C (p.A15P) in the PSAT1 gene, which was cosegregated in the two families.

Conclusions: The skin and nervous system may be the main affected targets in serine deficiency disorders. Our patients show a more simple and mild phenotype of PSAT1-related serine deficiency disorder. The pathological changes and regenerative ability of skin and peripheral nerves determine their response to serine supplements.

Molecular and cytoskeletal regulations in epidermal development

At the surface of the body, the epidermis covers great depth in its developmental regulation. While many genes have been shown to be important for skin development through their associations with disease phenotypes in mice and human, it is in the past decade that the intricate interplay between various molecules become gradually revealed through sophisticated genetic models and imaging analyses. In particular, there is increasing evidence suggesting that cytoskeleton-associated proteins, including adhesion proteins and the crosslinker proteins may play critical roles in regulating epidermis development. We here provide a broad overview of the various molecules involved in epidermal development with special emphasis on the cytoskeletal components.

KDF1, encoding keratinocyte differentiation factor 1, is mutated in a multigenerational family with ectodermal dysplasia

Ectodermal dysplasia is a highly heterogeneous group of disorders that variably affect the derivatives of the ectoderm, primarily skin, hair, nails and teeth. TP63, itself mutated in ectodermal dysplasia, links many other ectodermal dysplasia disease genes through a regulatory network that maintains the balance between proliferation and differentiation of the epidermis and other ectodermal derivatives. The ectodermal knockout phenotype of five mouse genes that regulate and/or are regulated by TP63 (Irf6, Ikkα, Ripk4, Stratifin, and Kdf1) is strikingly similar and involves abnormal balance towards proliferation at the expense of differentiation, but only the first three have corresponding ectodermal phenotypes in humans. We describe a multigenerational Saudi family with an autosomal dominant form of hypohidrotic ectodermal dysplasia in which positional mapping and exome sequencing identified a novel variant in KDF1 that fully segregates with the phenotype. The recapitulation of the phenotype we observe in this family by the Kdf1-/- mouse suggests a causal role played by the KDF1 variant.

On the phenotypic spectrum of serine biosynthesis defects

L-serine is a non-essential amino acid that is de novo synthesized via the enzymes phosphoglycerate dehydrogenase (PGDH), phosphoserine aminotransferase (PSAT), and phosphoserine phosphatase (PSP). Besides its role in protein synthesis, L-serine is a precursor of a number of important compounds. Serine biosynthesis defects result from deficiencies in PGDH, PSAT, or PSP and have a broad phenotypic spectrum ranging from Neu-Laxova syndrome, a lethal multiple congenital anomaly disease at the severe end to a childhood disease with intellectual disability at the mild end, with infantile growth deficiency, and severe neurological manifestations as an intermediate phenotype. In this report, we present three subjects with serine biosynthesis effects. The first was a stillbirth with Neu-Laxova syndrome and a homozygous mutation in PHGDH. The second was a neonate with growth deficiency, microcephaly, ichthyotic skin lesions, seizures, contractures, hypertonia, distinctive facial features, and a homozygous mutation in PSAT1. The third subject was an infant with growth deficiency, microcephaly, ichthyotic skin lesions, anemia, hypertonia, distinctive facial features, low serine and glycine in plasma and CSF, and a novel homozygous mutation in PHGDH gene. Herein, we also review previous reports of serine biosynthesis defects and mutations in the PHGDH, PSAT1, and PSPH genes, discuss the variability in the phenotypes associated with serine biosynthesis defects, and elaborate on the vital roles of serine and the potential consequences of its deficiency.

Role of SV40 integration site at chromosomal interval 1q21.1 in immortalized CRL2504 cells

We have applied a functional gene transfer strategy to show the importance of viral integration site in cellular immortalization. The large tumor antigen of SV40 is capable of extending the cellular life span by sequestering tumor suppressor proteins pRB and p53 in virus-transformed human cells. Although SV40 large T antigen is essential, it is not sufficient for cellular immortalization, suggesting that additional alterations in cellular genes are required to attain infinite proliferation. We show here that the disruption of human chromosomal interval at 1q21.1 by SV40 integration can be an essential step for cellular immortalization. The transfer of a 150-kb bacterial artificial chromosome (BAC) clone, RP364B14, corresponding to viral integration site in CRL2504 cells, reverted their immortal phenotype. Interestingly, the BAC transfer clones of CRL2504 cells displayed characteristics of either senescence as shown by beta-galactosidase activity or apoptosis as revealed by positive staining with M30 CytoDEATH antibody. The SV40 integration at 1q21.1, in the vicinity of epidermal differentiation complex (EDC) genes, resulted in the down-regulation of the filaggrin (FLG) gene that is part of the EDC. FLG gene expression was increased in BAC transfer senescent and apoptotic clones. Our results suggest that the disruption of native genomic sequence by SV40 may alter expression of genes involved in senescence and apoptosis by modulating chromatin structure. These studies imply that identification of genes located in the vicinity of viral integration sites in human cancers may be helpful in developing new diagnostic and therapeutic strategies.

IRF6 in development and disease: a mediator of quiescence and differentiation

Post utero development of the mammary gland is a complex developmental process characterized by states of rapid cell proliferation (branching morphogenesis) followed by functional differentiation (lactation) and the consequent apoptosis (involution) of the secretory mammary epithelial cell. This process is cyclical, such that involution returns the mammary gland to a near-virgin-like state capable of responding to morphogenic cues with each consecutive pregnancy. Importantly, many of the regulatory processes which oversee mammary gland development are corrupted or otherwise compromised during the development of breast cancer. For example, Interferon Regulatory Factor 6 (IRF6) is a novel protein with growth inhibitory properties that was initially identified in mammary epithelial cells through its interaction with maspin, a known tumor suppressor in normal breast tissue. Recent findings from our laboratory suggest that IRF6 functions synergistically with maspin to regulate mammary epithelial cell differentiation by acting on the cell cycle. This perspective focuses on the possible involvement of IRF6 in promoting differentiation by regulating exit from the cell cycle and entry into the G(0) phase of cellular quiescence, and how these new findings shed light on normal mammary gland development and the initiation and progression of breast cancer.

Identification of 14-3-3sigma mutation causing cutaneous abnormality in repeated-epilation mutant mouse

Repeated-epilation (Er) mutation in the mouse is inherited as an autosomal and semidominant mutation. Major defects in heterozygous adults and homozygous fetuses were associated with skin and were caused by abnormal ectodermal differentiation. Heterozygous mice are characterized by repeated hair loss and regrowth, and homozygous fetuses die at birth with severe abnormality in skin, limb, tail, and face. To identify the gene causing Er mutation, we have performed gene-expression profiles of skins and mouse embryonic fibroblasts from WT and mutant Er mice by using Affymetrix (Santa Clara, CA) chip analysis. By analyzing the candidate genes generated from gene-expression profiling, we identified a Sfn mutation in Er mice. A single nucleotide insertion in the Sfn (Stratifin, also called 14-3-3sigma) coding region results in a truncated protein lacking 40 amino acid residues at the C terminus. The mutation is linked with phenotypes of Er-heterozygous and -homozygous mice. Ectopic overexpression of WT 14-3-3sigma in Er/Er keratinocytes rescues defects in keratinocyte differentiation. Our study demonstrates that 14-3-3sigma is a crucial regulator for skin proliferation and differentiation.

Increased sensitivity to two-stage skin carcinogenesis of mice heterozygous for the repeated epilation mutation (Er)

Mice heterozygous for repeated epilation mutation (Er) have cutaneous abnormalities that result in repeated loss of hair. Skin papillomas and carcinomas occur spontaneously in such Er/+ mice. BALB/c mice are generally resistant to induced skin cancers. We investigated whether Er/+ heterozygous mice of BALB/c genetic background exhibit increased susceptibility to spontaneous and induced skin tumors. Although none of the Er/+ CXB(N5) mice spontaneously developed skin tumors, they exhibited increased sensitivity to the development of skin papillomas induced by an initiation-promotion regimen. Er/+ mice developed papillomas after 20 micrograms DMBA initiation in the absence of TPA promotion, but the same dose of DMBA was subtumorigenic in +/+ (sibling) mice. Although 15 weeks of TPA promotion resulted in similar tumor susceptibilities, tumor latencies and tumor frequencies in the 2 groups of initiated mice, the papillomas were qualitatively different. Er/+ mice developed more papillomas of the delayed promoter-independent type, which occur after termination of promotion. In contrast, +/+ mice developed more promoter-dependent papillomas, which regress after termination of promotion. Therefore Er/+ mice had a significantly higher number of papillomas than +/+ mice at the termination of the experiment. These results suggest that Er-mutation-induced skin defects not only lead to the repeated loss of hair, but also influence the mode of development of skin papillomas from carcinogen-initiated cells.

Characterization of a cDNA clone encoding human filaggrin and localization of the gene to chromosome region 1q21

Filaggrins are an important class of intermediate filament-associated proteins that interact with keratin intermediate filaments of terminally differentiating mammalian epidermis. They show wide species variations and their aberrant expression has been implicated in a number of keratinizing disorders. We have isolated a cDNA clone encoding human filaggrin and used this to demonstrate that the human gene encodes a polyprotein precursor containing numerous tandem filaggrin repeats. This structure is similar to that of mouse; however, the human filaggrin repeat is much longer (972 base pairs; 324 amino acids) and shows little sequence homology to the mouse protein. Also, data presented here reveal that the human filaggrin repeats show considerable sequence variations; such polymorphism is not found in the mouse. Furthermore, chromosomal mapping data revealed that the human gene is located at 1q21, indicating that the polymorphism is confined to a single locus. By peptide mapping, we define a short linker sequence within the human filaggrin repeat that is excised by proteolysis to yield functional molecules. Finally, we show by in situ hybridization that human filaggrin precursor gene expression is tightly regulated at the transcriptional level in terminally differentiating epidermis and that this represents a useful system in which to study intermediate filament-intermediate filament-associated protein interactions as well as disorders of keratinization.

The immunology of contact dermatitis. A review with special reference to the pathophysiology of eczema

This review summarizes current knowledge of the pathophysiological events which lie behind the development of contact dermatitis. The clinical distinction between allergic and irritant eczema is discussed. New observations are evaluated on our understanding of how allergic and irritant eczema may in many respects be similar, evolving through common physiological pathways of immune inflammation.

Abnormal expression and processing of keratins in pupoid fetus (pf/pf) and repeated epilation (Er/Er) mutant mice

The pupoid fetus (pf) and repeated epilation (Er) mutations of mice result in a failure of epidermal differentiation in homozygotes. Expression of the epidermal keratins has been followed in pf/pf and Er/Er mice by two-dimensional gel electrophoresis, and by immunohistochemistry and Western blotting using polyclonal antibodies that are monospecific for individual keratin polypeptides. Our results show that expression of the differentiation-specific keratins (K1 and K10) is delayed in both the pf/pf and Er/Er mutants and that, when these keratins do appear later in development, they are localized in the deeper layers of the thickened mutant epidermis. Conversely, K6 and K16, two keratins found in low abundance in normal epidermis, are abundant in mutant epidermis. In newborn mutant epidermis, K6 and K16 are found to be most abundant in the outermost epidermal cells, a distribution opposite to that of K1 and K10. These findings suggest that the expression of these hyperplastic keratins in mutant mice may occur to the exclusion of the differentiation-specific keratins both during development and in newborn animals. Differentiation, and an apparently normal pattern of keratin expression, occur when whole pf/pf or Er/Er skin is grafted to normal mice. These results suggest that the pf and Er genes may be expressed systemically and that transfer of the mutant skin to a "normal" environment results in the recovery of a normal phenotype.

Eye development in the normal and Pupoid foetus (pf/pf) mutant mouse

Embryonic development of the mammalian eye has not been studied in such great detail as that of the avian eye, and preliminary observations have suggested that the sequence of events may differ. It is therefore likely that the relative importance of the cell and tissue interactions involved also differs and it would be interesting to compare these two systems. The pupoid foetus mutation in the mouse shows disruption of eye development due to abnormal epidermal properties and so the relative importance of individual events in triggering subsequent development can be studied by seeing what happens when the situation is modified in the mutant. The behaviour of the pupoid foetus epidermal cells in the interactive system of the eye may also help to further characterise the phenotype of the mutation.

Immunologic localization of filaggrin in human oral epithelia and correlation with keratinization

Localization of filaggrin, a human epithelial structural protein, was investigated by indirect-immunofluorescence microscopy of oral mucosa. Thirty specimens were tested, 10 each of palate, gingiva, and buccal mucosa. Orthokeratinized and parakeratinized specimens displayed immunofluorescence within the stratum corneum, stratum granulosum, and upper stratum spinosum. Within the stratum corneum, the reaction was diffuse. Within the stratum granulosum and spinosum, the reaction was in a granular pattern, in a perinuclear position. Several of the nonkeratinized specimens had a negative reaction; however, most displayed a very weak, scattered reaction in a granular pattern within the most superficial cells. The presence of filaggrin in keratinized palate was confirmed by immunoblot studies with the same antibody. Profilaggrin was detectable in representative nonkeratinized and parakeratinized oral tissues, as well as in keratinized palatal epithelium. The localization of filaggrin is consistent with its possible function as the interfilamentous matrix protein within cells of the stratum corneum, and with its derivation from a cross-reactive precursor protein stored in keratohyaline granules. A strong positive correlation was found between the degree of keratinization and the amount of immunofluorescence; therefore, filaggrin and related antigens may be useful and sensitive marker proteins for epithelial keratinization.

Tissue interactions in development of teeth and related ectodermal derivatives

A number of traditional techniques have been used to examine epithelial-mesenchymal interactions; they remain to this day the only means to examine the problems of tissue interactions. Problems at the tissue level of organization require that the approaches to these problems ask questions and use techniques that address the issues of tissue organization and cellular morphology in the context of three-dimensional organization. Manipulating the embryo is exactly that--manipulating a three-dimensional organized organism that must express its differentiated functional organs and organ systems. When the parallel and correlative studies of cell biological function achieve new insights, they will have to be tested in terms of the cellular response at the tissue level of organization.

The differentiation of repeated epilation (Er/Er) mouse mutant skin in organ culture and in grafts

The homozygous repeated epilation (Er/Er) mouse mutant dies at birth and shows a variety of malformations, one of which is a skin defect. The developmental abilities of skin fragments from these Er/Er mouse embryos were studied in organ culture and in grafts performed either under the renal capsule of young mouse hosts or under the skin of mouse fetuses. In organ culture, the skin fragments differentiated in accordance with their genetic origin. The most characteristic feature was the abnormally thickened spinous layer and the formation of numerous epidermal nodules in Er/Er skin pieces removed from 13- to 16-day-old embryos, and cultured for 4 to 6 days, whereas the normal skin showed a constant layered organization. As in normal skin, keratin fibers developed within 4 to 6 days of culture. However, in contrast to normal skin, where keratin sheaths developed all over the surface of the epidermis, the Er/Er skin exhibited keratin masses inside the nodules. Combinations of mutant Er/Er epidermis with normal dermis resulted in abnormal skin differentiation, with formation of nodules similar to those observed for unseparated Er/Er skin fragments, whereas the reciprocal combination (normal epidermis with Er/Er dermis) produced normal skin differentiation. Cornified layers developed in both types of explants. Grafts of Er/Er and, for comparison, of normal skin fragments under the renal capsule or under a fetus's skin showed that the development of Er/Er skin in a normal or Er/+ host was similar to that of a normal skin. Thus, if isolated from the mutant organism, and inserted into a normal environment, the skin recovered within 6 to 7 days after transplantation and then developed normally. Our experiments suggest that the abnormal skin development of the Er/Er mutant might be caused by environmental influences.

A high performance liquid chromatography method to obtain rat epidermal filaggrin

Sodium pyrophosphate-glycerol buffer, a non-denaturing buffer has been used to solubilize epidermal proteins. The extracted proteins are different in their electrophoretic profile in various mammalian species, 47 K in rat and guinea pig, 31 K and 60 K in mouse and 43 K and 60 K proteins in human epidermis were most predominant. Electrophoretic analyses show synthesis of new proteins as a function of age in rat tissue. Purification of the major rat protein fraction was achieved using a reverse phase high-performance liquid chromatography using a gradient of 5-50% acetonitrile. Based upon the molecular size, amino acid data and immunodiffusion analysis, we conclude the purified rat protein to be filaggrin.

Embryonic development of the mouse mutant pupoid foetus (pf/pf)

The pupoid foetus mutation in the mouse is a recessive lethal mutation causing death of homozygous (pf/pf) embryos immediately after birth. From 11.3 days gestation onwards, these embryos are characterised externally by the development of a tail twist, followed by apparent stunting of the limbs and tail (when compared with the development of these structures in normal embryos), lack of digits, distortion of facial features, and possession of a smooth, mottled skin. Embryos ranging in age from 11.3 days gestation to full term have been examined using light microscopy and scanning and transmission electron microscopy. The skeletal structure and internal organs of the embryo are normal, but abnormalities occur in the external epidermis, the dermis, and the peripheral sensory nerves. Development of the palate and the eyes are affected by the behaviour of these tissues. The epidermis undergoes hypertrophy and fails to differentiate, and, on the basis of morphological criteria and theoretical considerations, it is suggested that the pf gene is activated in the epidermis during the keratinization pathway, preventing differentiation and altering the cell surface characteristics of the cells. Other abnormalities are explained in terms of interactions with the epidermis. This mutant is compared with other similar mutants.

Abnormal keratinization in the pupoid fetus (pf/pf) mutant mouse epidermis

During its development the epidermis of the pf/pf mutant mouse is invaded by cells from the underlying dermis. These invading cells establish a network of cells including fibroblasts, endothelial cells, and nerve fibers, throughout the epidermis. Subsequent to these events the keratohyalin protein, filaggrin, is drastically reduced and keratinization fails to occur. Heterotypic tissue recombinations indicate that the pf gene is not expressed in the skin. After simply grafting whole mutant dorsal skin, filaggrin synthesis is initiated and an orderly process of epidermal differentiation is achieved. These results suggest that the pf gene acts systemically and that the failure of epidermal differentiation in the mutant occurs secondary to abnormal epidermal organization.

Identification of filaggrin in cultured mouse keratinocytes and its regulation by calcium

Filaggrin is a histidine-rich cationic protein present in cells of the stratum corneum in vivo and derived from a precursor in keratohyalin granules. Biochemical and immunologic methods were used to determine the presence of filaggrin in keratinocytes cultured in vitro and induced to differentiate by increasing the extracellular calcium concentration from 0.07 to 1.2 mM. Indirect immunofluorescence using antibody to rat filaggrin was negative in cells cultured in low-calcium medium but positive in cells switched to high-calcium medium. Large immunofluorescent granules were identified in a perinuclear distribution starting at 6 hours after the shift in calcium concentration, coinciding with the time of appearance of phase-dense cytoplasmic granules. Radiolabeled histidine was preferentially incorporated into proteins of 95, 37, and 27 K. The 37 and 27 K bands were not adsorbed by DE52 cellulose and therefore are cationic. A 27 K cationic, histidine-labeled protein was readily extracted from frozen pellets of cells cultured in high-calcium medium. It comigrates with purified mouse filaggrin (27 K) and reacts with antibody to rat filaggrin on immunoautoradiography. Only trace amounts of this protein could be detected in cells cultured in low-calcium medium. Our observation of filaggrin-immunoreactive granules confirms the previous ultrastructural identification of keratohyalin granules after the shift to high-calcium medium. The results suggest that filaggrin synthesis is stimulated in keratinocytes induced to differentiate by the shift to high extracellular calcium concentration.

Epidermolytic hyperkeratosis: ultrastructure and biochemistry of skin and amniotic fluid cells from two affected fetuses and a newborn infant

Skin biopsy samples and amniotic fluid cells obtained in utero from two fetuses at risk for epidermolytic hyperkeratosis were examined by light and electron microscopy. Both fetuses were affected; the second was carried to term. Epidermal extracts were prepared from blisters of the newborn for analysis of keratin and filaggrin proteins. Abnormal clumps of keratin filaments were present in all layers of the prekeratinized fetal epidermis except the periderm and stratum germinativum. A significant population of amniotic fluid cells also contained the filament aggregations. Prenatal diagnosis of the disease should be possible using cells obtained at amniocentesis, thus avoiding fetal skin biopsy. Biochemical studies showed abnormalities in keratin and filaggrin proteins. The structural alterations in the tissue might be a consequence of altered interaction between these two abnormal epidermal proteins.

Identification of a rapidly labelled 350K histidine-rich protein in neonatal mouse epidermis

The major histidine-rich protein (HRP) found in the stratum corneum of neonatal mouse epidermis (band 2 protein, molecular weight 27,000) is a relatively late product of epidermal differentiation and incorporates labelled amino acids in vivo only after a 6-9 h lag period. A number of putative precursor HRPs in the 70-300 K molecular weight range were initially identified using short pulse labeling times and our previously described methods for isolation of epidermis and extraction of proteins. However, when steps were taken to minimise proteolysis during preparation, a single species of approximately 350 K molecular weight was the most strongly labelled protein following a 1 h in vivo pulse of [3H]-histidine. This protein was stable in sodium dodecyl sulphate dithiothreitol at 100 degrees C and in 4 M urea, suggesting a single covalently linked polypeptide. The kinetics of labelling and the localisation of the 350 K HRP in the lower granular layers suggest that it is a precursor of the stratum corneum HRP. The processing of the 350 K HRP to the stratum corneum species appears to involve a complex series of specific cleavage steps which give rise to a number of HRPs of intermediate molecular weight.