Interactions of the Vitamin D Receptor with the Corepressor Hairless

Consensus Statement on Vitamin D Status Assessment and Supplementation: Whys, Whens, and Hows

The 6th International Conference, "Controversies in Vitamin D," was convened to discuss controversial topics, such as vitamin D metabolism, assessment, actions, and supplementation. Novel insights into vitamin D mechanisms of action suggest links with conditions that do not depend only on reduced solar exposure or diet intake and that can be detected with distinctive noncanonical vitamin D metabolites. Optimal 25-hydroxyvitamin D (25(OH)D) levels remain debated. Varying recommendations from different societies arise from evaluating different clinical or public health approaches. The lack of assay standardization also poses challenges in interpreting data from available studies, hindering rational data pooling and meta-analyses. Beyond the well-known skeletal features, interest in vitamin D's extraskeletal effects has led to clinical trials on cancer, cardiovascular risk, respiratory effects, autoimmune diseases, diabetes, and mortality. The initial negative results are likely due to enrollment of vitamin D-replete individuals. Subsequent post hoc analyses have suggested, nevertheless, potential benefits in reducing cancer incidence, autoimmune diseases, cardiovascular events, and diabetes. Oral administration of vitamin D is the preferred route. Parenteral administration is reserved for specific clinical situations. Cholecalciferol is favored due to safety and minimal monitoring requirements. Calcifediol may be used in certain conditions, while calcitriol should be limited to specific disorders in which the active metabolite is not readily produced in vivo. Further studies are needed to investigate vitamin D effects in relation to the different recommended 25(OH)D levels and the efficacy of the different supplementary formulations in achieving biochemical and clinical outcomes within the multifaced skeletal and extraskeletal potential effects of vitamin D.

Vitamin D: Newer Concepts of Its Metabolism and Function at the Basic and Clinical Level

The interest in vitamin D continues unabated with thousands of publications contributing to a vast and growing literature each year. It is widely recognized that the vitamin D receptor (VDR) and the enzymes that metabolize vitamin D are found in many cells, not just those involved with calcium and phosphate homeostasis. In this mini review I have focused primarily on recent studies that provide new insights into vitamin D metabolism, mechanisms of action, and clinical applications. In particular, I examine how mutations in vitamin D metabolizing enzymes—and new information on their regulation—links vitamin D metabolism into areas such as metabolism and diseases outside that of the musculoskeletal system. New information regarding the mechanisms governing the function of the VDR elucidates how this molecule can be so multifunctional in a cell-specific fashion. Clinically, the difficulty in determining vitamin D sufficiency for all groups is addressed, including a discussion of whether the standard measure of vitamin D sufficiency, total 25OHD (25 hydroxyvitamin) levels, may not be the best measure—at least by itself. Finally, several recent large clinical trials exploring the role of vitamin D supplementation in nonskeletal diseases are briefly reviewed, with an eye toward what questions they answered and what new questions they raised.

Atrichia with papular lesions: A case report

We present a case of clinically suspected atrichia with papular lesions in an 8-year-old male presenting with alopecia universalis and keratotic papules. We review the literature available on this rare genodermatosis.

Identification of a novel nonsense mutation in the ligand-binding domain of the vitamin d receptor gene and clinical description of two greek patients with hereditary vitamin d-resistant rickets and alopecia

BACKGROUND/AIMS

We analyzed the vitamin D receptor (VDR) gene in 2 Greek patients who exhibited the classical features of hereditary vitamin D-resistant rickets (HVDRR) type II, including severe bone deformities and alopecia. We also describe the clinical phenotypes and the response to treatment of our patients.

METHODS

Genomic DNA was extracted from peripheral blood samples of both patients. Coding region and flanking introns of VDR gDNA was amplified and direct sequenced.

RESULTS

A unique cytosine to thymine (C>T) transition was identified at nucleotide position 1066 (c.1066C>T) in the ligand-binding domain of the VDR gene of both patients, predicting the substitution of a glutamine to a terminal codon at position 356 (Gln356stop).

CONCLUSIONS

The novel nonsense mutation c.1066C>T (Gln356stop) is expected to result in a VDR protein 71 amino acids shorter and thus to affect the normal VDR function. In particular, the missing protein part alters the VDR heterodimerization with the retinoid X receptor which has been correlated with the presence of alopecia. Both patients were introduced to treatment with supraphysiological doses of 1α-calcidiol which improved their clinical phenotypes except for alopecia.

Mutations in the vitamin D receptor and hereditary vitamin D-resistant rickets

Heterogeneous loss of function mutations in the vitamin D receptor (VDR) interfere with vitamin D signaling and cause hereditary vitamin D-resistant rickets (HVDRR). HVDRR is characterized by hypocalcemia, secondary hyperparathyroidism and severe early-onset rickets in infancy and is often associated with consanguinity. Affected children may also exhibit alopecia of the scalp and total body. The children usually fail to respond to treatment with calcitriol; in fact, their endogenous levels are often very elevated. Successful treatment requires reversal of hypocalcemia and secondary hyperparathyroidism and is usually accomplished by administration of high doses of calcium given either intravenously or sometimes orally to bypass the intestinal defect in VDR signaling.

Vitamin D receptor-mediated control of Soggy, Wise, and Hairless gene expression in keratinocytes

The vitamin D receptor (VDR), but not its hormonal ligand, 1,25-dihydroxyvitamin D3 (1,25D), is required for the progression of the mammalian hair cycle. We studied three genes relevant to hair cycle signaling, DKKL1 (Soggy), SOSTDC1 (Wise), and HR (Hairless), to determine whether their expression is regulated by VDR and/or its 1,25D ligand. DKKL1 mRNA was repressed 49-72% by 1,25D in primary human and CCD-1106 KERTr keratinocytes; a functional vitamin D responsive element (VDRE) was identified at -9590 bp in murine Soggy. Similarly, SOSTDC1 mRNA was repressed 41-59% by 1,25D in KERTr and primary human keratinocytes; a functional VDRE was located at -6215 bp in human Wise. In contrast, HR mRNA was upregulated 1.56- to 2.77-fold by 1,25D in primary human and KERTr keratinocytes; a VDRE (TGGTGAgtgAGGACA) consisting of an imperfect direct repeat separated by three nucleotides (DR3) was identified at -7269 bp in the human Hairless gene that mediated dramatic induction, even in the absence of 1,25D ligand. In parallel, a DR4 thyroid hormone responsive element, TGGTGAggccAGGACA, was identified at +1304 bp in the human HR gene that conferred tri-iodothyronine (T3)-independent transcriptional activation. Because the thyroid hormone receptor controls HR expression in the CNS, whereas VDR functions in concert with the HR corepressor specifically in skin, a model is proposed wherein unliganded VDR upregulates the expression of HR, the gene product of which acts as a downstream comodulator to feedback-repress DKKL1 and SOSTDC1, resulting in integration of bone morphogenic protein and Wnt signaling to drive the mammalian hair cycle and/or influencing epidermal function.

Hairless modulates ligand-dependent activation of the vitamin D receptor-retinoid X receptor heterodimer

The active form of vitamin D, 1α,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], binds to the vitamin D receptor (VDR) and regulates various physiological and pharmacological processes. Secondary bile acids, such as lithocholic acid (LCA), also act as endogenous VDR ligands. The molecular basis of ligand-selective VDR action remains largely unknown. Hairless (HR) acts as a coregulator of VDR through a direct interaction. HR mutations confer an alopecia phenotype similar to VDR mutations in mice and humans, but the underlying molecular mechanisms have not been elucidated. We examined the effect of HR on VDR activation induced by 1,25(OH)(2)D(3) and LCA. HR repressed VDR transactivation induced by both 1,25(OH)(2)D(3) and LCA. HR also repressed transactivation of VDR E269A and R391A mutants, but less effectively than that of wild-type VDR. These residues are involved in retinoid X receptor (RXR) heterodimer allosteric communication, through which information from ligands is transmitted to dimer and coactivator interfaces. In the presence of HR cotransfection, LCA activated these VDR mutants more effectively than wild-type VDR. In mammalian two-hybrid assays, HR enhanced the association of VDR with a corepressor, nuclear receptor corepressor. These findings indicate that HR affects VDR-RXR heterodimer allosteric communication and corepressor complex formation. Interestingly, HR knockdown in keratinocyte-derived HaCaT cells increased ligand-induced cytochrome P450, family 24, subfamily A, polypeptide 1 (CYP24A1) expression but suppressed expression of cathelicidin antimicrobial peptide, indicating that HR acts not only as a corepressor but also as a coactivator. HR may be a VDR modulator that affects the RXR allosteric communication network in order to regulate transcription in a gene-selective manner.

Genetic disorders and defects in vitamin D action

The biochemical and genetic analysis of the VDR in patients with HVDRR has yielded important insights into the structure and function of the receptor in mediating 1,25(OH)2D3 action. Similarly, study of children affected by HVDRR continues to provide a more complete understanding of the biologic role of 1,25(OH)2D3 in vivo. A concerted investigative approach to HVDRR at the clinical, cellular, and molecular levels has proved valuable in gaining knowledge of the functions of the domains of the VDR and elucidating the detailed mechanism of action of 1,25(OH)2D3. These studies have been essential to promote the well-being of the families with HVDRR and in improving the diagnostic and clinical management of this rare genetic disease.

The role of vitamin D receptor mutations in the development of alopecia

Hereditary Vitamin D Resistant Rickets (HVDRR) is a rare disease caused by mutations in the vitamin D receptor (VDR). The consequence of defective VDR is the inability to absorb calcium normally in the intestine. This leads to a constellation of metabolic abnormalities including hypocalcemia, secondary hyperparathyroidism and hypophosphatemia that cause the development of rickets at an early age in affected children. An interesting additional abnormality is the presence of alopecia in some children depending on the nature of the VDR mutation. The data indicate that VDR mutations that cause defects in DNA binding, RXR heterodimerization or absence of the VDR cause alopecia while mutations that alter VDR affinity for 1,25(OH)(2)D(3) or disrupt coactivator interactions do not cause alopecia. The cumulative findings indicate that hair follicle cycling is dependent on unliganded actions of the VDR. Further research is ongoing to elucidate the role of the VDR in hair growth and differentiation.

Hereditary vitamin D-resistant rickets (HVDRR) owing to a heterozygous mutation in the vitamin D receptor

Hereditary vitamin D-resistant rickets (HVDRR) is a rare autosomal recessive disease caused by mutations in the vitamin D receptor (VDR). Patients exhibit severe rickets and hypocalcemia. Heterozygous parents and siblings appear normal and exhibit no symptoms of the disease. We analyzed the VDR gene of a young girl who exhibited the clinical features of HVDRR without alopecia. The patient had clinical and radiographic features of rickets, hypocalcemia, and elevated serum concentrations of 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. A single heterozygous missense mutation was found in the VDR gene that substituted glutamic acid with alanine at amino acid 420 (E420A). Sequencing of the girl's VDR cDNAs showed that the f/M1 allele contained the E420A mutation, whereas the F/M4 allele was completely normal. The girl's father, who was also heterozygous for the E420A mutation on the f/M1 allele, exhibited minor symptoms of vitamin D resistance. In contrast, the mother had no signs of the disease and had no mutations in her VDR gene. Both the girl and the father's skin fibroblasts showed resistance to 1,25(OH)(2)D(3) by their severely reduced induction of CYP24A1 gene expression. In transactivation assays, the E420A mutant VDR showed dominant-negative activity towards the wild-type VDR. This is the first report that we are aware of describing a patient with HVDRR caused by a single heterozygous missense mutation in the VDR gene. The E420A mutant appears to act in a dominant-negative fashion, silencing the wild-type VDR and resulting in an attenuated response to 1,25(OH)(2)D(3).

Mechanism of JmjC-containing protein Hairless in the regulation of vitamin D receptor function

The JmjC-domain-containing protein Hairless (HR) and the vitamin D receptor (VDR) play a critical role in the maintenance of hair growth. Mutations in HR or VDR cause alopecia in humans and mice. Here we show that HR interacts with VDR and induces VDR relocalization in the nuclei. HR associates and colocalizes with nuclear receptor co-repressor (N-CoR) which is localized to subnuclear structures termed matrix-associated deacetylase (MAD) bodies. It is found that the HR mutants (C622G, N970S, D1012N, V1136D), associated with alopecia universalis congenita (AUC) or atrichia with papular lesions (APL), exhibit an abnormal subcellular distribution in addition to the impaired co-repressor activity with VDR. Studies on deletion mutants of HR indicate that the JmjC domain contributes to the co-repressor activity of HR. Our work provides new clues and evidence for the understanding on the role of HR in hair growth.

Detection of a novel missense mutations in atrichia with papular lesions

BACKGROUND

Atrichia with papular lesions (APL) is a rare inherited disease characterized by early onset of total hair loss, followed by papular lesions over the extensor areas of the body. Recently, mutations in the human hairless (HR) gene have been implicated in its pathogenesis. The identification of mutations in the HR gene is important for differentiating between APL and alopecia universalis (AU).

OBJECTIVE

We compared the HR genes of patients with presumed AU who showed minimal or no response to treatment with the HR genes of healthy controls.

METHODS

The subjects were 11 patients with presumed AU who had not responded to treatments. Fifty healthy people were included as controls for molecular analysis. To screen for mutations, polymerase chain reaction was performed.

RESULTS

DNA analysis identified a novel heterozygous G-to-A transition at nucleotide position 191 in exon 5. The mutation was not found in the controls, other AU patients, or any unaffected family members except for the patients' mother and maternal grandfather, who were heterozygous HR gene carriers.

CONCLUSION

Our study identifies a novel missense mutation in exon 5 of the HR gene in a Korean APL patient previously diagnosed as AU.

Disruption of the hedgehog signaling pathway contributes to the hair follicle cycling deficiency in Vdr knockout mice

Mice null for the Vitamin D receptor (VdrKO) have a disrupted first hair follicle cycle and aborted subsequent hair follicle cycling. We examined the expression of different markers and mediators of hair follicle cycling in the hair follicle of the VdrKO mouse during days 13-22 when the hair follicle normally initiates and completes the first catagen. We compared the expression of those genes in mice with a nonsense mutation in hairless (Rhino), which have a similar alopecia phenotype, and to Cyp27b1 null mice which are deficient in the production of 1,25(OH)2D3, the Vdr ligand, but display normal hair follicle cycling. Our results demonstrate the down regulation of hair follicle markers and the alteration of expression of hedgehog (Hh), Wnt, Fgf, and Tgfbeta pathways in VdrKO and Rhino mice, but not in Cyp27b1KO mice. Treatment of VdrKO mice with an agonist to the Hh pathway partially restored hair follicle cycling, suggesting a role of this pathway in the regulation of hair follicle cycling by VDR. These results suggest that Vdr regulates directly or indirectly the expression of genes required for hair follicle cycling, including Hh signaling, independent of 1,25(OH)2D3.

Hairless expression attenuates apoptosis in a mouse model and the COS cell line; involvement of p53

BACKGROUND

Neurons are more likely to die through apoptosis in the immature brain after injury whereas adult neurons in the mature brain die by necrosis. Several studies have suggested that this maturational change in the mechanism of cell death is regulated, in part, by thyroid hormone. We examined the involvement of the hairless (Hr) gene which has been suspected of having a role in cell cycle regulation and apoptosis in the hair follicle and is strongly regulated by the thyroid hormone in the brain.

METHODOLOGY

Forced expression of Hr by transfection decreased the number of apoptotic nuclei, levels of caspase-3 activity, and cytosolic cytochrome C in COS cells exposed to staurosporine and tunicamycin. Similarly, caspase-3 activity was lower and the decrease in mitochondrial membrane potential was smaller in cultures of adult cerebellar granule neurons from wild type mice compared to Hr knockout mice induced to undergo apoptosis. In vivo, apoptosis as detected by positive TUNEL labeling and caspase 3 activity was lower in wild-type mice compared to Hr knockouts after exposure to trimethyltin. Hr expression lowered levels of p53, p53 mediated reporter gene activity, and lower levels of the pro-apoptotic Bcl2 family member Bax in COS cells. Finally, Hr expression did not attenuate apoptosis in mouse embryonic fibroblasts from p53 knockout mice but was effective in mouse embryonic fibroblasts from wild type mice.

CONCLUSIONS/SIGNIFICANCE

Overall, our studies demonstrate that Hr evokes an anti-apoptotic response by repressing expression of p53 and pro-apoptotic events regulated by p53.

Analysis of hairless corepressor mutants to characterize molecular cooperation with the vitamin D receptor in promoting the mammalian hair cycle

The mammalian hair cycle requires both the vitamin D receptor (VDR) and the hairless (Hr) corepressor, each of which is expressed in the hair follicle. Hr interacts directly with VDR to repress VDR-targeted transcription. Herein, we further map the VDR-interaction domain to regions in the C-terminal half of Hr that contain two LXXLL-like pairs of motifs known to mediate contact of Hr with the RAR-related orphan receptor alpha and with the thyroid hormone receptor, respectively. Site-directed mutagenesis indicates that all four hydrophobic motifs are required for VDR transrepression by Hr. Point mutation of rat Hr at conserved residues corresponding to natural mutants causing alopecia in mice (G985W and a C-terminal deletion DeltaAK) and in humans (P95S, C422Y, E611G, R640Q, C642G, N988S, D1030N, A1040T, V1074M, and V1154D), as well as alteration of residues in the C-terminal Jumonji C domain implicated in histone demethylation activity (C1025G/E1027G and H1143G) revealed that all Hr mutants retained VDR association, and that transrepressor activity was selectively abrogated in C642G, G985W, N988S, D1030N, V1074M, H1143G, and V1154D. Four of these latter Hr mutants (C642G, N988S, D1030N, and V1154D) were found to associate normally with histone deacetylase-3. Finally, we identified three regions of human VDR necessary for association with Hr, namely residues 109-111, 134-201, and 202-303. It is concluded that Hr and VDR interact via multiple protein-protein interfaces, with Hr recruiting histone deacetylases and possibly itself catalyzing histone demethylation to effect chromatin remodeling and repress the transcription of VDR target genes that control the hair cycle.

Genetic disorders and defects in vitamin d action

Two rare genetic diseases can cause rickets in children. The critical enzyme to synthesize calcitriol from 25-hydroxyvitamin D, the circulating hormone precursor, is 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-hydroxylase). When this enzyme is defective and calcitriol can no longer be synthesized, the disease 1alpha-hydroxylase deficiency develops. The disease is also known as vitamin D-dependent rickets type 1 or pseudovitamin D deficiency rickets. When the VDR is defective, the disease hereditary vitamin D-resistant rickets, also known as vitamin D-dependent rickets type 2, develops. Both diseases are rare autosomal recessive disorders characterized by hypocalcemia, secondary hyperparathyroidism, and early onset severe rickets. In this article, these 2 genetic childhood diseases, which present similarly with hypocalcemia and rickets in infancy, are discussed and compared.

Hereditary 1,25-dihydroxyvitamin D-resistant rickets with alopecia resulting from a novel missense mutation in the DNA-binding domain of the vitamin D receptor

The rare genetic recessive disease, hereditary vitamin D resistant rickets (HVDRR), is caused by mutations in the vitamin D receptor (VDR) that result in resistance to the active hormone 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3) or calcitriol). In this study, we examined the VDR from a young boy with clinical features of HVDRR including severe rickets, hypocalcemia, hypophosphatemia and partial alopecia. The pattern of alopecia was very unusual with areas of total baldness, adjacent to normal hair and regions of scant hair. The child failed to improve on oral calcium and vitamin D therapy but his abnormal chemistries and his bone X-rays normalized with intravenous calcium therapy. We found that the child was homozygous for a unique missense mutation in the VDR gene that converted valine to methionine at amino acid 26 (V26M) in the VDR DNA-binding domain (DBD). The mutant VDR was studied in the patient's cultured skin fibroblasts and found to exhibit normal [(3)H]1,25(OH)(2)D(3) binding and protein expression. However, the fibroblasts were unresponsive to treatment with high concentrations of 1,25(OH)(2)D(3) as demonstrated by their failure to induce CYP24A1 gene expression, a marker of 1,25(OH)(2)D(3) responsiveness. We recreated the V26M mutation in the WT VDR and showed that in transfected COS-7 cells the mutation abolished 1,25(OH)(2)D(3)-mediated transactivation. The mutant VDR exhibited normal ligand-induced binding to RXRalpha and to the coactivator DRIP205. However, the V26M mutation inhibited VDR binding to a consensus vitamin D response element (VDRE). In summary, we have identified a novel V26M mutation in the VDR DBD as the molecular defect in a patient with HVDRR and an unusual pattern of alopecia.

Hairless is a nuclear receptor corepressor essential for skin function

The activity of nuclear receptors is modulated by numerous coregulatory factors. Corepressors can either mediate the ability of nuclear receptors to repress transcription, or can inhibit transactivation by nuclear receptors. As we learn more about the mechanisms of transcriptional repression, the importance of repression by nuclear receptors in development and disease has become clear. The protein encoded by the mammalian Hairless (Hr) gene was shown to be a corepressor by virtue of its functional similarity to the well-established corepressors N-CoR and SMRT. Mutation of the Hr gene results in congenital hair loss in both mice and men. Investigation of Hairless function both in vitro and in mouse models in vivo has revealed a critical role in maintaining skin and hair by regulating the differentiation of epithelial stem cells, as well as a putative role in regulating gene expression via chromatin remodeling.

Modulation of vitamin d receptor activity by the corepressor hairless: differential effects of hairless isoforms

The vitamin D receptor (VDR) and its corepressor Hairless (HR) are thought to regulate key steps in the hair cycle because mutations in VDR or HR cause alopecia in humans and mice. Many mammalian cells express two major HR isoforms due to alternative splicing of exon 17. HR isoform-a encodes an 1189-amino acid protein (full-length HR), and isoform-b encodes an 1134-amino acid protein (HRDelta1072-1126). We demonstrated that both HR isoforms are expressed in primary human keratinocytes and in the human keratinocyte cell line HaCaT. In transfected COS-7 cells, the full-length HR repressed VDR-mediated transactivation. In contrast, HRDelta1072-1126 failed to suppress and even stimulated VDR-mediated transactivation. In coimmunoprecipitation, both HR isoforms interacted with the VDR, but only the full-length HR interacted with histone deacetylase 1 (HDAC1). Alanine mutagenesis of two conserved glutamic acids residues (E1100A/E1101A) encoded by exon 17 completely eliminated HR corepressor activity and interactions with HDAC1. When the two HR isoforms were coexpressed in COS-7 cells, the corepressor activity of the full-length HR was not antagonized by the HRDelta1072-1126 isoform. When transfected into HaCaT cells, the full-length HR inhibited endogenous CYP24A1 basal gene expression as well as 1,25-dihydroxyvitamin D3-stimulated CYP24A1 expression. HRDelta1072-1126 failed to suppress basal or 1,25-dihydroxyvitamin D3-stimulated CYP24A1 gene expression. In conclusion, we have demonstrated that both HR isoforms are expressed in keratinocytes and that the HRDelta1072-1126 isoform lacks corepressor activity and is unable to bind HDACs. HRDelta1072-1126 may function as a coactivator in some settings by inhibiting HDAC recruitment to the VDR transcriptional complex.

A novel mutation in Hr causes abnormal hair follicle morphogenesis in hairpoor mouse, an animal model for Marie Unna Hereditary Hypotrichosis

Hairpoor mice (Hr(Hp)) were derived through N-ethyl-N-nitrosourea (ENU) mutagenesis. These mice display sparse and short hair in the Hr(Hp)/+ heterozygous state and complete baldness in the Hr(Hp)/Hr(Hp) homozygous state. This phenotype was irreversible and was inherited in an autosomal semidominant manner. Hair follicles (HFs) of Hr(Hp)/+ mice underwent normal cycling and appeared normal, although smaller than those of the wild-type mice. In contrast, HFs of Hr(Hp)/Hr(Hp) mice became cyst-like structures by postnatal day (P) 21. The number and length of vibrissae decreased in a dose-dependent manner as the number of mutant alleles increased. A positional candidate gene approach was used to identify the gene responsible for the hairpoor phenotype. Genetic linkage analysis determined that the hairpoor locus is 2 cm from D14Mit34 on chromosome 14. Sequence analysis of the exons of the candidate gene hairless revealed a T-to-A transversion mutation at nucleotide position 403 (exon 2), presumably resulting in abolishment of an upstream open reading frame (uORF). In addition, we also found that the near-naked mouse (Hr(N)), a spontaneously arising mutant, harbors a A402G transition in its genome. Both mutations were in the uATG codon of the second uORF in the 5' UTR and corresponded to the mutations identified in Marie Unna Hereditary Hypotrichosis (MUHH) patients. In the present study we describe the phenotype, histological morphology, and molecular etiology of an animal model of MUHH, the hairpoor mouse.

Vitamin D receptor: molecular signaling and actions of nutritional ligands in disease prevention

The human vitamin D receptor (VDR) is a key nuclear receptor that binds nutritionally derived ligands and exerts bioeffects that contribute to bone mineral homeostasis, detoxification of exogenous and endogenous compounds, cancer prevention, and mammalian hair cycling. Liganded VDR modulates gene expression via heterodimerization with the retinoid X receptor and recruitment of coactivators or corepressors. VDR interacts with the corepressor hairless (Hr) to control hair cycling, an action independent of the endocrine VDR ligand, 1,25-dihydroxyvitamin D(3). We report novel dietary ligands for VDR including curcumin, gamma-tocotrienol, and essential fatty acid derivatives that likely play a role in the bioactions of VDR.

Involvement of SMRT corepressor in transcriptional repression by the vitamin D receptor

To repress the expression of target genes, the unliganded nuclear receptor generally recruits the silencing mediator of retinoid and thyroid hormone receptor (SMRT)/nuclear receptor corepressor via its direct association with the conserved motif within bipartite nuclear receptor-interaction domains (IDs) of the corepressor. Here, we investigated the involvement of the SMRT corepressor in transcriptional repression by the unliganded vitamin D receptor (VDR). Using small interference RNA against SMRT in human embryonic kidney 293 cells, we demonstrated that SMRT is involved in the repression of the VDR-target genes, osteocalcin and vitamin D(3) 24-hydroxylase in vivo. Consistent with this, VDR and SMRT are recruited to the vitamin D response element of the endogenous osteocalcin promoter in the absence of 1alpha,25-(OH)(2)D(3) in chromatin immunoprecipitation assays. To address the involvement of the VDR-specific interaction of SMRT in this repression, we identified the molecular determinants of the interaction between VDR and SMRT. Interestingly, VDR specifically interacts with ID1 of the SMRT/nuclear receptor corepressor and that ID1 is required for their stable interaction. We also identified specific residues in the SMRT-ID1 that are required for VDR binding, using the one- plus two-hybrid system, a novel genetic selection method for specific missense mutations that disrupt protein-protein interactions. These mutational studies revealed that VDR interaction requires a wide range of the residues within and outside the extended helix motif of SMRT-ID1. Notably, SMRT mutants defective in the VDR interaction were also defective in the repression of endogenous VDR-target genes, indicating that the SMRT corepressor is directly involved in the VDR-mediated repression in vivo via an ID1-specific interaction with the VDR.

Erasing the methyl mark: histone demethylases at the center of cellular differentiation and disease

The enzymes catalyzing lysine and arginine methylation of histones are essential for maintaining transcriptional programs and determining cell fate and identity. Until recently, histone methylation was regarded irreversible. However, within the last few years, several families of histone demethylases erasing methyl marks associated with gene repression or activation have been identified, underscoring the plasticity and dynamic nature of histone methylation. Recent discoveries have revealed that histone demethylases take part in large multiprotein complexes synergizing with histone deacetylases, histone methyltransferases, and nuclear receptors to control developmental and transcriptional programs. Here we review the emerging biochemical and biological functions of the histone demethylases and discuss their potential involvement in human diseases, including cancer.

(Neuro-)endocrinology of epithelial hair follicle stem cells

The hair follicle is a repository of different types of somatic stem cells. However, even though the hair follicle is both a prominent target organ and a potent, non-classical site of production and/or metabolism of numerous polypetide- and steroid hormones, neuropeptides, neurotransmitters and neurotrophins, the (neuro-)endocrine controls of hair follicle epithelial stem cell (HFeSC) biology remain to be systematically explored. Focussing on HFeSCs, we attempt here to offer a "roadmap through terra incognita" by listing key open questions, by exploring endocrinologically relevant HFeSC gene profiling and mouse genomics data, and by sketching several clinically relevant pathways via which systemic and/or locally generated (neuro-)endocrine signals might impact on HFeSC. Exemplarily, we discuss, e.g. the potential roles of glucocorticoid and vitamin D receptors, the hairless gene product, thymic hormones, bone morphogenic proteins (BMPs) and their antagonists, and Skg-3 in HFeSC biology. Furthermore, we elaborate on the potential role of nerve growth factor (NGF) and substance P-dependent neurogenic inflammation in HFeSC damage, and explore how neuroendocrine signals may influence the balance between maintenance and destruction of hair follicle immune privilege, which protects these stem cells and their progeny. These considerations call for a concerted research effort to dissect the (neuro-)endocrinology of HFeSCs much more systematically than before.

Mutations in the hairless gene underlie APL in three families of Pakistani origin

BACKGROUND

Atrichia with papular lesions (APL) (OMIM#209500) is a rare autosomal recessively inherited form of irreversible alopecia characterized by papular lesions of keratin-filled cysts on various regions of the body. Males and females are equally affected and present with a distinct pattern of total hair loss on scalp, axilla and body. It begins shortly after birth with the development of hair loss, and patients are normally devoid of eyelashes and eyebrows. Mutations in the hairless (HR) gene have been previously shown to be responsible for APL.

OBJECTIVE

In this study, we studied the molecular basis of APL in three unrelated families of Pakistani origin.

METHOD

Molecular analysis of the HR genes was performed on genomic DNA from probands and family members.

RESULTS

DNA sequencing of the HR gene in family A revealed a novel homozygous 2bp deletion in exon 6 leading to a frameshift and a downstream premature termination codon in exon 8 (1782-83delAG). In family B, we identified a novel homozygous deletion of a G nucleotide at the exon 15-intron 15 boundary, termed 3097delG. Family C carries a previously reported missense mutation consisting of an A-to-G transition at nucleotide 276 resulting in the mutation N970S in exon 14.

CONCLUSION

Two mutations identified in this study are novel mutations in the HR gene and extend the body of evidence implicating the hairless gene family in the pathogenesis of human skin disorders. The one previously reported mutation suggests it may represent a recurrent mutation, or alternatively, an allele that is widely dispersed around the world.

Ligand-independent regulation of the hairless promoter by vitamin D receptor

The characteristic alopecia associated with mutations in the hairless (hr) and vitamin D receptor (VDR) genes defines the resulting genetic disorders, known as atrichia and VDRRIIa rickets, as phenocopies. In both cases, the separation of the dermal papilla from the regressing hair follicle at the onset of the first catagen phase of the hair cycle and the development of dermal cysts and utricules subsequent to mutation of either gene suggests that their activities affect the same regulatory pathways. VDR functions as a hormonally activated transcription factor, and a role in transcription has been postulated for Hr due in part to its nuclear localization and homology with the GATA-1 zinc-finger domain. Therefore, we examined the hypothesis that VDR and Hr have a direct regulatory effect on each other via a transcriptional mechanism. Ectopic expression of the VDR repressed hr promoter activity in HaCaT cells and primary human keratinocytes (PHKs). While this repression occurs in the absence of 1,25 dihydroxyvitamin D3 (D3), the addition of ligand greatly augments the effect. However, we also demonstrate the rare phenomenon of ligand-independent promoter transactivation by VDR. We show that the full-length promoter is transactivated by VDR in a ligand-independent and cell type-specific manner, suggesting that direct transcriptional regulation of hr by the VDR accounts in part for the phenotypic overlap between atrichia and VDRRIIa rickets.