Hairless Gene Nonsense Mutations in Alopecia Universalis: A Case Report

Alopecia universalis (AU) congenital, known as generalized atrichia, is a severe form of autosomal recessive alopecia that results in complete hair loss of scalp and body. Mutations in the human hairless gene (HR) are associated with the phenotype of the disease. A consanguineous couple who had a child with the generalized atrichia sign referred to us for genetic counseling. According to the patient's symptoms and after thorough examination and history taking, the HR gene was the candidate gene to be assessed and analyzed. For this purpose targeted primers were designed for all exons of the HR gene followed by running PCR for exons amplification. Finally, the PCR products were sequenced. Whole-gene sequence analysis revealed a nonsense homozygous mutation in exon 6 that, according to the ACMG guide, is a pathogenic variant. Sequence analysis of the exon in parents reveals that they are heterozygout for the non-sense mutation, as well.

Functional mapping of the mouse hairless gene promoter region

The mouse hairless gene (Hr) encodes a protein of 127 kDa, acting as corepressor of nuclear hormone receptors. The Hairless protein (HR) is involved in the control of the cellular transition to the first hair cycle in adult Mammals. In its absence hair follicles disintegrate leading to a complete and irreversible hair loss with formation of cutaneous cysts. The hairless phenotype is therefore linked to defective proliferation and migration of the hair follicle stem cells apparently unable to respond to various signalling molecules. The Hr gene is expressed at high levels in skin and brain, and hairless transcripts were detected in gonads, thymus and colon. Although the patterns of Hr expression appear to be spatially and temporally regulated, very little is known about the molecular basis of the transcriptional control underlying Hr gene function. In this work we determine the precise transcriptional initiation start site of the mouse Hr gene and identify a new 1,1 kb cis-control element (RE1) that encompasses the promoter region and is able to drive luciferase reporter expression in skin and brain derived cell lines. We performed a deletion analysis and explored functionally regulatory motifs within this fragment to show that the role of this upstream regulatory region is linked to the presence of TRE and VDRE binding sites. We find that a TRE situated at –300 bp from the cap site is essential for gene expression in both skin NIH 3T3 and GHFT1 cells, while a VDRE positioned 94 bp upstream of the TRE modulates reporter expression specifically in skin derived cell lines. In addition, we define a novel cis-regulatory motif UE60, situated at the 5’-end of RE1 and likely to interact with both TRE and VDRE. Our data complete previous results on the possible existence of an autoregulatory pathway, implicated in Hr gene regulation. Taken together these findings reveal a complex molecular network that potentially links several signalling pathways in hair follicle formation. We discuss the organisation of the regulatory modules in the mouse Hr gene upstream DNA sequences in the light of the high homology of this region in mouse, rat and human.

Molecular cloning of the tomato Hairless gene implicates actin dynamics in trichome-mediated defense and mechanical properties of stem tissue

Trichomes are epidermal structures that provide a first line of defense against arthropod herbivores. The recessive hairless (hl) mutation in tomato (Solanum lycopersicum L.) causes severe distortion of trichomes on all aerial tissues, impairs the accumulation of sesquiterpene and polyphenolic compounds in glandular trichomes, and compromises resistance to the specialist herbivore Manduca sexta. Here, we demonstrate that the tomato Hl gene encodes a subunit (SRA1) of the highly conserved WAVE regulatory complex that controls nucleation of actin filaments in a wide range of eukaryotic cells. The tomato SRA1 gene spans a 42-kb region containing both Solyc11g013280 and Solyc11g013290. The hl mutation corresponds to a complex 3-kb deletion that removes the last exon of the gene. Expression of a wild-type SRA1 cDNA in the hl mutant background restored normal trichome development, accumulation of glandular trichome-derived metabolites, and resistance to insect herbivory. These findings establish a role for SRA1 in the development of tomato trichomes and also implicate the actin-cytoskeleton network in cytosolic control of specialized metabolism for plant defense. We also show that the brittleness of hl mutant stems is associated with altered mechanical and cell morphological properties of stem tissue, and demonstrate that this defect is directly linked to the mutation in SRA1.

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.