Regulation of hair follicle development by the TNF signal ectodysplasin and its receptor Edar

Death receptor signaling giving life to ectodermal organs

A new tumor necrosis factor (TNF) pathway has been identified that has an important function in the regulation of embryonic development. Three key components of this pathway are previously unknown proteins: the TNF ligand ectodysplasin (also known as EDA), its death domain-containing receptor EDAR, and the death domain adapter molecule EDARADD. This pathway was discovered and delineated through the cloning of genes that cause human hypohidrotic ectodermal dysplasia (HED) syndromes and by analysis of the corresponding mouse mutants (Tabby, downless, and crinkled) showing defects in hair, teeth, and several exocrine glands. EDAR signaling is mediated by the activation of nuclear factor kappa B, but other downstream targets are not known. Ectodysplasin-EDAR signaling mediates cell interactions within the ectoderm and regulates the initiation and morphogenesis of hair and teeth. It is also necessary for the development of fish scales, indicating that this pathway and its function have been conserved during the evolution of ectodermal organs.

Alopecia areata in families: association with the HLA locus

Alopecia areata (AA) is a T cell mediated disease directed against hair follicles that results in bald patches. It can range in severity from patchy (AA), to total scalp hair loss (alopecia totalis; AT) or body hair loss (alopecia universalis; AU). We have previously shown that HLA-DR4 and DR11 as well as HLA-DQ*03 alleles are increased in unrelated AA patients compared with controls. To study whether class II HLA alleles are linked to AA, we investigated 81 extended families that included 192 AA patients, including 89 with AT or AU. We also performed the transmission disequilibrium test (TDT) in 143 nuclear families. Results showed an association between alleles of HLA-DQB (p = 0.014) and HLA-DR (p = 0.010). We also performed linkage analysis in 75 families whose members' genomic DNA were available for HLA typing. Results from this analysis support linkage between AA and class II loci with a maximal LOD score of 2.42 to HLA-DQB at 5% recombination, and with a maximal LOD score of 2.34 to HLA-DR at 0% recombination. There was an increased incidence of atopic dermatitis and autoimmune thyroiditis in families. AA appears to be a class II HLA restricted organ specific immune response to the hair follicle.