毛囊小型化是一个退化过程，它减小上皮细胞和间叶细胞的腔室尺寸，将粗头发变为细而柔的头发。
它最常见于“雄激素源性脱发”，但也是一种被称为“遗传性单纯少毛症”（HHS）的罕见脱发症的特征。现在，HHS中所涉及的基因在受影响家族中通过基因关联分析已被识别为APCDD1，该基因编码一种与膜结合在一起的糖蛋白，该蛋白在人的毛囊中有大量表达。
功能研究表明，APCDD1抑制Wnt信号作用，因而是毛囊中信号传导通道的一个潜在重要构成部分。(生物谷Bioon.com)
生物谷推荐原文出处：
Nature doi:10.1038/nature08875
APCDD1 is a novel Wnt inhibitor mutated in hereditary hypotrichosis simplex
Yutaka Shimomura1,7, Dritan Agalliu3,7, Alin Vonica4,7, Victor Luria1,2,7, Muhammad Wajid1, Alessandra Baumer5, Serena Belli6, Lynn Petukhova1, Albert Schinzel5, Ali H. Brivanlou4, Ben A. Barres3 & Angela M. Christiano1,2
1. Department of Dermatology,
2. Department of Genetics and Development, Columbia University, College of Physicians and Surgeons, 630 West 168th Street, VC15 204A, New York, New York 10032, USA
3. Department of Neurobiology, Stanford University, Stanford, California 94305, USA
4. The Laboratory of Vertebrate Embryology, The Rockefeller University, New York, New York 10065, USA
5. Institute of Medical Genetics, University of Zurich, Schwerzenbach CH-8603, Switzerland
6. Struttura Semplice Genetica Medica APSS, Trento 138100, Italy
7. These authors contributed equally to this work.
Hereditary hypotrichosis simplex is a rare autosomal dominant form of hair loss characterized by hair follicle miniaturization1, 2. Using genetic linkage analysis, we mapped a new locus for the disease to chromosome 18p11.22, and identified a mutation (Leu9Arg) in the adenomatosis polyposis down-regulated 1 (APCDD1) gene in three families. We show that APCDD1 is a membrane-bound glycoprotein that is abundantly expressed in human hair follicles, and can interact in vitro with WNT3A and LRP5—two essential components of Wnt signalling. Functional studies show that APCDD1 inhibits Wnt signalling in a cell-autonomous manner and functions upstream of β-catenin. Moreover, APCDD1 represses activation of Wnt reporters and target genes, and inhibits the biological effects of Wnt signalling during both the generation of neurons from progenitors in the developing chick nervous system, and axis specification in Xenopus laevis embryos. The mutation Leu9Arg is located in the signal peptide of APCDD1, and perturbs its translational processing from the endoplasmic reticulum to the plasma membrane. APCDD1(L9R) probably functions in a dominant-negative manner to inhibit the stability and membrane localization of the wild-type protein. These findings describe a novel inhibitor of the Wnt signalling pathway with an essential role in human hair growth. As APCDD1 is expressed in a broad repertoire of cell types3, our findings indicate that APCDD1 may regulate a diversity of biological processes controlled by Wnt signalling.
