Title: Genetic Mechanisms Controlling Human Hair Growth
Description: The genetic underpinnings of human hair growth are complex, relying upon several mechanisms to regulate gene expression. As such, inherited skin and hair disorders can arise from a variety of mutational events in the genome, from changes in single nucleotides to structural rearrangements of chromosomes. Importantly, inherited conditions affecting hair growth can be used as models to interrogate the molecular basis of the disease, and obtain novel insight into mechanisms and pathways required for normal hair growth. The approaches used to identify pathogenic mutations in a given disorder will depend on the inheritance pattern and disease prevalence in the population. In rare, Mendelian disorders of the skin and hair, the genetic architecture is often composed of rare variants with large effects, detected by linkage or whole-exome/genome sequencing. In contrast, polygenic disorders are composed of common and rare variants that contribute small to moderate effects and can be detected using Genome-Wide Association Studies (GWAS).

The primary goal of my thesis research is to identify and characterize genetic mechanisms controlling human hair growth. To accomplish this, I have studied three inherited conditions affecting human hair growth as genetic models in which I performed detailed functional and molecular analyses of the causal genetic lesions and their downstream effects on gene expression in the hair follicle. My thesis work revolved around the use of three major approaches to identify and characterize genetic mechanisms underlying human hair growth: 1. Identifying genomic effects on human hair growth in a rare, sporadic Mendelian disorder (Chapter II), 2. Characterizing single-gene effects on human hair growth in a rare, familial condition (Chapter III), and 3. Functional analysis of rare, non-coding variants in a complex polygenic autoimmune disease, alopecia areata (Chapter IV). In the first part of my thesis work, I investigated the genetic mechanism associated with X-linked hypertrichosis (XLH), a very rare condition of excessive hair overgrowth.

We identified a large interchromosomal insertion at chrXq27.1 that completely cosegregated with the phenotype, and was consistent with findings of interchromosomal insertions in two previously reported XLH families. Remarkably, the insertions in all three families occur at the exact same palindromic sequence, and because the sequences contained within each insertion are distinct, we hypothesized that the presence of the insertion (rather than its content) may be responsible for the excessive hair overgrowth phenotype. I then tested the impact of the insertion on the expression of the surrounding genes and found that FGF13 levels were selectively and dramatically reduced in patient hair follicles, suggesting a position effect as a result of the interchromosomal insertion. We postulate that the presence of this insertion disrupts key inter- and intrachromosomal interactions required for normal hair growth. In the second part of my thesis, I identified single-gene mutations that affect human hair growth by investigating the genetic basis of autosomal recessive congenital generalized hypertrichosis terminalis (CGHT) in a consanguineous family.

We performed whole-exome sequencing and identified a novel, rare splice variant in ABCA5 that cosegregates with the phenotype in a homozygous recessive manner. I found that ABCA5 is highly expressed in human skin and hair follicles, and its expression pattern is conserved in mouse tissues as well. The ABCA5 mutation in CGHT leads to a complete loss-of-function in patient hair follicles, as well as reduced lysosome function and cholesterol transport, a finding consistent with defects in Abca5-/- mice. Moreover, we identified a deletion spanning ABCA5 in an unrelated sporadic CGHT case and found that ABCA5 levels were dramatically reduced in patient hair follicles. Collectively, our findings point to a novel role for ABCA5 in regulating hair growth. In the third part of my thesis, I characterized the genetic architecture of a complex, polygenic disease affecting hair growth by studying rare, non-coding variants in alopecia areata. The Christiano lab previously performed the first GWAS, identifying the ULBP3/6 locus on chr.6q25.1 encoding NKG2D T cell receptor ligands as the most significant association outside the HLA region.

A strong upregulation of these ligands was observed on both human and mouse hair follicles, and we recently showed that T cells bearing the NKG2D receptor are both necessary and sufficient to induce disease in the mouse model. To identify the susceptibility variants at ULBP3/6 in human AA, we performed targeted deep resequencing and functional genomics studies and identified three rare, novel variants that reside within ULBP6 regulatory elements and CTCF binding sites. I found that these variants disrupt CTCF binding and regulatory activity in vitro, and CTCF binding is enriched at ULBP6 in vivo. Future studies examining long-range reporter activity and CTCF-mediated interactions will define the role of CTCF in the repression of the ULBP3/6 genes in the human hair follicle. Collectively, I used a variety of genetic approaches to identify novel genes and mechanisms controlling human hair growth.