Chromodomain proteins in development: lessons from CHARGE syndrome

Abstract

In humans, heterozygous mutations in the adenosine triphosphate-dependent chromatin remodeling gene CHD7 cause CHARGE syndrome, a common cause of deaf-blindness, balance disorders, congenital heart malformations, and olfactory dysfunction with an estimated incidence of approximately 1 in 10,000 newborns. The clinical features of CHARGE in humans and mice are highly variable and incompletely penetrant, and most mutations appear to result in haploinsufficiency of functional CHD7 protein. Mice with heterozygous loss of function mutations in Chd7 are a good model for CHARGE syndrome, and analyses of mouse mutant phenotypes have begun to clarify a role for CHD7 during development and into adulthood. Chd7 heterozygous mutant mice have postnatal delayed growth, inner ear malformations, anosmia/hyposmia, and craniofacial defects, and Chd7 homozygous mutants are embryonic lethal. A central question in developmental biology is how chromodomain proteins like CHD7 regulate important developmental processes, and whether they directly activate or repress downstream gene transcription or act more globally to alter chromatin structure and/or function. CHD7 is expressed in a wide variety of tissues during development, suggesting that it has tissue-specific and developmental stage-specific roles. Here, we review recent and ongoing analyses of CHD7 function in mouse models and cell-based systems. These studies explore tissue-specific effects of CHD7 deficiency, known CHD7 interacting proteins, and downstream target sites for CHD7 binding. CHD7 is emerging as a critical regulator of important developmental processes in organs affected by human CHARGE syndrome.

Figure 1. Chd7 is expressed in CHARGE related tissues during development

Frozen sections from E14.5 Chd7^Gt/+ embryos demonstrate β-galactosidase (X-gal staining) in CHARGE-related organs including (A) olfactory bulbs (ob), (B) olfactory epithelium (oe), (C) limb, (D) heart (hrt), (E) kidney (kd), (F) enteric neurons of the gut (en), (G) crista (ca) and cochlea (coc) of the inner ear and (H) lens (le) and retina (re) of the eye. All sections are in the coronal orientation.

Figure 2. Model of inner ear development in wild type and Chd7 heterozygous mutant mice

The E11.5 otocyst extends dorsally to form the endolymphatic duct (ed, grey) and ventrally to form the cochlea (coc), and neuroblasts delaminate from the ventral epithelium to occupy the statoacoustic ganglion (sag, shown in blue). By E12.5, the epithelium of the canal pouches fuses to form the anterior (asc), posterior (psc) and lateral (lsc) semicircular canals. Between E13.5 and adulthood, the vestibular apparatus continues to mature, the cochlea completes its coiling, and the sensory epithelium becomes completely innervated. In Chd7 heterozygous mutant ears the lateral and posterior semicircular canals are truncated (lt), small, or misshapen. In addition, the nerve to the posterior ampulla (npa) is disrupted. Normal developmental structures are shown by a dashed line in Chd7 heterozygous mutants.

Figure 3. Model of olfactory function in wild type and Chd7^Gt/+ mice

Following inhalation, odorants are detected in the olfactory epithelium by binding to specific odorant receptors located on the surface of olfactory cilia. Bound odorants activate olfactory sensory neurons, which are bipolar neurons that project axons to the glomeruli in the olfactory bulb. Each olfactory sensory neuron (ex. blue, red, green, and purple) contains one type of odorant receptor in the cilia and each neuron with that type of odorant receptor projects an axon to the same glomerulus in the olfactory bulb. Electrical signals sent by olfactory sensory neurons are detected by mitral cell dendrites in the glomeruli. These signals are then sent to higher brain regions in the central nervous system. Wild type mouse olfactory epithelium contains densely packed olfactory sensory neurons which project to the olfactory bulb. However, Chd7^Gt/+ young adult mice have 30% fewer olfactory sensory neurons and olfactory bulb hypoplasia. We hypothesize that aged Chd7^Gt/+ mice have a greater reduction in both olfactory sensory neurons and size of the olfactory bulb.

Figure 4

The chromodomain family of proteins contains 9 members that are subdivided into 3 classes on the basis of shared protein motifs. CHD7 is a member of the third class together with CHD5, CHD6, CHD8, and CHD9.

Figure 5. Model for CHD7 transcriptional regulation

CHD7 binds to enhancer regions of target genes together with a tissue-specific complex of proteins. The CHD7 tissue-specific complex either activates or represses downstream target gene expression in a developmental stage specific manner.