Millions across the world are currently living with one of 7,000 rare diseases, the majority of which are thought to be monogenic in origin. Due to their rarity, many of these diseases are not well studied. As a result, patients with a monogenic disease often face limited treatment options and a slim chance towards remission. However, with recent advances in genome editing, it is now possible to model many inherited diseases in a variety of organisms. Given this, we are utilizing Caenorhabditis elegans as an avenue to model monogenic diseases and develop treatments for patients. The nature of C. elegans makes it an efficient, inexpensive, and simple system to accomplish this task. In this study, we have applied this novel approach to explore Barber-Say, Ablepharon Macrostomia, and novel Saethre-Chotzen-like syndromes, which arise due to mutations in the two human TWIST genes. These syndromes are autosomal dominant in nature. We use C. elegans to model conserved mutations in hlh-8, the TWIST ortholog.
Human TWIST1 and TWIST2 are bHLH transcription factors involved in many important pathways, including craniofacial development. As a result, humans with TWIST dysfunction have severe bone development defects, most obvious in the skull manifested as craniofacial deformation. Using our system, we modeled six mutations found in patients with the above three syndromes. The mutations, all at codon 29 of hlh-8, disrupt the basic domain, which binds DNA. Interestingly, we observed different, distinct phenotypes for each allele, even though these mutants only differed by the amino acid at codon 29. The three phenotypes we observed were one or a combination of the following: egg-laying defective (Egl), constipation (Con), and a tail deformation. The most severe allele exhibited all three phenotypes whereas the mildest only displayed an Egl phenotype. Medium severity alleles exhibited an Egl phenotype with a variably penetrant Con and tail deformation phenotype. This unexpected spectrum of alleles poses some interesting biochemical questions for future investigation. A previously studied null hlh-8 mutation behaved as a recessive allele with all three of these phenotypes. Our genetic analysis is consistent with these new alleles having a dominant negative nature, which suggests the human syndromes are due to defective TWIST proteins interfering with wild-type TWIST function. Future plans include suppressor screens of these alleles and modeling other diseases using this methodology.