Genetic and Developmental Basis for Coordinated Growth of the Limb

How is growth and proportion regulated during development to ensure appropriate scaling, or proportion of elements within a structure? Our lab used genetic screens in zebrafish to identify the potassium channels KCNH2 and KCNK5 as sufficient to cause overgrowth of appendage structures in a coordinated fashion. Further, we investigate a unique presentation of growth dysregulation, macrodactyly, as a case study to understand how integrated, non-cancerous, growth occurs even in the case of potent oncogenic driver mutations. In a broad cohort of patients with macrodactyly, we identified genetic factors associating with patterned digital overgrowth in the PI3K signaling cascade. In parallel, using genetic screens in zebrafish, we identified the potassium channels KCNH2 and KCNK5 as sufficient to cause overgrowth of appendage structures in a coordinated fashion. Of note, we demonstrate functional synergy between PI3K signaling and altered KCNH2 variants identified in patients, sufficient to lead to patterned overgrowth of appendages. These findings highlight unknown role of potassium channels in mediating growth potential of oncogenic mutations. The lack of knowledge on the role of these genetic factors during development limits our current ability to effectively address and alleviate disease progression. To detail the intricate regulation of somatic clone growth, we outline three specific aims that leverage clinical and experimental analyses to address changes in gene regulation, role of potassium channel function in modifying growth potential, and tissue and genetic constraints on clonal dynamics. In Aim 1, we take advantage of gain- and loss-of-function lines to address function of the potassium channels Knch2 and Kcnk5 in regulating differentiation, growth and proportion of the limbs. These mouse models permit detailed tissue-specific analysis of potassium channel function mirroring mutant analysis in the zebrafish. Through histological and anatomical analyses, we will assess the effect of altering potassium channel function on the differentiation of osteochondral progenitors in long bones of the developing limb. In Aim 2, we will use zebrafish models to observe cell behavior in real time to specifically address the role of defined genetic context and developmental constraint in clonal expansion during development. We will extend and complement this work in Aim 3 to systematically identify genetic regulators of growth using new unbiased, zebrafish somatic-mosaic screens to identify modifiers of overgrowth regulation. Through these integrated, and separate innovative approaches, we can distill the mechanisms by which overgrowth is shaped, and potentially alleviated.