Intellectual disability refers to a broad group of neurodevelopmental disorders defined by a low IQ and significant impairments to adaptive functioning that affect approximately 1% of individuals worldwide1. Despite the enormous monetary cost2 and public health challenge these disorders represent, scientific understanding of the etiology of intellectual disability remains remarkably incomplete. However, emerging evidence suggests that many of the hundreds of intellectual-disability-linked genes converge on only a few molecular pathways, including those regulating neuronal plasticity by controlling synaptic protein translation3-6. This convergence suggests that scientific and therapeutic insight gained in monogenic, experimentally tractable forms of intellectual disability could be applicable to forms whose precise etiology is complex or unknown. Recently, we have shown that one such monogenic form of intellectual disability is caused in humans by loss-of-function mutations in ZC3H14, a gene encoding a ubiquitously expressed RNA binding protein (RBP) whose molecular function, protein binding partners, and RNA targets are largely unknown. To understand how mutations in human ZC3H14 induce an intellectual disability, and thus to gain insight into the development of intellectual disability in general, we have developed a unique Drosophila model of this monogenic form of intellectual disability by deleting dNab2, the fly ortholog of ZC3H14. Critically, many of the phenotypes caused by this dNab2 loss are rescued by neuronal expression of human ZC3H14, suggesting that ZC3H14 is functionally well conserved between flies and humans and supporting the feasibility of understanding ZC3H14 function through the study of dNab2. While the molecular function of dNab2 is not yet well understood, our preliminary data demonstrates that dNab2 plays a key role in neuronal development, possibly through control of neuronal translation. Consistent with this hypothesis, dNab2 exhibits strong genetic interactions with Ataxin-2 (Atx2), a neurodegenerative-disease-associated RNA binding protein that regulates synaptic translation. Building on these data, we will test the hypothesis that dNab2, a conserved RNA binding protein associated with intellectual disability, directly regulates translation in neurons in conjunction with Atx2.
The Specific Aims o this project are: 1) to define the physical, spatial, and functional connections between dNab2 and the neuronal translational regulator Atx2 and 2) to identify the cytoplasmic, neuronal RNA targets of dNab2 and to functionally characterize the role of dNab2 in controlling their translatio in neurons in vivo. To accomplish the former, we will test whether dNab2 co-immunoprecipitates from fly heads with Atx2, whether dNab2 and Atx2 co-localize in primary cultured fly brain neurons, and whether dysregulation of dNab2 perturbs the translation of canonical RNA targets of Atx2. To accomplish the latter, we will combine a discovery-based RBP-RNA complex immunoprecipitation and sequencing approach with a candidate-based analysis of the effects of dNab2 dysregulation on translational reporters in vivo.
Intellectual disabilities, defined by an IQ below 70 and significant impairments to learning or activities of daily life, can be caused by mutations in hundreds of different genes. However, the normal function of many of these genes converge on one of only a few molecular pathways, including the control of protein synthesis at the junctions between neurons, suggesting that insight into the molecular mechanisms of a single intellectual disability will be applicable to many others. Thus, to enhance our understanding of intellectual disability in general, we will define the roles in neuronal protein synthesis of a modl system version of the human protein ZC3H14, mutations in which cause a form of intellectual disability.
