The goal of this proposal is to understand the genetic and functional anatomical basis underlying autosomal recessive horizontal gaze palsy with progressive scoliosis (HGPPS;OMIM 607313), the disease locus of which we recently mapped to 11q23-25. This condition is characterized by a complete absence of conjugate horizontal eye movement at birth, with a delayed development of progressive scoliosis during infancy and childhood. Absent horizontal gaze likely results from maldevelopment of the abducens nuclei, involving both abducens moto- and inter-neurons. The recruitment of additional HGPPS patients has enabled the confirmation as well as narrowing of the candidate region in six ethnically diverse inbred families. Continuing effort to identify new patients may further narrow the region. The specific aims for this proposal are: 1) To test the hypothesis that the HGPPS gene is a novel patterning gene. Already underway, candidate gene sequencing will be prioritized based on neuronal expression pattern, putative function in neurodevelopment, and orthologous genes in mice and other organisms. 2) To test the hypothesis that maldevelopment of the abducens nucleus is the anatomical basis of HGPPS. Innovative high-resolution conventional, diffusion tensor, and functional MRI studies will be performed in normal and genetically characterized patients to define the functional anatomical basis of horizontal gaze dysfunction localizing to the brainstem, which has not been well visualized. 3) To test the hypothesis that the HGPPS gene is important in the normal development of the abducens nucleus and other brainstem structures. We will examine expression of the HGPPS gene in embryonic brain tissue from human and mouse (wildtype and developmental mutants) to study its role in the cascade of genetically programmed signaling pathways mediating neurogenesis. Neuroimaging techniques that we develop will be applicable to the study of other brainstem structures important in oculomotor control. Understanding the anatomical and molecular basis of HGPPS will provide insight into the genetically programmed neurodevelopment of the conjugate horizontal gaze center and other cranial nuclei in the brainstem.