Our section has a major effort on understanding the molecular and cellular basis of stuttering, a common but poorly understood speech disorder. Stuttering has long been known to have a genetic component. We have previously identified mutations in the GNPTAB, GNPTG and NAGPA genes that are associated with stuttering in populations worldwide. These results indicate that non-syndromic stuttering can be associated partial loss-of-finction mutations in lysosomal targeting and transport, a cellular function that is well-studied in health and disease. More recently we have identified mutations in the AP4E1 gene that cause stuttering. AP4E1 encodes a cellular component that serves to direct movement of vesicles within the cell, including those destined to go to the lysosome. The product of the AP4E1 genes recognizes and directs the trafficking of the product of the NAGPA gene in particular, thus tying this finding to our previous gene findings, and highlighting the importance of this cellular process in the genesis of stuttering. We find a mutation in one of these four genes in 20% of unrelated individuals with familial persistent stuttering from populations around the world.. However this indicates a large fraction of presumably genetic cases of stuttering have a causative gene that is not yet identified. We know from previous genetic linkage studies in our lab that other stuttering genes exist, and that additional genes that can cause stuttering reside on chromosomes 2, 3, 10, 14, and 16. Efforts are underway to identify these genes, with the goal of broadening our knowledge of the underlying causes of stuttering at the molecular and cellular level. Another goal of our current research is to identify how the cell metabolic defects caused by mutations in these genes lead to stuttering without any other discernible symptoms. These studies are focused on using neuroimaging and neuropathology methods the identify brain tissue pathologies in individuals carrying different mutations in these genes. We are collaborating with Dr. Peter Vogel at St. Jude's Hospital in Memphis, TN to examine any microscopically observable neuropathology that may accompany the vocalization deficits we observe in mice carrying human stuttering mutations. We are also working to develop a mouse model of human stuttering. This is being done by creating so-called knock-in strains of mice that carry the mutations identified in humans who stutter. These experiments require a detailed acoustical analysis of mouse vocalization, which is largely ultrasonic in nature. In these experiments, we are working with Drs. Terra Barnes and Tim Holy at Washington University in St. Louis. Our studies have demonstrated that the presence of human stuttering mutations causes reproducible alterations in mouse vocalizations, and that these alterations show parallels with the alterations present in the speech of individuals who stutter. We are now using mouse genome engineering technologies to express stuttering mutations within specific neuronal cell types and cell lineages within the brain. The goal of these studies is to identify specifically which cell types within the brain mediate these alterations in vocalization. Our studies of taste perception are focused on the role of genetic differences in taste perception in tobacco use, particularly the use of mentholated cigarettes, which are disproportionately used by African Americans. Our goal is to determine whether this disproportionate use is associated with genetic differences, specific to African Americans, in genes that encode taste perception machinery. This study is being done in collaboration with the University of Texas Southwestern Medical Center, using the well-characterized Dallas Heart Study population. In the past year, we have also entered into a collaboration with Dr. Thomas Kirchner at New York University and Dr. Carla Berg of Emory University, who together have provided >1700 DNA samples from individuals with known smoking phenotypes. Whole exome sequencing, which simultaneously evaluates variation in all human genes, is now being performed in these subjects, followed by statistical analysis of these very large data sets.
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