The root causes of Autism Spectrum Disorders (ASD) remain almost entirely unknown. Despite strong evidence for genetic involvement, no specific genes have yet been identified. The co-occurrence of ASD and Tuberous Sclerosis Complex (TSC) has been recognized for many years. Features of ASD are present in 25- 50 percent of individuals with TSC, a neurodevelopmental disorder caused by mutations in tumor suppressor genes TSC1 and TSC2, encoding hamartin and tuberin respectively. Tuberin and hamartin function together to inhibit mTOR signaling, which regulates protein synthesis and cell growth. In addition to being a critical regulator of cell growth, mTOR signaling plays an essential role in neural plasticity and synapse function. Naturally occurring mutations, resulting in inactivation or downregulation of the tuberin-hamartin complex through phosphorylation, lead to aberrant activation of mTOR signaling. Haploinsufficiency or a reduction in TSC proteins has been shown to be sufficient for perturbations of synapse function. Neurofibromatosis type 1 (NF1), is another common inherited neurocutaneous disorder associated with cognitive, attention and learning deficits. NF1 deficiency results in tuberin inactivation through phosphorylation, and subsequent mTOR activation. Similarly, brain-derived neurotrophic factor (BDNF) phosphorylates tuberin and induces mTOR- dependent local protein synthesis in neurons. Pam (Protein Associated with Myc), a large protein that we identified as an interactor of the TSC2 protein tuberin, is highly conserved across many species and has the highest expression in brain. Pam homologs in Drosophila and C. elegans are neuron-specific and function as synaptic growth regulators. PTEN is another important upstream regulator of mTOR signaling, and mutations in PTEN result in tuberin phosphorylation and mTOR activation. Based on these observations, we hypothesize that aberrant hyperactivation of mTOR in neurons is a common causal pathway for learning and other cognitive deficits associated with TSC and NF1, and increases risk for ASD. Further, we hypothesize that inherited variations in one or more genes that influence mTOR signaling, TSC1, TSC2, NF1, BDNF, Pam and PTEN will be associated with risk for ASD. We will explore these hypotheses by conducting family-based association studies using single-nucleotide polymorphisms (SNPs) and haplotype analyses spanning the entire TSC1, TSC2, NF1, BDNF, Pam, and PTEN genes in 777 AGRE families with ASD. In addition, we will carry out re-sequencing of the entire coding region of PTEN in 500 ASD cases and 250 controls. If one or more of these genes is found to be associated with ASD, it would not only break new ground for understanding the pathogenesis of ASD, but would also indicate an effective therapeutic strategy, since rapamycin and its analogs are effective in blocking mTOR signaling. Autism, a disorder that involves many genetic factors, is also seen in other genetic diseases such as Fragile X syndrome and Tuberous Sclerosis Complex (TSC). TSC genes regulate signaling mediated through mammalian target of rapamycin, referred to as mTOR. mTOR signaling plays an essential role in determining how neurons in brain communicate with each other. This project will test whether genes that control mTOR signaling are associated with an increased risk for Autism Spectrum Disorders, and thus has direct relevance to public health. ? ? ? ?
| James, Marianne F; Han, Sangyeul; Polizzano, Carolyn et al. (2009) NF2/merlin is a novel negative regulator of mTOR complex 1, and activation of mTORC1 is associated with meningioma and schwannoma growth. Mol Cell Biol 29:4250-61 |
| Han, Sangyeul; Witt, Rochelle M; Santos, Tulio M et al. (2008) Pam (Protein associated with Myc) functions as an E3 ubiquitin ligase and regulates TSC/mTOR signaling. Cell Signal 20:1084-91 |
