Abnormal Cellular/Synaptic Growth Hypothesis

From Pheno Wiki
Jump to: navigation, search

Abnormal Cellular/Synaptic Growth Hypothesis

This hypothesis came about from studies on single gene disorders where patients with the disorder have a high incidence rate of ASD, like fragile X with an ASD prevalence rate of 15% and Tuberous sclerosis, which has a prevalence rate anywhere between 25%-60%. Studies of these disorders can illuminate which biological pathways may be responsible for ASD phenotypes.

Studies of Tuberouse Schlerosis and Cowden/Lhermitte-Cuclos Syndromes found that the two syndromes associate susceptibility to Autism Spectrum Disorders with macrocephaly.1 Macrocephaly is relatively common among ASD patients with an estimated prevalence rate of 15-35%.2

Tuberouse Schlerosis and Cowden/Lhermitte-Cuclos Syndrome are caused by NF1, TSC1/TSC2, and PTEN. The tuberous schlerosis complex has GAP activity against GTP binding protein Rheb which results in the inhibition of rapamycin kinase (mTOR). These proteins act as negative effectors of mTORC1, which is a major regulator of cellular growth in mitotic cells. It is thought that mutations enhance the MTORC1 complex which is activated by a sequential kinase cascade downstream of the phosphoinositide-3 kinase pathway. This pathway can be modulated by serotonin.1

Brain MRI T1 movie.gifNeuroimaging

During the first year of birth, brains of people with ASD often show increased brain volume and head circumference. Most of the additional growth is in the frontal lobes. Retrospective studies have shown that head circumference of people with ASD are normal at birth, there have been no studies on size of prenatal or neonatal ASD brain volume. 3

Anaphase IF.gifGenetics

FGF signaling is important in regulating cortical regional properties and growth, particularly in the frontal cortex. Fgf8 is most active in this respect, but other Fgf proteins also play a factor. For example, Fgf17 KO mice only have a subtle reduction in their prefrontal cortex. Of note is the fact that the lesion of a signaling gene (Fgf17) can disrupt the neural systems that are abnormal in people with ASD.

This shows that the FGF-signaling pathway is perhaps overactive in some forms of ASD, which could be a result of increased expression of FGF or increased signaling through other elements downstream of this pathway, including mutations in PTEN phosphatase and TSC1/TSC2.3

Animal Models

Home Page


1. Bourgeron T. A synaptic trek to autism. Curr Opin Neurobiol. 2009 Apr;19(2):231-4 PMID 19545994

2. Williams CA et. al. Genetic disorders associated with macrocephaly. Am J Med Genet A. 2008 Aug 1;146A(15):2023-37. PMID 18629877

3. Rubenstein JL. Three hypotheses for developmental defects that may underlie some forms of autism spectrum disorder.Curr Opin Neurol. 2010 Apr;23(2):118-23. PMID 20087182