Changes in language pathways in tuberous sclerosis complex patients with autism
Tuberous sclerosis complex (TSC) is an autosomal-dominant neurocutaneous disease caused by loss of the TSC1 (encoding hamartin) or TSC2 (encoding tuberin) genes. Neurologic symptoms are common and varied in TSC and include epilepsy and behavioral conditions like autism spectrum disorders (ASD). Between 17 and 61% of children with TSC exhibit symptoms of ASD. The purpose of this study was to investigate a potential correlate of poor neurological outcome in TSC by assessing the integrity of brain language pathways and the relationship to ASD. 42 patients with TSC and 42 age-matched control subjects were scanned with advanced diffusion-weighted MRI. White matter language pathways were identified with a validated automatic method and analyzed for microstructural characteristics, including fractional anisotropy (FA) and mean diffusivity (MD). Well-defined white matter pathways in the brain are characterized by high FA and low MD. During normal development, brain white matter pathways increase in FA and decrease in MD. Out of 42 patients with TSC, 12 had ASD (29%). After controlling for age, TSC patients without ASD showed a small decrease in FA of the arcuate fasciculus compared to control subjects, and TSC patients with ASD had much lower FA than both control subjects and TSC patients without ASD. Similarly, while TSC patients without ASD had only a small increase in MD compared to control subjects in the arcuate fasciculus, TSC patients with ASD had much higher MD than control subjects and TSC patients without ASD. A new method for assessing the microstructure of young patients showed similar results with decreased compactness in language pathways of TSC patients with ASD. Another new method designed to better analyze regions with crossing pathways showed modifications in language pathway microstructure that correlated with ASD diagnosis in the TSC patients. Preliminary analysis of neuropsychiatric data also showed a trend toward an association of arcuate fasciculus MD with verbal IQ, although the result was not significant after multiple comparisons correction. It remains unclear why some patients with TSC develop ASD, while others have better language outcomes. Our results suggest that aberrant development of language pathways may act as a marker for poor neurological outcome in TSC patients. The impaired microstructure in language pathways of TSC patients may be responsible for the development of ASD, although prospective studies examining the development of language pathways and subsequent ASD diagnosis in this patient population remain essential. It is also possible that a primary problem with language leads to decreased use and subsequent poor development of language pathways. Early diagnosis of ASD is crucial for improving the outcomes of affected children.
