The rate of ASD in TSC varies widely in published reports from 17% to 63%.6,7,11
In our cohort of patients with TSC evaluated by a single neuropsychologist, the prevalence of ASD was 40%. We also report a lower prevalence of TSC1
mutations in those with TSC and ASD. Although we sampled from a tertiary clinic population, our findings correlate well with prior investigations examining larger groups of patients with TSC.25–27
Moreover, we feel that sampling bias has been minimized by implementing a policy at our TSC clinic to have all children and young adults with TSC undergo a formal NP evaluation to better serve our population.
We report that TSC mutations inactivating the hamartin domain of the TSC2
gene were associated with ASD in TSC. Although unlikely to be a causal relationship, mutations in these regions of the TSC genes may markedly increase risk of ASD, and in combination with mutations in other modifier genes of ASD, the phenotype can be observed. Here, our methods detected germline mutations in TSC genes in peripheral blood. The somatic, “second-hit” mutation in the remaining TSC allele may also contribute to phenotypic variability in TSC. It is possible that the timing and cellular location of the second hit mutation can have downstream effects on a developing brain, including learning, behavior, and epileptogenesis. Although the pathogenesis of some brain lesions in TSC may not be dependent on a second-hit mutation,28
further investigations interrogating the relationship between neurologic phenotype and somatic mutations in TSC pathology, including cyst-like tubers, are warranted.
Here, we found that interictal epileptiform features in the temporal lobe are associated with ASD, specifically, the left temporal lobe. This laterality has not been reported previously. Interestingly, the prevalence of interictal epileptiform features was markedly high in both groups, demonstrating the epileptogenicity of the brain in TSC. Localization of focal interictal epileptiform features in patients with TSC has been demonstrated to be stable for more than 10 years.29
Thus, although the dates of our EEG data and NP evaluations were often disparate in a given patient, our data may still accurately depict the electrographic nature of his or her brain at the time of NP evaluation.
Intriguingly, we report that timing of seizure onset and increased seizure frequency were associated with ASD. From this, it is plausible that early detection and treatment of both clinical seizures and epileptiform features in TSC could change the course of disease progress. Indeed, evidence from our group and recent evidence from a small cohort in Italy suggest that early and effective treatment may improve neurodevelopmental outcomes.30,31
Routine EEG in the first years of life may provide a useful screening technique in all patients with TSC to assess both subclinical seizures and epileptiform features. However, delineating the role that such abnormalities may have on the progression of ASD in TSC will pose substantial methodologic challenges.
Despite our use of the MRI FLAIR sequences, which have an increased sensitivity for tuber identification,23,24
we failed to find a significant relationship with tuber localization and ASD. A number of explanations exist for these findings, including methodologic flaws that may be inherent to assessment of cortical tubers by MRI in TSC. Histologic evidence suggests that brain pathology in TSC extend beyond the limits of cortical tubers visualized on MRI.32,33
Our inability to demonstrate a clear relationship between cortical tuber burden and ASD could represent our inability to accurately evaluate the widespread cortical disorganization in these patients. Indeed, a study assessing gray and white matter volumes, in conjunction with tuber burden, found that only abnormalities in volume and in not burden were predictive of memory deficits in patients with TSC.34
Thus, although cortical tuber burden evaluated by MRI may be a better marker of brain disease in TSC than cortical tuber size or number alone, it still may fall short of ascertaining the full extent of the neuropathology of TSC.19,25,35
Cyst-like cortical tubers are a neuroanatomic finding in TSC, the prognostic significance of which is under investigation.36
The mechanism by which cyst-like cortical tubers develop is unknown, as is their natural history. Here we find that cyst-like tubers are more common and more numerous and affect more brain regions in patients with TSC/ASD. Further investigations examining the relationship between cyst-like tubers and epileptogenic foci could reveal a pathophysiologic link between these lesions and ASD. Because interictal epileptiform activity has been hypothesized to alter cognition and behavior, persistent electrophysiologic abnormalities related to cyst-like tubers could contribute to the development of ASD in TSC.37,38
Epileptiform features, separate from or in concert with ictal events, could provide considerable dysfunction or “static” in the brain and contribute to the high prevalence of ASD in TSC.38,39
One cannot rule out the possibility that cyst-like tubers, EEG abnormalities, or the other risk factors for ASD identified herein are markers for more significant brain pathology in TSC, rather than the etiologic basis for ASD in TSC. Several investigations have demonstrated a more severe neurologic phenotype in patients with TSC2
including increased prevalence of cyst-like tubers.36
Here, results of our multiple regression analysis demonstrate that TSC mutation type as well as temporal lobe epileptiform activity were associated with ASD, providing evidence that these variables are independent predictors of ASD in TSC. However, this finding does not exclude that possibility that the pathogenesis of ASD in this population may result from a more global brain dysfunction.
Taken together, our data lend support for an underlying role of genetic, physiologic, and structural abnormalities in the development of ASD in TSC, although none of these can fully explain the high prevalence of ASD in this population. Further investigations aiming to elucidate the relationship between neuropathology in TSC and ASD may be served well by using mouse models of TSC that exhibit some degree of neuronal disorganization, including the Tsc1
conditional knockout mouse.40