**5.2 Tuberous sclerosis (TSC1/TSC2 genes)**

Tuberous sclerosis (TS) is another rare syndromic disorder associated with ASD. TS is characterized by the development of non-cancerous tumors in the brain and other organs leading to neurological symptoms such as developmental delay, epilepsy, and ASD. It is an autosomal dominant condition caused by mutations in either the TSC1 or TSC2 genes, which form a tumor suppressor complex involved in the regulation of the mTOR signaling pathway. Approximately 40% of TS patients are co-diagnosed with ASD, and, interestingly, those with cerebellar lesions have been found to have a more severe ASD diagnosis [61]. Further, PC loss has been found in postmortem cerebellum samples from TSC patients [62].

Several lines of mutant TSC mice have been generated and studied in detail. A mutant mouse in which the Tsc2 gene was selectively deleted from PCs starting at postnatal day 6 was generated to mimic patients with one nonfunctioning TSC2 allele [62]. The haploinsufficiency of TSC2 caused a progressive increase in PC cell size and subsequent death from apoptosis. TSC2-null PCs showed increased endoplasmic reticulum and oxidative stress, which were rescued by treatment with the mTOR inhibitor rapamycin. In a subsequent study, the authors reported that PC-TSC2-haploinsufficient mice showed social deficits and repetitive behaviors [64]. These observations indicate that selective loss of TSC2 in PCs in a TSC2 haploinsufficient background is enough to lead to autistic-like behavioral deficits.

*The Cerebellum and Autism: More than Motor Control DOI: http://dx.doi.org/10.5772/intechopen.85897*

A conditional PC-TSC1 KO mouse has also been generated. Both heterozygous and homozygous losses of TSC1 in mouse cerebellar PCs result in autistic-like behaviors, including abnormal social interaction and repetitive behavior and vocalizations, in addition to decreased PC excitability. Similar to TSC2 mutants, treatment of TSC1 mutant mice with the mTOR inhibitor, rapamycin, prevented the pathological and behavioral deficits. Strikingly, PC-TSC1 homozygous mice, but not PC-TSC1 heterozygous mice, showed PC loss at 2 months of age. The fact that PC-TSC1 heterozygous mice showed autistic symptoms and PC excitability alterations in the absence of PC loss suggests that the decrease in PC excitability is likely driving the phenotype [69]. Further, this model has also been reported to show deficits in eyeblink conditioning; they specifically show lower percentage of conditioned response in this test [70]. These findings demonstrate new roles for TSC1/TSC2 in PC function and define a molecular basis for a cerebellar contribution to cognitive disorders such as autism.

### **5.3 Phelan-McDermid syndrome (SHANK3 gene)**

Phelan-McDermid syndrome (PMS) is due to heterozygous chromosome 22q13 deletions and is often co-diagnosed with ASD. The clinical manifestations of PMS include global developmental delay/intellectual disability and absent or delayed speech [71]. Although the deletion encompasses numerous genes, a good candidate that could account for ASD symptoms is SHANK3, a gene within which mutations have independently been associated with non-syndromic ASD. ASD patients with SHANK3 deletions are also known to have severe core symptoms and mental disabilities [72]. Although to our knowledge there is no data addressing the effect of SHANK3 mutations on cerebellar anatomy and function, recent research suggests that mutations in *SHANK3* may be related to cerebellar abnormalities. SHANK3 is highly expressed in cerebellar granule cells [73] and has been suggested to play a role in the recruitment of axon terminals to cerebellar granule cell dendrites [74]. In addition, cerebellar vermis hypoplasia has been found in patients with PMS, suggesting that SHANK3 may be involved in cerebellar development [75].

Multiple mouse lines with SHANK3 mutations exist, and several display behaviors analogous to the core symptoms of autism, including isoform-specific SHANK3B KO [76], SHANK3 (∆exons4–9) deleting major isoforms of the gene [77], and SHANK3 (∆C), deleting the C-terminal region of the gene [78]. SHANK3 (∆C) mice present a decreased density of PC compared to controls [70], and they show deficits in the eyeblink conditioning task, showing lower percentage of conditioning response and a delay in the response latency [70].

Interestingly, mutations in other proteins from the SHANK family, such as SHANK2, have been also linked to ASD. A KO mouse for SHANK2 shows alterations in social and repetitive behaviors and presents changes in PC electrophysiological characteristics, such as decreased intrinsic PC plasticity, synaptic strength at the PC-parallel fiber synapse, and enhanced inhibitory input into PC. Further PC-specific SHANK2 KO replicated these findings, arguing for a cerebellar role in autistic-like behaviors [79].

## **6. Conclusion**

The cerebellum has been recently indicated as a key structure not only for sensorimotor control but also for language, social cognition, and emotion, via its extensive connections with cortical areas. In the present work, we aimed to provide an up-to-date overview of current findings on cerebellar involvement

in the pathophysiology of ASD. Anatomical studies report cerebellar abnormalities in postmortem brain tissue from autistic individuals, neuroimaging studies indicate abnormal cerebellar activation when performing social paradigms, and animal models of monogenic forms of autism converge on the cerebellum as one of the common sites of abnormalities. The cerebellum represents an emerging field of interest for ASD research, based on the hypothesis that ASD is a connectivity disorder and cerebellar dysfunction could impact other brain areas within the social network, leading to the core ASD symptoms. Although the literature in this new field is at a very early stage, based on the presented data, future studies should not exclude the cerebellum in analyses of structural and functional differences in ASD.
