**5.1 Fragile X syndrome (FMR1 gene)**

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability, and ~30% of FXS patients are also diagnosed with ASD. FXS is considered also as the most common genetic cause of ASD, representing around 3% of ASD individuals. FXS arises from loss of function mutations in the X-linked FMR1 gene, which result in either total absence or functional inactivation of the encoded protein (FMRP), an mRNA-binding protein involved in translational regulation [65]. Although FXS affects many brain regions, cerebellar PC loss and cell displacement, as seen in idiopathic autism, have been reported in human postmortem studies of FXS [66]. Interestingly, imaging studies have identified specific abnormalities in the posterior cerebellar vermis (lobules VI–VII) in FXS patients with ASD that are not seen in FXS patients without comorbid ASD diagnosis. Further, these


**Table 1.** *Cerebellar abnormalities in mouse models of ASD.*

lobules are also abnormal in non-syndromic ASD [57]. Moreover, positive correlations between the size of the posterior vermis and several subscales of the autism behavior checklist, a list of nonadaptive behaviors that represent an individual's challenges to respond appropriately to daily life situations, in persons with FXS, have been reported [58]. On a different note, recent postmortem work has shown reductions in FMRP in cerebella and frontal cortices of subjects with autism who do not carry a mutation for FXS [59].

The Fmr1 knockout (KO) mouse is a validated and widely studied animal model of ASD. By means of high-resolution MRI imaging to study brain structure, the cerebellum in Fmr1 KO mice was found to show significant volume alterations comparing with wild-type controls. Specifically, the deep cerebellar nuclei, which transfer the output of the cerebellar cortex to the thalamus and cerebral cortex, were smaller in Fmr1 mice. Moreover, this reduced volume was accompanied by loss of neurons and increase in astrocytes, as measured by immunohistochemical techniques [60]. Later, the same authors also identified a volume reduction in the cerebellar cortex in these mice [67]. Further, a detailed study by diffusion tensor imaging of postnatal development in the Fmr1 KO mouse showed reduced cerebellar volume in the first few weeks after birth [68]. In addition to the full KO, a conditional Fmr1 KO mouse (Fmr1 cKO) has also been generated. Deletion of Fmr1 specifically in PC leads to several structural and functional abnormalities in the cerebellum also seen in the full Fmr1 KO, such as a reduction of cerebellar volume, cellular loss in the cerebellar nuclei, and longer spines in PC. Functionally, an enhanced LTD induction at the parallel fiber synapses that innervate these spines is seen [63]. In addition, both Fmr1 KO and PC-Fmr1 cKO mice present alterations in classical delay eyeblink conditioning, a Pavlovian associative learning where subjects learn to execute an appropriately timed eyeblink in response to a previously neutral conditioning stimulus in which the cerebellum plays a key role. Specifically, the percentage of conditioned responses and their peak amplitude and peak velocity were reduced. [63]. Interestingly, FXS patients were found to display the same cerebellar deficits in eyeblink conditioning as mutant mice [63]. Together these results suggest that the deficits in eyeblink conditioning are likely due to the loss of FMRP in PC.
