The challenges in identifying biomarkers in neurodevelopmental disorders, particularly in autism, stem from the difficulties researchers have in considering the inter-individual phenotypic variability. It ranges from those with a developmental trajectory in the normal range (although carrying environmental and/or genetic vulnerability risk factors) to individuals with mild to severe impairment. The purpose of our study was, therefore, to account for this inter-individual variability by exploring the spectrum of RRBI assessed in a large population of individuals with autism, their unaffected first-degree relatives, and controls from the general population. Interestingly, our results underlined the potential implication of the putamen in the RRBI: an increase in the volume of the putamen was associated with 'low-order' symptoms (mainly stereotyped behaviors) and on the opposite, a decrease in the putamen volume with cognitively driven symptoms - (mainly obsessive-compulsive symptoms).
Dimensionality of the RRBI symptoms
The factor analysis revealed the multidimensionality of RRBI by identifying three dimensions: FA1 gathered the RBS-R-related items typically observed in autism (Jiujias, Kelley & Hall, 2017, Chaxiong, Dimian & Wolff, 2022); FA2 put together the items related to rigidity/insistence on sameness from the Y-BOCS, symptoms which were trait features of the individuals with autism but also those with obsessive-compulsive symptoms (Uljarevic et al., 2017; Zandt, Prior & Kyrios, 2007); and FA3 that included only Y-BOCS related items usually displayed by individuals with OCD (Delorme et al., 2006). Interestingly, FA2 put together the symmetry and ordering of obsessions/compulsions and the need to repeat things. These symptoms were described in the literature as settled clinical characteristics of autistic children with comorbid OCD (Ruzzano et al., 2015). Symmetry & ordering symptoms are frequently reported in the developmental subtype of OCD, which - beyond its juvenile onset - is characterized by an increased proportion of neurodevelopmental comorbidities, including Tourette's syndrome, ADHD, and autism, but also by significant impairment in executive functions (Darrow et al., 2017; Grassi et al., 2021; Bragdon et al., 2018). A few studies have highlighted the link between the intensity of symmetry/ordering symptoms and poor verbal working memory, visuospatial planning, inhibitory control, and cognitive flexibility abilities (Bragdon et al., 2018). Thus, among the characteristics shared by children with autism and OCD, executive dysfunction may play a critical role in these disorders (Boyd et al., 2009; Iversen & Lewis, 2021). Interestingly, unaffected first-degree parents of children with OCD or autism also showed executive impairments (Delorme et al., 2007; Seng et al., 2021; Mc Lean et al., 2014). Overall, FA2 dimension we identified may be a testimony of the cumulative impact of executive impairment and the presence of RRBI. Finally, the FA3 dimension was driven by OCD-related symptoms since gathering the washing, checking, contamination, and aggressive symptoms (Ruzzano et al. 2015; Lamothe et al., 2022). This dimension was probably less related to autism but more a proxy of obsessive-compulsive symptoms (Lewin et al., 2011; McDougle et al., 1995).
Correlation between structural brain volumes and RRBI-related dimensions
Our results were in line with publications stressing the critical role of the cortico-thalamic-striatal-cortical loop in RRBI. Our study revealed a central implication of the putamen across the 3 distinct dimensions we reported, as well as specific roles of subcortical and cortical structures per FA, which may shape the diversity of symptoms, agglomerated on the dimensions. The positive association of the putamen and the left amygdala with FA1 was consistent with the role of the putamen in autonomic movements, described in complex motor stereotypies (Mahone et al., 2016) and in autism (Langhen et al., 2014, van Rooij et al., 2018, Mc Kinnon et al., 2019, Ferhat et al., 2023). Including the putamen, the basal ganglia play important roles in regulating repetitive behaviors (notably in autism) in association with the hippocampus, the hypothalamus, but also other neuroanatomical structures of the limbic system (including the amygdala) (Gandhi et al., 2021). Obviously, amygdala volume abnormalities were more usually related to anxiety, and more specifically to social anxiety in autism (Bellani et al., 2013, Gandhi et al., 2021, Jayakar et al, 2020).
In contrast to the positive association of the putamen volume with FA1, we observed a negative relationship with FA2 and FA3. Interestingly, our results replicated those of the ENIGMA-OCD consortium, which also reported a reduced putamen volume in OCD (Tang et al., 2016, Szeko et al., 2008). This reduced putamen volume may relate more to the compulsive component than the obsessive one in OCD (Bowen et al., 2021). FA2 was associated in addition to the decreased volume of the parietal cortex. Similar parietal abnormalities in cortical thickness, volume or surface area, may reflect the cortical dysmaturation in this area, frequently reported in children with OCD and autism (Boedhoe et al, 2018, Rus et al., 2017, Rooij et al., 2018). Interestingly, the parietal cortex is involved in social interactions, motor learning and repetitive behaviors in autism (Travers et al, 2014) but also participates in cognitive in/flexibility, which is coherent with the sameness nature of RRBI-related symptoms encompassed by FA2 (Gruner et al. 2017).
Limitations
The lack of power of the brain imaging part of our study did not allow us to explore the brain asymmetry structures associated with the dimensions we reported. We, however, observed a trend for a leftward asymmetry (based on p-value and coefficients L > R). A similar leftward brain asymmetry involving the putamen was previously reported in autism and OCD (Posterma et al., 2019; Rus et al., 2017, Kong et al., 2020; Boedhoe et al., 2017; Van den Heuvel et al., 2022). The role and impact of this brain asymmetry on the symptomatology of ASD and OCD remained unclear but may result from aberrant brain development trajectories.
When exploring the dimensions gathering the RRBI phenotypic variability, we did not include the effect of potential covariates, such as the whole pattern of comorbidities or the effect of executive dysfunctions, as mentioned above. For example, planning strategy impairments consistently reported in probands, and their first-degree relatives may specifically participate in FA2 (Bora, 2020).
Finally, one additional limitation may result from the wide range of ages of individuals enrolled in our study. This might have biased the volume estimates of the small brain structures, specifically subcortical structures such as the putamen or the amygdala, since it relied on a limited number of voxels (Herten et al., 2019; Lidauer et al., 2022; Schoemaker et al., 2016; Mulder et al., 2014). This effect may result from the opposite coefficient direction of the regression analysis we reported between FA1 and the neuroanatomical structures (positive regression), and between FA2 or FA3 (negative regression) and these. FA1 more reflected the autistic-related symptoms and thus was mainly based on symptoms displayed by probands with autism, younger than relatives and controls enrolled in our study