Significant neuronal soma volume deficit in the limbic system in subjects with 15q11.2-q13 duplications

Abstract

Introduction: Autism is diagnosed in numerous genetic and genomic developmental disorders associated with an overlap in high-risk genes and loci that underlie intellectual disability (ID) and epilepsy. The aim of this stereological study of neuronal soma volume in 25 brain structures and their subdivisions in eight individuals 9 to 26 years of age who were diagnosed with chromosome 15q11.2-13.1 duplication syndrome [dup(15)], autism, ID and epilepsy; eight age-matched subjects diagnosed with autism of unknown etiology (idiopathic autism) and seven control individuals was to establish whether defects of neuronal soma growth are a common denominator of developmental pathology in idiopathic and syndromic autism and how genetic modifications alter the trajectory of neuronal soma growth in dup(15) autism.

Results: Application of the Nucleator software to estimate neuronal size revealed significant neuronal soma volume deficits in 11 of 25 structures and their subregions (44 %) in subjects diagnosed with dup(15) autism, including consistent neuronal soma volume deficits in the limbic system (sectors CA2, 3 and 4 in Ammon's horn, the second and third layers of the entorhinal cortex and in the amygdala), as well as in the thalamus, nucleus accumbens, external globus pallidus, and Ch3 nucleus in the magnocellular basal complex, and in the inferior olive in the brainstem. The second feature distinguishing dup(15) autism was persistent neuronal soma deficits in adolescents and young adults, whereas in idiopathic autism, neuronal volume deficit is most prominent in 4- to 8-year-old children but affects only a few brain regions in older subjects.

Conclusions: This study demonstrates that alterations in the trajectory of neuronal growth throughout the lifespan are a core pathological features of idiopathic and syndromic autism. However, dup(15) causes persistent neuronal volume deficits in adolescence and adulthood, with prominent neuronal growth deficits in all major compartments of the limbic system. The more severe neuronal nuclear and cytoplasic volume deficits in syndromic autism found in this study and the more severe focal developmental defects in the limbic system in dup(15) previously reported in this cohort may contribute to the high prevalence of early onset intractable epilepsy and sudden unexpected death in epilepsy.

Fig. 1

Neuronal size in control, dup(15) autism and idiopathic autism. Panel of micrographs demonstrates smaller neurons in the nucleus accumbens, amygdala (lateral nucleus), thalamus (dorsal medial nucleus), CA3 sector in the Ammon’s horn, the second layer of the entorhinal cortex (islands of stellate neurons; ECIsl) and the external globus pallidus in a 24-year-old male diagnosed with dup(15), compared to a 32-year-old control male and a 23-year-old male diagnosed with idiopathic autism (IAutism). Stereology revealed significant (from 17 % to 23 %) neuronal soma volume deficit in these structures in dup(15) autism. Staining with cresyl violet. Calibration bar, 20 μm

Fig. 2

Neuronal soma volume deficit in dup(15) autism. Comparison of neuronal soma volume between dup(15) autism and control subjects revealed a significant neuronal soma volume deficit in affected subjects in 11 brain structures or their subdivisions (marked with arrows) per 25 examined (44 %), but a trend to lower cell soma volume is reflected in the insignificantly lower soma volume in another 11 brain structures in dup(15) autism cases from 9 to 26 years of age

Fig. 3

Neuronal soma volume frequency distribution. Comparison of neuronal soma frequency distribution in dup(15) autism group (9–26 years old), idiopathic autism (IA) group (8–39 years old) and control group (8–33 years old). Graphs arranged in order from the smallest to the largest neurons demonstrate a significant (asterisks) shift towards small neurons in the nucleus accumbens (Acc), amygdala (Amy), thalamus (Th), CA3 and CA4 sectors in the Ammon’s horn, second layer in the entorhinal cortex (EC2) and the globus pallidus external (GPEx) in the dup(15) group (segment-line) in comparison to age-matched control subjects (continuous line). However, at this age, the difference in neuronal body volume distribution in these structures is not significant when subjects with idiopathic autism (IA) (dot-line) are compared to control subjects (C). The shift to smaller neurons was significant in the dup(15)/A cohort when compared with IA in the amygdala, CA3 and CA4 sectors, and the globus pallidus external. A similar shift in the thalamus did not reach significance level but is consistent with the trend observed in other structures

Fig. 4

Neuronal soma volume distributions. Box plots illustrate neuronal body volume distributions in 10 structures with significantly smaller neurons (marked with a star) in 8- to 26-year-old autistic individuals with dup(15) compared to age-matched control subjects. Purkinje cells and neurons in the dentate nucleus were also smaller, but the difference did not reach the level of significance. Volume measures on the Y axis correspond to x1000 of μm³. The upper and lower boundaries of each box represent the interquartile range (IQR), whereas the whisker above and below each box marks the maximum and minimum values unless any data point lies more than 1.5 times of the IQR above the 75^th percentile or 1.5 times below the 25^th percentile. The characteristic features of neuron volume distribution in the dup(15) autism group in all 12 structures were lower median values (marked with horizontal line within box), lower range of 50 % of data (box range), and fewer large neurons (much lower maximum values marked by whiskers above the box). A few neurons in range above the 75^th percentile are indicated by circles; however, as a result of much lower maximum values outliers occur more often in dup(15) autism