Clinical genetic microarray testing; ASD neuropathology


▪ Clinical Genetic Microarray Testing [Shen et al., 2010]

The American Academy of Pediatrics provided a very useful guide to autism screening and evaluation [Johnson & Myers, 2007]. This was the first professional guideline to recommend genetic testing in ASD on a routine basis. Fragile X testing and G-banded karyotype was recommended. Subsequently, Marshall et al. [2008] reported a relatively large sample in which both G-banded karyotype and single-nucleotide polymorphism microarray testing were done in comparison to 1,652 controls. In that study it was reported that the microarray test was more sensitive than the G-banded karyotype and that the findings that were missed by microarray were a relatively small number of balanced translocations. This study largely replicates previous study and is important in the context of solidifying the role of clinical genetic microarray testing in ASD.

This study further clarifies that there are situations in which genetic microarray for deletion or duplication of genomic material is not only more sensitive but that its better resolution can clarify the benign nature of some karyotypic abnormalities. As an example, although maternal duplications of the 15q11-q13 region are frequently associated with ASD and/or other developmental disorders, a polymorphism of a more proximal region of 15q11 can lead to an erroneous karyotypic finding of 15q11-q13 duplication. A case in this collection demonstrated a false-positive 15q11-q13 karyotypic finding that was corrected by the microarray result.

The challenge is that so many of the findings are rare, in this use of the term not recurring in other cases of up to 900 other patients with ASD. This decreases the certainty of attributing causality to the variant. This emphasizes the need for larger samples of cases and controls to allow better predictive value of many of the rare chromosomal variants being identified. It also emphasizes the role of clinical geneticists and genetic counselors in helping interpret the results of genetic tests in this rapidly evolving field.

▪ ASD Neuropathology [Wegiel et al., online in advance of print]

The first neuropathology studies in autism were published in the 1980s. Although the number of brains from individuals who had ASD is still relatively small, this study represents the largest systematic case-controlled study of autism neuropathology. The authors used brains from several brain banks. They started with 20 brains from those who had ASD and 18 brains from controls. Two cases in the ASD group were dropped because post-mortem administration of the ADI-R to the parents did not lead to ADI-R classification of autism. Five brains from individuals with ASD and four brains from controls were removed because of pathology due to hypoxic encephalopathy related to the cause of death or because of post-mortem deterioration of the condition of the brains. One brain from an individual with ASD was excluded because of multiple microinfarcts. After these exclusions, 13 ASD and 14 age-matched control brains were available for inclusion in the study.

After at least 3 weeks of formalin fixation, each brain hemisphere was scanned by structural MRI with 1.5 mm coronal slices. Subsequently, serial 200-µm thick coronal sections were prepared as celloidin blocks in cases and controls. They were then stained with cresyl violet. Blind to diagnosis, one neuropathologist examined over 100 sections per case with a 1.2 mm distance between examined sections. All slides for which pathology was detected in this first screening were examined by the first and senior authors.

In total, 12 of 13 (92%) of brains from those with ASD had abnormalities related to neurogenesis, neuronal migration, and/or dysplasia (abnormal microstructure related to neuronal migration). There was heterogeneity of the nature and location of the changes. As seen in previous studies of the neuropathology of ASD, the cerebellum was the most commonly affected region (7 of 12 with cerebellar dysplasia in the ASD group and in one control).

Two cases had striking changes in neurogenesis that led to the presence of tubers (nodules) of dysplastic neurons. Notably, these did not have the characteristic pathology of tuberous sclerosis (tuberous sclerosis is rare in ASD, but ASD is common in tuberous sclerosis). Specifically, giant neurons and ballooned cells characteristic of tuberous sclerosis were not present in the tubers found in the two subjects in this study. In these two cases, there was evidence of these nodules on structural MRI scans.

Heterotopias refer to the presence of neurons in a location that represents the failure of the neurons to migrate to their expected location. Heterotopias have been previously reported in study of the neuropathology of ASD [Bailey et al., 1998]. They are interesting in relationship to the finding of rare variants in genes coding for proteins involved in cell–cell communication in the brain related to migration. For example, evidence for both rare and common susceptibility variants in the gene, contactin associated binding protein 2 (CNTNAP2), in ASD is interesting because of the finding of neuronal migration abnormalities and dysplasia seen with CNTNAP2 mutations [Strauss et al., 2006].

One question for consideration is whether the neuropathological changes represented here and in other studies are more symptomatic of a primary signaling abnormality in ASD that interferes with learning mechanisms in a more consistent manner than the neuropathological changes, which may be more feasibly studied given limitations in ante-mortem study of synaptic plasticity with sufficient temporal and spatial resolution.