Posted in Cerebellum, CNTNAP2, Frontal cortex, Frontal pole, Fusiform gyrus, Rostral fronto-occipital fasciculus, Thalamus, White matter, tagged 23andMe, Add new tag, autism, Autism spectrum, Brain, Development, Frontal lobe, Functional magnetic resonance imaging, Genetic testing, Genetics, Grey matter, Health, Mental disorder, Mental health, Neural development, Neurodevelopmental, synaptogenesis, White matter on March 5, 2010 |
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The A-to-T SNP rs7794745 in the CNTNAP2 gene was found to be associated with increased risk of autism (see Arking et al., 2008). Specifically, the TT genotype, found in about 15% of individuals, increases these folks’ risk by about 1.2-1.7-fold. Sure enough, when I checked my 23andMe profile, I found that I’m one of these TT risk-bearing individuals. Interesting, although not alarming since me and my kids are beyond the age where one typically worries about autism. Still, one can wonder if such a risk factor might have exerted some influence on the development of my brain?
The recent paper by Tan et al., “Normal variation in fronto-occipital circuitry and cerebellar structure with an autism-associated polymorphism of CNTNAP2” [doi:10.1016/j.neuroimage.2010.02.018 ] suggests there may be subtle, but still profound influences of the TT genotype on brain development in healthy individuals. According to the authors, “homozygotes for the risk allele showed significant reductions in grey and white matter volume and fractional anisotropy in several regions that have already been implicated in ASD, including the cerebellum, fusiform gyrus, occipital and frontal cortices. Male homozygotes for the risk alleles showed greater reductions in grey matter in the right frontal pole and in FA in the right rostral fronto-occipital fasciculus compared to their female counterparts who showed greater reductions in FA of the anterior thalamic radiation.”
The FA (fractional anisotropy – a measurement of white-matter or myelination) results are consistent with a role of CNTNAP2 in the establishment of synaptic contacts and other cell-cell contacts especially at Nodes of Ranvier – which are critical for proper function of white-matter tracts that support rapid, long-range neural transmission. Indeed, more severe mutations in CNTNAP2 have been associated with cortical dysplasia and focal epilepsy (Strauss et al., 2006).
Subtle changes perhaps influencing long-range information flow in my brain – wow!
More on CNTNAP2 … its evolutionary history and role in language development.
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Like “Joe the Plumber” (whose real name is Samuel), CNTNAP2 (whose real name is CASPR2) has achieved a bit of fame lately. While recently appearing almost everywhere (here, here, here) except FOX News, CNTNAP2 (not Joe the Plumber) is apparently a transcriptional target of the infamous FOXP2 “language gene” – so says Sonja C. Vernes & colleagues [doi: 10.1056/NEJMoa0802828] who precipitated DNA-protein complexes using anti-FOXP2 antibodies from a cell line transiently expressing FOXP2. The team later evaluated measures of expressive and receptive language abilities and nonsense-word repetition and found that a series of snps – most significantly rs17236239 – were associated with performance of children from a consortium of families at risk for language impairment. This adds to several previous reports of CNTNAP2 and risk for autism, a disorder where language ability is severely impaired.
So what’s all the fuss ? How can something so insignificant (rs17236239 not Joe the Plumber) stir up so much trouble ? Well, as reported in a previous post, the expression of CNTNAP2 in the developing superior temporal cortex may be a relevant clue since this brain region is activated by language tasks. Also, this gene encodes a rather massive protein which (as reported by Coman et al.,) seems to participate in the establishment of myelination and “nodes” that permit rapid neural transmission and long-range coordination across neural structures in the brain. Interestingly, this gene shows evidence for recent positive selection in humans (as posted on here and here) although the newly derived G-allele at rs17236239 seems to be the allele that is causing the language difficulties. My own 23andMe profile shows a middling A/G here which makes it slightly hard to recall and repeat “Samuel Wurzelbacher”.
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Posted in CNTNAP2, tagged autism, evolution on March 7, 2008 |
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Image via Wikipedia I was just browsing the recent paper “Natural selection has driven population differentiation in modern humans” by Barreiro and colleagues (doi:10.1038/ng.78) and noticed in their supplementary table that the autism risk factor CNTNAP2 (as blogged about earlier here) contains at least one non-synonymous or 5′-UTR SNP with a high Fst value. Yann Klimenidis has a great post on this paper.
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- Image by pfv. via Flickr
The acquisition of language in humans remains a complex and fascinating mystery from both a neuro- and evolutionary-biological perspective. Attempts to identify genetic regulators of neural processes that are involved in language acquisition have the potential to shed light, not only on the natural history of homo sapiens, but also, to help understand the complex neurodevelopmental disorder, Autism, often associated with profound language impairments. So, it is very exciting to read, “Genome-wide analyses of human perisylvian cerebral cortical patterning” by Abrahams et al., (DOI) who examined human gene expression in frontal vs. superior temporal cortex at a developmental period where neurogenesis and neuronal migration are particularly active. The authors went looking for differential gene expression during a critical developmental time point and in a critical brain region – since the superior temporal cortex is an area that is reliably activated by linguistic tasks as well as social cognition tasks. According to the article, a total of 345 differentially expressed genes were identified, with 61 enriched and 284 down-regulated in superior temporal cortex across two microarray platforms, with 13 genes identified by both microarray array platforms. One of the genes identified is LDB1, a regulator of the asymmetrically expressed LIM domain-only 4 (LMO4) a known mediator of calcium-dependent transcription in cortical neurons and known to regulate thalamocortical connectivity. Another gene, CNTNAP2, a member of the neurexin transmembrane superfamily of proteins that mediate cellular interactions in the nervous system has been previously associated with autism. Both of these genes seem to have important developmental roles and should provide access to the fine-scale wiring that occurs during the development of neural networks involved in language.
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