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Posts Tagged ‘cognitive development’

Just a pointer to onetime University of Edinburgh Professor C.H. Waddington’s 1972 Gifford Lecture on framing the genes vs. environment debate of human behavior.  Although Waddington is famous for his work on population genetics and evolutionary change over time, several of his concepts are experiencing some resurgence in the neuroimaging and psychological development literatures these days.

One term, CHREOD, combines the Greek word for “determined” or “necessary” and the word for “pathway.” It describes a system that returns to a steady trajectory in contrast to homeostasis which describes a system which returns to a steady state.  Recent reviews on the development of brain structure have suggested that the “trajectory” (the actual term “chreod” hasn’t survived) as opposed to any specific time point is the essential phenotype to use for understanding how genes relate to psychological development.  Another term, CANALIZATION, refers to the ability of a population to produce the same phenotype regardless of variability in its environment or genotype.  A recent neonatal twin study found that the heritability of grey matter in neonatal humans was rather low.  However it seems to then rise until young adulthood – as genetic programs presumably kick-in – and then decline again.  Articles by neurobiologist Jay N. Giedd and colleagues have suggested that this may reflect Waddington’s idea of canalization.  The relative influence of genes vs. environment may change over time in ways that perhaps buffer against mutations and/or environmental insults to ensure the stability and robustness of functions and processes that are both appropriate for survival and necessary for future development.  Another Waddington term, EPIGENETIC LANDSCAPE, refers to the limitations on how much influence genes and environment can have on the development of a given cell or structure.  Certainly the environment can alter the differentiation, migration, connectivity, etc. of neurons by only so much.  Likewise, most genetic mutations have effects that are constrained or compensated for by the larger system as well.

Its amazing to me how well these evolutionary genetic concepts capture the issues at the nexus of of genetics and cognitive development.  From his lecture, it is clear that Waddington was not unaware of this.  Amazing to see a conceptual roadmap laid out so long ago.  Digging the book cover art as well!

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Some quick sketches that might help put the fast-growing epigenetics and cognitive development literature into context.  Visit the University of Utah’s Epigenetics training site for more background!

The genome is just the A,G,T,C bases that encode proteins and other mRNA molecules.  The “epi”genome are various modification to the DNA – such as methylation (at C residues) – and acetylation of histone proteins.   These changes help the DNA form various secondary and tertiary structures that can facilitate or block the interaction of DNA with the transcriptional machinery.

When DNA is highly methylated, it generally is less accessible for transcription and hence gene expression is reduced.  When histone proteins (purple blobs that help DNA coil into a compact shape) are acetylated, the DNA is much more accessible and gene expression goes up.

We know that proper epigenetic regulation is critical for cognitive development because mutations in MeCP2 – a protein that binds to methylated C residues – leads to Rett syndrome.  MeCP2 is normally responsible for binding to methylated DNA and recruiting histone de-acetylases (HDACs) to help DNA coil and condense into a closed form that is inaccessible for gene expression (related post here).

When DNA is accessible for gene expression, then it appears that – during brain development – there are relatively more synaptic spines produced (related post here).  Is this a good thing? Rett syndrome would suggest that – NO – too many synaptic spines and too much excitatory activity during brain development may not be optimal.  Neither is too little excitatory (too much inhibitory) activity and too few synaptic spines.  It is likely that you need just the right balance (related post here). Some have argued (here) that autism & schizophrenia are consequences of too many & too few synapses during development.

The sketch above illustrates a theoretical conjecture – not a scenario that has been verified by extensive scientific study. It tries to explain why epigenetic effects can, in practice, be difficult to disentangle from true (changes in the A,G,T,C sequence) genetic effects.  This is because – for one reason – a mother’s experience (extreme stress, malnutrition, chemical toxins) can – based on some evidence – exert an effect on the methylation of her child’s genome.  Keep in mind, that methylation is normal and widespread throughout the genome during development.  However, in this scenario, if the daughter’s behavior or physiology were to be influenced by such methylation, then she could, in theory, when reaching reproductive age, expose her developing child to an environment that leads to altered methylation (shown here of the grandaughter’s genome).  Thus, an epigenetic change would look much like there is a genetic variant being passed from one generation to the next, but such a genetic variant need not exist (related post here, here) – as its an epigenetic phenomenon.  Genes such as BDNF have been the focus of many genetic/epigenetic studies (here, here) – however, much, much more work remains to determine and understand just how much stress/malnutrition/toxin exposure is enough to cause such multi-generational effects.  Disentangling the interaction of genetics with the environment (and its influence on the epigenome) is a complex task, and it is very difficult to prove the conjecture/model above, so be sure to read the literature and popular press on these topics carefully.

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