How do you remember?

Memory processes like formation, maintenance and consolidation have been the subjects of extensive research and, as a result, we know quite a bit about them. And just when we thought that we are getting a pretty clear picture of the memory tableau and all that is left is a little bit of dusting around the edges and getting rid of the pink elephant in the middle of the room, here comes a new player that muddies the waters again.

DNA methylation. The attaching of a methyl group (CH3) to the DNA’s cytosine by a DNA methyltransferase (Dnmt) was considered until very recently a process reserved for the immature cells in helping them meet their final fate. In other words, DNA methylation plays a role in cell differentiation by suppressing gene expression. It has other roles in X-chromosome inactivation and cancer, but it was not suspected to play a role in memory until this decade.

Oliveira (2016) gives us a nice review of the role(s) of DNA methylation in memory formation and maintenance. First, we encounter the pharmacological studies that found that injecting Dnmt inhibitors in various parts of the brain in various species disrupted memory formation or maintenance. Next, we see the genetic studies, where mice Dnmt knock-downs and knock-outs also show impaired memory formation and maintenance. Finally, knowing which genes’ transcription is essential for memory, the researcher takes us through several papers that examine the DNA de novo methylation and demethylation of these genes in response to learning events and its role in alternative splicing.

Based on these here available data, the author proposes that activity induced DNA methylation serves two roles in memory: to “on the one hand, generate a primed and more permissive epigenome state that could facilitate future transcriptional responses and on the other hand, directly regulate the expression of genes that set the strength of the neuronal network connectivity, this way altering the probability of reactivation of the same network” (p. 590).

Here you go; another morsel of actual science brought to your fingertips by yours truly.

99-dna-copy

Reference: Oliveira AM (Oct 2016, Epub 15 Sep 2016). DNA methylation: a permissive mark in memory formation and maintenance. Learning & Memory,  23(10): 587-593. PMID: 27634149, DOI: 10.1101/lm.042739.116. ARTICLE

By Neuronicus, 22 September 2016

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One parent’s gene better than the other’s

Not all people with the same bad genetic makeup that predisposes them to a particular disease go and develop that disease or, at any rate, not with the same severity and prognosis. The question is why? After all, they have the same genes…

Here comes a study that answers that very important question. Eloy et al. (2016) looked at the most common pediatric eye cancer (1 in 15,000) called retinoblastoma (Rb). In the hereditary form of this cancer, the disease occurs if the child carries mutant (i.e. bad) copies of the RB1 tumour suppressor gene located on chromosome 13 (13q14). These copies, called alleles, are inherited by the child from the mother or from the father. But some children with this genetic disadvantage do not develop Rb. They should, so why not?

The authors studied 57 families with Rb history. They took blood and tumour samples from the participants and then did a bunch of genetic tests: DNA, RNA, and methylation analyses.

They found out that when the RB1 gene is inherited from the mother, the child has only 9.7% chances of developing Rb, but when the gene is inherited from the father the child has only 67.5% chances of developing Rb.

The mechanism for this different outcomes may reside in the differential methylation of the gene. Methylation is a chemical process that suppresses the expression of a gene, meaning that less protein is produced from that gene. The maternal gene had less methylation, meaning that more protein was produced, which was able to offer some protection against the cancer. Seems counter-intuitive, you’d think less bad protein is a good thing, but there is a long and complicated explanation for that, which, in a very simplified form, posits that other events influence the function of the resultant protein.

Again, epigenetics seem to offer explanations for pesky genetic inheritance questions. Epigenetic processes, like DNA methylation, are modalities through which traits can be inherited that are not coded in the DNA itself.

RB - Copy

Reference: Eloy P, Dehainault C, Sefta M, Aerts I, Doz F, Cassoux N, Lumbroso le Rouic L, Stoppa-Lyonnet D, Radvanyi F, Millot GA, Gauthier-Villars M, & Houdayer C (29 Feb 2016). A Parent-of-Origin Effect Impacts the Phenotype in Low Penetrance Retinoblastoma Families Segregating the c.1981C>T/p.Arg661Trp Mutation of RB1. PLoS Genetics, 12(2):e1005888. eCollection 2016. PMID: 26925970, PMCID: PMC4771840, DOI: 10.1371/journal.pgen.1005888. ARTICLE | FREE FULLTEXT PDF

By Neuronicus, 24 July 2016