Posts Tagged ‘Conditions and Diseases’

Where's Waldo in Google Maps?
Image by Si1very via Flickr

In an earlier post on Williams Syndrome, we delved into the notion that sometimes a genetic variant can lead to enhanced function – such as certain social behaviors in the case of WS.  A mechanism that is thought to underlie this phenomenon has to do with the way in which information processing in the brain is widely distributed and that sometimes a gene variant can impact one processing pathway, while leaving another pathway intact, or even upregulated.  In the case of Williams Syndrome a relatively intact ventral stream (“what”) processing but disrupted dorsal stream (“where”) processing leads to weaker projections to the frontal cortex and amygdala which may facilitate gregarious and prosocial (a lack of fear and inhibition) behavior.  Other developmental disabilities may differentially disrupt these 2 visual information processing pathways.  For instance, developmental dyspraxia contrasts with WS as it differentially disrupts the ventral stream processing pathway.

A recent paper by Woodcock and colleagues in their article, “Dorsal and ventral stream mediated visual processing in genetic subtypes of Prader–Willi syndrome” [doi:10.1016/j.neuropsychologia.2008.09.019] ask how another developmental disability – Prader-Willi syndrome – might differentially influence the development of these information processing pathways.  PWS arises from the lack of expression (via deletion or uniparental disomy) of a cluster of paternally expressed genes in the 15q11-13 region (normally the gene on the maternally inherited chromosome is silent, or imprintedrelated post here).  By comparing PWS children to matched controls, the team reports evidence showing that PWS children who carry the deletion are slightly more impaired in a task that depends on the dorsal “where” pathway whilst some sparing or relative strength in the ventral “what” pathway.

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Kim Kardashian
Image by BiggerPictureImages.com via Flickr

Sometimes, when flipping channels late at night, its hard NOT to stop and gawk at the various spectacles on reality-trash-TV.  No self-respecting scientist would admit to being smitten by all the vanity and preening – right?  Well, back in 2002, there was a mouse whose homeobox-B8 gene was disrupted – who caused a minor media sensation in the community – for its tendency toward, “excessive grooming … not unlike that of humans suffering from the OC-spectrum disorder”.  Hunh?  A mouse not-unlike trash-TV celebs who can’t stop fixing their hair?  An interesting genetic effect to be sure.

A recent paper, “Loss of Hoxb8 alters spinal dorsal laminae and sensory responses in mice” reports a closer look at this mouse mutation and provides evidence that the excessive grooming is, instead, a consequence merely of “itch perception” which arises from disrupted development of itch specific GrpR-positive neurons in lamina I of the dorsal spinal cord“.  Indeed, when the investigators applied sub cutaneous lidocaine to the peripheral nerve endings in the groomed regions – the excessive grooming stopped.  If you are interested in the development of the peripheral nervous system, the paper is well worth a read!  If you are into the psychology of excessive grooming, the Kardashian sisters always provide a steady stream of data.

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The hydrophobic cell membrane prevents charged...
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Few genes have been studies as intensely as apolipoprotein E (APOE).  In particular, one of its variants, the epsilon-4 allele has been especially scrutinized because it is correlated with an earlier onset (about 10 years earlier than average) of Alzheimer’s Disease.  Among the many roles of APOE – its just a tiny cholesterol binding protein – are those as participant in synaptic plasticity, early neural development, damage-response and other processes – all of which share the need for the synthesis and movement of neuronal membranes (see the fluid mosaic model) and their component parts – such as cholesterol.   Hence, whenever neural membranes are being synthesized (plasticity & development) or damaged (overstimulation and other sources of oxidative damage) the tiny APOE is there to help with its membrane stabilizing cholesterol molecule in hand. Over the course of a lifetime, routine damage to neuronal membranes adds up (particularly in the hippocampus where constant storage-recall memory functions place enormous demands on synaptic plasticity systems), and individuals (such as epsilon-4 carriers) may simply show more wear-and-tear because their version of APOE is not as optimal as the other forms (epsilon-2 and -3).

apoeWith this etiological model in mind, perhaps you would like to take better care of you cell membranes (much like your car mechanic implores to change your car’s spark plugs and oil to keep the engine clean on the inside).  Moreover, perhaps you would like to do-so especially if you knew that your APOE system was less optimal than average.  Indeed, results from the recent REVEAL study suggest that folks who are in their 50’s are not unduly distressed to make this genetic inquiry and find out their genotypic status at this APOE polymorphism – even though those who discovered that they were epsion-4 carriers reported more negative feelings, understandably.  Still, with a number of education and intervention strategies available, an optimistic outlook can prevail.

Furthermore, there are ever newer diagnostic strategies that can improve the rather weak predictive power of the genetic test.  For example, cognitive assessments that measure hippocampal-dependent aspects of memory or visual orienting have been shown to be valid predictors of subsequent dementia – even moreso in populations that carry the APOE epsilon-4 allele.  Other forms of neuroimaging that directly measure the structure and function of the hippocampus also have tremendous sensitivity (here for a broad review of imaging-genetics of AD) and can, in principle, provide a more predictive view into one’s distant future.

On the very cutting edge of this imaging-genetic crystal ball technology, lies a recent paper entitled, “Distinct patterns of brain activity in young carriers of the APOE-e4 allele” by Fillippini and colleagues [doi: 10.1073/pnas.0811879106].  Here, the research team asks whether individuals in their late 20’s show structural/functional brain differences that are related to APOE genotype.  They employ various forms of imaging analysis such as a comparison of brain activity when subjects were performing a novel vs. familiar memory task and also an analysis of so-called resting state networks – which reflect a form of temporal coherence (brain areas that oscillate in-sync with each other when subjects are lying still and doing nothing in the scanner).  For the analysis of the memory task, the team found that APOEe4 carriers showed more activation in the hippocampus as well as other brain regions like the anterior midbrain and cerebellum.  When the team analysed a particular resting state network – the default mode network – they found differences in the medial temporal lobe (containing head of the hippocampus and amygdala) as well as the medial prefronal cortex.  According to the paper, none of these differences could be explained by differences in the structure or resting perfusion of the young-adult brains in the study.

Wow, these results seem to suggest that decades before any mild cognitive impairments are observable, there are already subtle differences in the physiology of the APOEe4 brain – all of which could be detected using the data obtained in 6 minutes of rest. 6 minutes of rest and spit in a cup – what does the future hold?

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