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Posts Tagged ‘Twin’

We hope, that you choke, that you choke.
Image by Corrie… via Flickr

Coping with fear and anxiety is difficult.  At times when one’s life, livelihood or loved one’s are threatened, we naturally hightenen our senses and allocate our emotional and physical resources for conflict.  At times, when all is well, and resources, relationships and relaxation time are plentiful, we should unwind and and enjoy the moment.  But most of us don’t.  Our prized cognitive abilities to remember, relive and ruminate on the bad stuff out there are just too well developed – and we suffer – some more than others  (see Robert Saplosky’s book “Why Zebras Don’t Get Ulcers” and related video lecture (hint – they don’t get ulcers because they don’t have the cognitive ability to ruminate on past events).  Such may be the flip side to our (homo sapiens) super-duper cognitive abilities.

Nevertheless, we try to understand our fears and axieties and understand their bio-social-psychological bases. A recent paper entitled, “A Genetically Informed Study of the Association Between Childhood Separation Anxiety, Sensitivity to CO2, Panic Disorder, and the Effect of Childhood Parental Loss” by Battaglia et al. [Arch Gen Psychiatry. 2009;66(1):64-71] brought to mind many of the complexities in beginning to understand the way in which some individuals come to suffer more emotional anguish than others.  The research team addressed a set of emotional difficulties that have been categorized by psychiatrists as “panic disorder” and involving sudden attacks of fear, sweating, racing heart, shortness of breath, etc. which can begin to occur in early adulthood.

Right off the bat, it seems that one of the difficulties in understanding such an emotional state(s) are the conventions (important for $$ billing purposes) used to describe the relationship between “healthy” and “illness” or “disorder”.  I mean, honestly, who hasn’t experienced what could be described as a mild panic disorder once or twice?  I have, but perhaps that doesn’t amount to a disorder.  A good read on the conflation of normal stress responses and disordered mental states is “Transforming Normality into Pathology: The DSM and the Outcomes of Stressful Social Arrangements” by Allan V. Horwitz.

Another difficulty in understanding how and why someone might experience such a condition has to do with the complexities of their childhood experience (not to mention genes). Child development and mental health are inextrictably related, yet, the relationship is hard to understand.  Certainly, the function of the adult brain is the product of countless developmental unfoldings that build upon one another, and certainly there is ample evidence that when healthy development is disrupted in a social or physical way, the consequences can be very unfortunate and long-lasting. Yet, our ability to make sense of how and why an individual is having mental and/or emotional difficulty is limited.  Its a complex, interactive and emergent set of processes.

What I liked about the Battaglia et al., article was the way in which they acknowledged all of these complexities and – using a multivariate twin study design – tried to objectively measure the effects of genes and environment (early and late) as well as candidate biological pathways (sensitivity to carbon dioxide).  The team gathered 346 twin pairs (equal mix of MZ and DZ) and assessed aspects of early and late emotional life as well as the sensitivity to the inhalation of 35% CO2 (kind of feels like suffocating and is known to activate fear circuitry perhaps via the ASC1a gene).   The basic notion was to parcel out the correlations between early emotional distress and adult emotional distress as well as with a very specific physiological response (fear illicited by breathing CO2).  If there were no correlation or covariation between early and late distress (or the physiological response) then perhaps these processes are not underlain by any common mechanism.

However, the team found that there was covariation between early life emotion (criteria for separation anxiety disorder) and adult emotion (panic disorder) as well as the physiological/fear response illicited by CO2.  Indeed there seems to be a common, or continuous, set of processes whose disruption early in development can manifest as emotional difficulty later in development.  Furthermore, the team suggests that the underlying unifying or core process is heavily regulated by a set of additive genetic factors.  Lastly, the team finds that the experience of parental loss in childhood increased (but not via an interaction with genetic variation) the strength of the covariation between early emotion, late emotion and CO2 reactivity.  The authors note several limitations and cautions to over-interpreting these data – which are from the largest such study of its kind to date.

For individuals who are tangled in persistent ruminations and emotional difficulties, I don’t know if these findings help.  They seem to bear out some of the cold, cruel logic of life and evolution – that our fear systems are great at keeping us alive when we’ve had adverse experience in childhood, but not necessarily happy.  On the other hand, the covariation is weak, so there is no such destiny in life, even when dealt unfortunate early experience AND genetic risk.  I hope that learning about the science might help folks cope with such cases of emotional distress.

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Last year I dug a bit into the area of epigenetics (indexed here) and learned that the methylation (CH3) and acetylation (OCCH3) of genomic DNA & histones, respectively, can have dramatic effects on the structure of DNA and its accessibility to transcription factors – and hence – gene expression.  Many of the papers I covered suggested that the environment can influence the degree to which these so-called “epigenetic marks” are covalently bonded onto the genome during early development.  Thus, the thinking goes, the early environment can modulate gene expression in ways that are long-lasting – even transgenerational.  The idea is a powerful one to be sure.  And a scary one as well, as parents who read this literature, may fret that their children (and grandchildren) can be epigenetically scarred by early nutritional, physical and/or psycho-social stress.  I must admit that, as a parent of young children myself, I began to wonder if I might be negatively influencing the epigenome of my children.

I’m wondering how much physical and/or social stress is enough to cause changes in the epigenome?  Does the concern about epigenetics only apply to exposure to severe stress?  or run of the mill forms of stress?  How much do we know about this?

This year, I hope to explore this line of inquiry further.  For starters, I came across a fantastic paper by Fraga et al., entitled, “Epigenetic differences arise during the lifetime of monozygotic twins” [doi:10.1073/pnas.0500398102].   The group carries out a remarkably straightforward and time honored approach – a twin study – to ask how much identical twins differ at the epigenetic level.  Since identical twins have the same genome sequence, any differences in their physiology, behavior etc. are, strictly speaking, due to the way in which the environment (from the uterus to adulthood) shapes their development.  Hence, the team of Fraga et al., can compare the amount and location of methyl (CH3) and acetyl (OCCH3) groups to see whether the environment has differentially shaped the epigenome.

An analysis of some 40 identical twin pairs from ages 3-74 years old showed that – YES – the environment, over time, does seem to shape the epigenome (in this case of lymphocytes).  The most compelling evidence for me was seen in Figure 4 where the team used a method known as Restriction Landmark Genomic Scanning (RLGS) to compare patterns of methylation in a genome-wide manner.  Using this analysis, the team found that older twin pairs had about 2.5 times as many differences as did the epigenomes of the youngest twin pairs.  These methylation differences also correlated with gene expression differences (older pairs also had more gene expression differences) and they found that the individual who showed the lowest levels of methylation also had the highest levels of gene expression.  Furthermore, the team finds that twin pairs who lived apart and had more differences in life history were more likely to have epigenetic differences.  Finally, measures of histone acetylation seemed consistent with the gradient of epigenetic change over time and life-history distance.

Thus it seems that, as everyday life progresses, the epigenome changes too.  So, perhaps, one does not need extreme forms of stress to leave long-lasting epigenetic marks on the genome?  Is this true during early life (where the team did not see many differences between pairs)?  and in the brain (the team focused mainly on lymphocytes)?  Are the differences between twins due to the creation of new environmentally-mediated marks or the faulty passage of existing marks from dividing cell-to-cell over time?  Will be fun to seek out information on this.

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Surgeon holding scalpel.
Image by bethd821 via Flickr

Whether you are a carpenter, plumber, mechanic, electrician, surgeon or chef, your livelihood depends on a set of sturdy, reliable, well-honed, precision tools.  Similarly, neuroscientists depend on their electrodes, brain scanners, microscopes and more recently their genome sequencers.  This is because they are not just trying to dissect the brain – the physical organ – but also the psychological one.  As the billions of neurons connected by trillions of synapses process electrical impulses – a kind of neural information – it is the great endeavor of cognitive-molecular-neuro-psychology (or whatever you wish to call the art) to figure out how all of those neurons and connections come into being and how they process information in ways that lead to your personality, self-image, hopes, dreams, memories and the other wonderful aspects of your mental life.  How and why does information flow through the brain in the way it does? and how and why does it do so in different ways for different people? Some, for instance, have informally related Sigmund Freud‘s models of mental structure to a kind of plumbing wherein psychic energy was routed (or misrouted) through different structural aspects of the mind (pipes as it were).  Perhaps such a model was fitting for the great industrial era in which he lived – but perhaps not in today’s highly information-based, inter-connected and network-oriented era.  If our understanding of mental life is a product of our tools, then perhaps we should be sure that our modern tools are up to the job.

One recent paper reminded me of how important it is to double check the accuracy and precision of one’s tools was the research article, “Quantifying the heritability of task-related brain activation and performance during the N-back working memory task: A twin fMRI study” [doi:10.1016/j.biopsycho.2008.03.006] by Blokland et al..  In this report, the team summarizes the results of measurments of the brain activity – not structure – but rather activity as measured by their chosen tool, the MRI scanner.  This research team, based in UCLA and known as one of the best in the field, asks whether the so-called BOLD response (an indirect measure of neural activity) shows greater concordance in identical (monozygotic) vs. fraternal (dizygotic) twins.  To generate brain activity, the research team asked the subjects to perform a task called an N-back  workng memory task, which entails having to remember something that happend “N” times ago (click here for further explanation of N-back task or play it on your iphone).  If you’ve done this, you’ll know that its hard – maddeningly so – and it requires a lot of concentration, which, the researchers were counting on to generate activity in the prefrontal cortex.

After looking at the brain activity patterns of some 29 MZ pairs and 31 DZ pairs, the team asked if the patterns of brain activity in the lateral frontal cortex were more similar in the MZ pairs vs. the DZ pairs.  If so, then it would suggest that the scanning technology (measurement of the BOLD response) is sufficiently reliable and precise enough to detect the fraction of individual differences in brain activty that arise from additive genetic variation.  If one actually had such super-precise tool, then one could begin to dissect and tease apart aspects of human cognition that are regulated by individual genetic variation – a very super-precise and amazing tool – that might allow us to understand mental life in biologically-based terms (and not Freud’s plumbingesque analogies).  If only such a tool existed! Somewhat amazingly, the scanning tools did seem to be able to detect differences between the BOLD response correlations of MZ pairs vs. DZ pairs.  The BOLD response correlations were greater for MZ vs. DZ in the middle frontal gyrus, angular gyrus, supramarginal gyrus when activity for the 2-back task was compared to the 0-back task.  The team were cautious to extend these findings too far, since the standard deviations are large and the estimates of heritability for the BOLD response are rather low (11-36%), but, overall, the team suggests that the ability to use the fMRI methods in conjunction with genetic markers shows future promise.

Meanwhile, the literature of so-called “imaging-genetic” findings begins to grow in the literature.  I hope the tools are reliable and trustworthy enough to justify conclusions and lessons about human genetic variation and its role in mental life.  Will certainly keep this cautionary report in mind as I report on the cognitive genetics literature in the future.

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morph_slicer_demoThe brain is a wonderfully weird and strange organ to behold.  Its twists and folds, magnificent, in and of themselves, are even moreso when we contemplate that the very emotional experience of such beauty is carried out within the very folds.  Now consider the possibility of integrating these beauteous structure/function relationships with human history – via the human genome – and ask yourself if this seems like fun.  If so, check out the recent paper, “Genetic and environmental influences on the size of specific brain regions in midlife: The VETSA MRI study” [doi:10.1016/j.neuroimage.2009.09.043].

Here the research team – members of the Biomedical Informatics research Network – have carried out the largest and most comprehensive known twin study of brain structure.  By performing structural brain imaging on 404 male twin pairs (important to note here that the field still awaits a comparable female study), the team examined the differences in identical (MZ) vs. fraternal (DZ) pair correlations of the structure of some 96 different brain regions.  The authors now provide an updated structural brain map showing what structures are more or less influenced by genes vs. environment. Some of the highlights from the paper are that genes accounted for about 70% of overall brain volume, while in the cortex, genes accounted for only about 45% of cortical thickness.  Much of the environmental effects were found to be non-shared, suggesting, as expected, that individual experience can have strong effects on brain structure.  The left and right putamen showed the highest additive genetic influence, while the cingulate and temporal cortices showed rather low additive genetic influences (below 50%).

If you would like to play around with a free brain structure visualization tool, check out Slicer 3D, which can be obtained from the BIRN homepage or directly here.  I had fun this morning digitally slicing and dicing grey matter from ventricles and blood vessels.

slicer

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Human Genome
Image by Dollar Bin via Flickr

pointer to: download Power Point presentation hosted on the HUGO website entitled, “From the human genome to human behaviour: how far have we travelled?” (both English and Russian text) – by Ian Craig and Nick Yankovsky, Education Council Human Genome Organisation.

Covers recent findings on MAOA and 5HTT several and others also covered here.

Congrats to Hsien on the new position!

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Genetic Data
Image by giumaiolini via Flickr

As the personal genomics era dawns, it becomes clear that the new genetic information will lead to more new questions than answers.  Consider a well-intentioned parent who finds any number of suspicious risk factors in the genome of their child.  Perhaps a genetic risk variant for mental illness – an anxiety disorder perhaps?  What can be done?  What, if anything, should be done?

Of course there is no simple answer to this question.  Nevertheless, the technology itself may create strong demand for answers in the near future.  If it were me, I certainly would want to know – something, anything – to help.  Furthermore, there are already examples of willful misinformation in the consumer genetic marketplace that seem to prey on anxieties of parents, and which could ultimately heighten the need for reliable, evidence-based guidance.

To this end, the recent research article entitled, “A Genetically Informed Study of the Association Between Childhood Separation Anxiety, Sensitivity to CO2, Panic Disorder, and the Effect of Childhood Parental Loss“[Arch Gen Psychiatry. 2009;66(1):64-71], caught my attention. In this article, the authors consider Panic Disorder, a condition which can lead to the disruption of a healthy personal and professional life.  Genetic studies have shown that specific genes can contribute to the risk of the disorder, but also that these genes interact with early life and adult life experience.  What might these genes be doing in early life – and if we knew – then might we intervene early on to prevent the onset of the disorder later in life?

Again, there are more questions than answers here, but the research team of Battaglia et al., show – using 712 young adult twins – that a common genetic factor underlies childhood separation anxiety and the adult onset of panic disorder.  Thus, it may be the case that the sames genes that contribute to the risk of panic disorder, also may contribute to a form of childhood anxiety.  Having found evidence for a particular form of developmental continuity, the research team is one step closer to learning how a genomically-guided child-based early intervention might be structured.

Because there are many pathways that can lead to mental illness and many ways in which the genome interacts with the environment – it will be complex, if not impossible, to design early interventions that prevent the onset of mental illness.  In most cases, it is rather likely that most children who carry risk for mental illness, will – due to the probablistic nature of gene-gene and gene-environment interactions – just develop typically and not develop mental illness.  Neverthess, some will and its worth learning more.

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Binocular Smile
Image by cobalt123 via Flickr

Is the human brain a blank slate? or a pre-programmed machine that is ready to take the S.A.T.s right out of the box? Obviously neither, or both as it were. Some have gingerly waded into the nature vs. nuture debate and suggested that the human brain comes pre-wired to receive certain experiences – experience expectant – and thus acknowledge the importance of natural selection in shaping an organism via heritable factors but also the need to be able to use the brain to learn from experience and adapt on the fly.

In their paper entitled, “Nature versus Nurture in Ventral Visual Cortex: A Functional Magnetic Resonance Imaging Study of Twins [DOI:10.1523/JNEUROSCI.4001-07.2007] Thad Polk and colleagues provide a wonderful example of this.  The team suggested that the brain (visual system) should be somewhat innately (genomically if you will) prepared to process visual stimuli such as faces and objects, but not so for stimuli such as pseudo words.  They proposed to test the role of the genome by comparing patterns of brain activity in identical vs. fraternal twins.  If the brain activity patterns were very similar for identical twins, and less so for fraternal twins, then it is likely that the genome plays some role in the generation of brain (at least with respect to blood flow) responses to such stimuli. The team used fMRI to assess 13 pairs of identical twins and 11 pairs of fraternal twins for their brain responses to pictures of faces, houses, chairs and non-word strings on letters as well as control “scrambled” images that were comparable in visuo-spatial frequency.

Interestingly, the team found that for faces and houses, there were significant identical vs. fraternal differences in the “activation maps” of the twins but no such differences for chairs and pseudowords.  Thus it seems that the genome plays a role in the way the brain processes faces and houses (or perhaps faces and places in general), but not so much for items that are not found (or weren’t found by our evolutionary ancestors) in a natural setting.

I’m surprised by the chair result … although perhaps being a couch potato is something evolution does not select for.

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