Cognitive Genetics developmental issues and an example from PKU Angela Brant

Scerif & Karmiloff-Smith (2005) 1 st decade of 21 st Century- “the dawn of cognitive genetics” (Pinker, 2001). Can the function of specific genes be linked to specific cognitive-level processes? Various methods used to examine this issue…. Scerif & Karmiloff-Smith point out that evidence of expression of gene A, accompanied by deficits in cognitive function B does not necessarily mean that A directly has a role in implementing B.

Developmental Issues Ignoring developmental complexities in cognitive processes carries some potentially erroneous assumptions This article reviews evidence of this in monogenic disorders

1.Can earlier cognitive outcome be inferred from the adult phenotype? Williams syndrome (WS)- adults show relative strengths in language and face processing and weaknesses in visuo- spatial cognition and numerical processing However, infants show a different profile, with grossly delayed vocabulary comprehension, and numerical abilities in line with their chronological age. Not necessarily the case (e.g. fragile X shows consistent difficulties at all stages) There may also be atypical interactions and compensations across processes over developmental time (WS again).

2.Single genes but uneven cognitive outcomes E.g. fragile X- neural changes caused by the silencing of one protein are more relevant to the neurocomputational requirements of some functions compared to others FMRP- altered dendritic spine morphology across neocortical areas. However, dendritic spines increase in density as you move forward in the cortex in human and non-human primates. Thus, a generalized change can cause apparently localized deficits. Mechanistic accounts of why particular deficits occur are important to understand these patterns. Selective deficits are not impossible, but need to be tested empirically.

All these considerations become more complicated when polygenetic, complex traits are examined!

Diamond (2002) Outlines various deficits found in the brains of even supposedly well-controlled PKU Gives an interesting example of how the global silencing of a single protein can cause apparently selective and unrelated deficits. Also shows an example of a deficit that is (perhaps) unrelated to abilities during development and one that is firmly caused by childhood phenotype.

The dopamine system and PKU Dopamine neurons projecting from VTA to DLPFC are acutely sensitive to changes in the supply of tyrosine as they have a higher firing rate and higher rate of dopamine turnover. Reductions in tyrosine supply to small to have an effect on other dopamine systems have been shown to profoundly reduce dopamine levels in prefrontal cortex.

PKU and executive functioning Strong effect of tyrosine availability on the DLPFC 1980s- problems in holding information in mind, problem solving and ‘executive functions’ in children with PKU on a low Phe diet (e.g. Pennington et al, 1985) Children with PKU have an average IQ in the 80s and 90s- lower than their peers and reminiscent of the deficits seen after damage to the prefrontal cortx

Developmental delay or lasting deficit? All children improved over time But…..between group differences reappeared when battery for next age group was implemented. Deficit showed no evidence of subsiding within age range studied. Data do not exist to answer this question, but extended cognitive delays likely to have profound effects even if the cognitive deficit is resolved.

Dopamine in the visual system Witkovsky, 2004

Structural effect on contrast sensitivity Contrast sensitivity not related to current Phe levels Contrast sensitivity usually improves with age but the reverse is seen in PKU siblings Earlier-born PKU siblings perform worse than their same-age siblings