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BACKGROUND: Several studies have shown that the in utero environment, which can be indexed by birth weight (BW), is associated with cortical morphology in adolescence and adulthood. Work in monozygotic (MZ) twins suggests that this association is driven by non‐shared environmental factors. This correlation could be the result of in utero impacts on DNA methylation. The aim of the present study with MZ twins is to replicate the association between discordance in BW and brain morphology and test whether discordance in DNA methylation mediates this relationship. METHODS: One hundred and four adolescent MZ twins (52 pairs, of which 42% were male pairs) who have been followed regularly since birth underwent T1 weighted structural MRI, and epigenome‐wide assessment of DNA methylation from saliva at age 15. RESULTS: Co‐twins had very similar measures of DNA methylation and cortical morphology. Higher BW members of a twin pair had increased total cortical surface area, and decreased cortical thickness compared to their lower BW sibling. BW Discordance was positively associated with both cortical surface area and cortical volume discordance. Genes involved in neurodevelopment were tentatively identified as mediators of both the BW ‐ cortical volume, and BW‐ cortical surface area relationships. CONCLUSIONS: The association between BW and cortical morphology in adolescence appears to be attributable to in utero environmental effects, and DNA methylation may play a role in mediating this relationship. Hum Brain Mapp 38:2037–2050, 2017. © 2017 Wiley Periodicals, Inc.

Contexte : la présence d’un biais cognitif négatif chez les individus qui souffrent de dépression majeure (DM) et ceux qui y sont à haut risque (e.g. enfants de mères qui souffrent de DM) est maintenant établie. Aucune étude portant sur la vulnérabilité cognitive (VC) des membres de la fratrie n’est rapportée. Objectifs : la présente étude a pour but de vérifier si la fratrie des adolescents qui souffrent de DM présentent une VC qui les prédisposent à la DM. Méthode : cette étude porte sur 49 adolescents (18 participants traités pour une DM, 16 membres de la fratrie et 15 participants témoin), âgés entre 12 et 20 ans. La VC de chaque participant est quantifiée via un questionnaire qui mesure la réactivité cognitive (RC), soit le LEIDS-R, et une tâche de reconnaissance des expressions faciale (REF). La cognition sociale des participants est mesurée par le MASC, un outil qui évalue la cognition sociale par médium vidéo et que notre équipe a traduit de l’allemand au français. Résultats : les résultats préliminaires de la présente étude indiquent qu’une différence de réactivité cognitive est présente entre les adolescents traités pour une DM et les participants du groupe témoin (p < 0,001). L’analyse préliminaire tend à indiquer qu’une différence est présente entre la fratrie et le groupe contrôle. Conclusion : plusieurs de nos résultats tendent en faveur de la présence d’une VC prédisposant à la DM chez la fratrie des adolescents souffrant de DM. Ces résultats préliminaires doivent être confirmés par des études longitudinales., Background: a negative cognitive bias is present among individuals who suffers from major depression. This bias is also reported among individuals at high risk of major depression (e.g. child of depressed mother). No study to date aimed to evaluate cognitive vulnerability of siblings of depressed individuals. Objectives: the present study aim to verify if siblings of depressed adolescents present a cognitive vulnerability that would predispose them to develop a major depression. Method: This study evaluates 49 adolescents (18 participants treated for depression, 16 siblings and 15 controls), aged between 12 and 20 years old. The cognitive vulnerability of every participant has been assessed using an auto-report questionnaire of symptoms (LEIDS-R) that evaluates cognitive reactivity and a task of facial recognition. Social cognition of participants is measured using the Movie for Assessment of Social Cognition (MASC) that we translated from german to french. Results: the preliminary analyses of this study concludes that a significant difference of cognitive reactivity is present between adolescents treated for depression and controls (p < 0.001). Conclusion: some of our results tend to confirm the presence of a cognitive vulnerability to depression among siblings of depressed adolescents. Those results are still preliminary and need to be confirmed by longitudinal studies., Réalisé sous la co-direction de Linda Booij, Catherine Herba et Patricia Garel