Aims: The overarching hypothesis is that circulating levels of pro-inflammatory cytokines and other inflammatory markers are genetically associated with depression in adolescents.Specifically, it is hypothesised that there will be a correlation between variation in genetic risk of depression and genetic variation of circulating pro-inflammatory cytokines and inflammatory markers.
Methods:A systematic review of the literature was conducted to establish the current understanding of the relationship between inflammatory markers and depression in adolescents and to inform study design. Multiple approaches (including different types of genetic analyses and multiple data sets) were used to address the overarching hypothesis.A pilot study measuring cytokines, inflammatory markers, and other biomarkers involved in immune regulation (including Vitamin D, antibodies to infectious agents, and gliadin antibodies (found in coeliac disease)) was conducted in 107 monozygotic (MZ) and 160 dizygotic (DZ) twin pairs (mean age 16.2 years, standard deviation (SD) 0.25 years) from the Brisbane Adolescent Twin Study. A clinical study was undertaken to collect biological samples and clinical data from an in-patient adolescent mental health unit.
Investigation of the relationship between iron measures (altered in inflammatory states)and measures of depression was undertaken in community cohorts of twins and their parents (3,416 adolescents from 1,688 families, and 9,035 adults from 4,533 families). In the adolescent cohort, depressive measures were assessed through the Somatic and Psychological Health Report (SPHERE) (mean age 15.1 years (SD 3.2 years)). In the adult cohort, a quantitative score of depression was measured by the Delusions Symptoms State Inventory (DSSI) (mean age 23.2 years (SD 2.2 years)). Heritabilities of, and phenotypic and genetic correlations between, traits were estimated. Association analyses, genetic profile risk score analyses, and LD score analyses were also used to investigate the genetic relationship between the iron and depression measures. Genetic profile risk score analyses were used to explore the genetic relationship between these variables.
Results:In the pilot study, cytokines that were successfully measured in plasma were found to be moderately heritable (transforming growth factor-β1 (TGFβ1), 0.57 (95% CI 0.26 -0.80) and tumour necrosis factor-receptor type 1 (TNFR1), 0.50 (95% CI 0.11-0.63)). A negative correlation between Vitamin D and the cytokine IL-18 (-0.14) was not statistically significant (p=0.054). However, major difficulties were encountered in measuring cytokines, in particular due to the low levels of circulating cytokines in healthy adolescents. Challenges were also encountered in the cytokines study of the clinical sample of adolescents, which were able to be broadly divided into patient factors, blood collection factors, and other data collection factors.Iron measures were found to be highly heritable in both adolescents and adults: The relationship between CRP and Major D...