Over the past decade, advances in genetics have led to identification of thousands of genetic loci in the human genome that contribute to complex traits such as the timing of pubertal onset. Following genome-wide association studies (GWAS), we currently know how a large proportion of the heritability of complex traits can be explained by common genetic variation in these loci. However, in many instances, we understand relatively little about the biology behind these genetic associations, and functional characterization of genetic variation has become a new bottleneck for genetic research. Prior to this thesis project, sequence variants nearby lin-28 homolog B (LIN28B) had become associated with pubertal timing in the general population. Remarkably, compared to all other common genetic variants, the variants in the LIN28B locus appeared to exert relatively large effects on pubertal timing: one pubertal timing advancing allele associating with ~1,5 months advancement in age at menarche (AAM) in females. Yet, the gene showed little evidence of affecting the established mechanisms behind pubertal onset. Exploring the molecular mechanisms by which LIN28B affects puberty therefore became the focus of this thesis, which is is founded on three original studies addressing the function of LIN28B. To move beyond the original GWAS results, we first utilized Finnish population cohorts to assess the potential pleiotropy of the gene in terms of body size and adult health. The second original study combined human gene expression data from the GTEx database with zebrafish models, evaluating the consequences of transient dysregulation of lin28b during embryogenesis. For the third study, we first created lin28b knockout zebrafish with CRISPR-Cas9 technology to evaluate the effects of permanent lin28b knockout, and utilized the GTEx data and the UK biobank resource to study whether LIN28B contributes to sex steroid signaling in humans. The results presented in this thesis suggest that LIN28B has pleiotropic actions on vertebrate phenotypes and may contribute to the timing and tempo of human growth in more complex ways than originally thought. Our studies demonstrate that the gene associates with variation in several body size parameters in adult humans, although showing little evidence of affecting metabolism. The results also suggest that the effects that LIN28B has on body size are evolutionarily conserved. Overexpression of lin28b during embryogenesis appears to stimulate zebrafish growth, and, intriguingly, lin28b knockout zebrafish show similar growth patterns as humans that carry sequence variants linked with lower LIN28B expression. Importantly, the data presented in this thesis indicates that the sequence variants associating with pubertal timing affect LIN28B expression mostly in the hypothalamus and the pituitary of adult humans. These changes in the LIN28B expression level may have further consequences: LIN28B expression at the hypothalamic-pituitary (HP) axis seems to correlate positively with the expression of several hormonal genes like ESR1 and POMC. Highlighting the gene’s potential to contribute to sex steroid signaling, we finally associated LIN28B with the regulation of testosterone levels in adult humans, which might be relevant in terms of explaining many of the GWAS associations including pubertal timing. Overall, the results presented in this thesis offer novel insight into LIN28B function in pubertal timing, disease and development, the project simultaneously serving as an example for follow-up studies of GWAS loci in general.
|Status||Publicerad - 2019|
|MoE-publikationstyp||G5 Doktorsavhandling (artikel)|
- 3111 Biomedicinska vetenskaper
- 1184 Genetik, utvecklingsbiologi, fysiologi