EJE 1. JUSTICIA PARA TODAS Y TODOS

Prior to the era of GWA studies, little was known about common variants which influence the timing of puberty in the general population, despite the wide variability in timing and tempo between individuals. In 2008, a genome-wide linkage study found no evidence for highly penetrant common variants influencing AAM in women from Australia, The Netherlands, and the United Kingdom (102). In 2009, a small GWA study identified associated variants in the gene SPOCK, but subsequent studies failed to replicate this locus (145).

This was the current state of knowledge about the genetics of puberty in the general population in 2009, when we began work on the first study included in this thesis, a gene mapping study of the pubertal growth spurt in several large Finnish population birth cohort studies. Later that same year, four studies, including between 5,000 and 25,000 study subjects each, were published (Table 1). These studies identified a robust association between SNPs near LIN28B, a heterochronic regulator of developmental timing, and AAM (146–149), the same locus which robustly associated with pubertal growth in our study. Variation near the gene also associated with earlier breast development in girls, earlier pubic hair development and voice break in boys, a faster tempo of height growth and shorter adult stature in both sexes (146). A secondary locus on chromosome 9 was also highlighted by these studies.

Realizing that increasing the sample size would also increase power to detect common variants with smaller effect sizes, we joined forces with other teams interested in the genetics of reproductive traits within the ReproGen Consortium, which came to include 175 researchers from 104 international research institutions. In 2010, we contributed data from several Finnish cohorts to a study of AAM including 87,802 women of European descent, which identified 30 new loci at the genome-wide significant level, including 4 that had previously been associated with body mass index, 3 loci in or near genes for energy homeostasis, and 3 loci implicated in hormonal regulation (150). A further 10 loci were associated at the suggestive level.

The most recent large-scale GWA study of AAM published by the ReproGen consortium included over 180,000 women of European ancestry, from 57 contributing cohort studies including our contribution from the Finnish population cohorts, and mapped 123 signals at 106 genomic loci as associated with the timing of menarche (133). All but one of the

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previously published signals showed association with menarche in this study, and 2.71% of the variance in AAM was explained by these 123 SNPs in an independent sample of 8,689 women. Interestingly, six independent signals were located in imprinted regions of the genome, and parent-of-origin effects were found at 14q32 which harbors the reciprocally imprinted genes DLK1 and MEG3, 15q11-13 in the imprinted region critical for Prader-Willi syndrome, and at KCNK9, representing an enrichment when compared to all published genome-wide significant alleles for any trait (6/123, 4.8% vs. 75/4332, 1.7%, P = 0.017).

Studies investigating the association between the timing of menarche in non-European populations have revealed a large degree of overlap among distant populations. The majority of these studies aimed to replicate European menarche loci in different populations. 8 loci were replicated in 3468 Hispanic women from the United States as part of the Women's Health Initiative SNP Health Association Resource (151), and 25 (60%) of 42 European loci also contained significant SNPs associated with the timing of menarche in African American women (n = 18,089) (152). One of these was the locus at RORA, which was suggestively associated in European women, and also associated with Tanner brest development stage in our third study included in this thesis. In 6,929 Chinese women from Shanghai, China, 32 variants had the same direction of effect as in European women, and 9 of them were statistically significant (153). Finally, a study of over 15,000 Japanese women showed that SNPs at two European loci also associated at a statistically significant level with menarche in this population, and that six more loci were suggestively associated (154).

Other markers of puberty, including measurements of the pubertal height growth spurt and secondary sex characteristics, have not been studied comprehensively in GWA studies. Candidate studies of the association between adult height-associated SNPs and the pubertal growth spurt have only revealed association for SNPs at LIN28B (155) and SOCS2 (156). Consequently, very little is known about the genetic underpinnings of male pubertal maturation and how menarche loci relate to pubertal initiation, tempo, or other markers of puberty.

In conclusion, it is clear that advances are being made in our understanding of the genetic architecture of pubertal timing in the general population, both in Europeans as well as other ethnicities. However, one limitation of these studies is that they are all primarily investigating the timing of menarche, which is a female-specific trait which occurs an average of two years after the onset of puberty. In the studies included in this thesis, we have looked at phenotypes that are present in both sexes, such as the pubertal growth spurt, and sexually dimorphic traits which closely follow the onset of puberty, female breast and male genital development. Therefore, our work has enabled the investigation of common variants influencing puberty in males, as well as allowing us to study the genetic

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overlap of different phases of pubertal development, both important contributions to the study of pubertal genetics.