Two other interesting observations from our GWAS results wer

Two other interesting observations from our GWAS results were the overlap of loci that previously had been mapped for HSPC phenotypes using linkage analysis and identification of genomic regions that harbored genes encoding cell surface markers used for flow cytometry. In this regard, we identified a major locus for LSK purchase Nanaomycin A over the genes encoding Sca-1 (Ly6e-Ly6a-Ly6f) on chromosome 15, which confirms previous reports that certain strains carry a Sca-1 haplotype associated with decreased expression (Spangrude and Brooks, 1993). Exclusion of these strains from the analysis did not significantly decrease the heritability of LSK variation or reveal additional loci, but yielded highly suggestive evidence for association with a chromosome 18 locus containing Zfp521. QTL analysis with BXD RI strains also identified a locus for HSC frequency on chromosome 18 near Zfp521, although the peak of that linkage signal mapped ∼20 Mb distal from our lead GWAS SNP (de Haan and Van Zant, 1997). Zfp521 encodes a transcription factor protein with 30 zinc fingers that was originally identified as being specifically expressed in primitive human CD34+ cells compared to more mature hematopoietic cells (Bond et al., 2004). More recently, Zfp521 also has been implicated in bone formation (Hesse et al., 2010; Kiviranta et al., 2013), but validation of Zfp521 as a gene directly influencing HSPCs will require additional functional studies. Notably, the loci on chromosomes 15 (Ly6e-Ly6a-Ly6f) and 18 (Zfp521-Ss18) were identified in the GWAS analyses for LSKCD150−CD48− cells as well. However, aside from these two regions, there was no overlap between the loci identified for the three HSPC populations or with those previously reported for blood cell traits in the HMDP (Davis et al., 2013). In addition to Ly6e-Ly6e-Ly6f, our GWAS analysis revealed highly significant association of LSKCD150−CD48− cells with the Slam locus on chromosome 1, the strength of which increased by two orders of magnitude after excluding the low Sca-1-expressing strains. Consistent with our results, Müller-Sieburg and Riblet (1996) also mapped a QTL for HSCs directly over the Slam locus using the CAFC assay and linkage analysis in the BXD RI panel. Although we cannot determine with certainty whether Cd48 and Slamf1 account for the signal on chromosome 1 due to extensive LD across this region, Cd48-deficient mice do exhibit increased numbers of short-term HSCs (Boles et al., 2011). While GWAS in humans has been successful in identifying genes for many different disease phenotypes, these types of analysis have had limited success for HSPCs. This is due, in part, to the logistical challenges of obtaining the relevant tissue in humans (i.e., BM) in large numbers of subjects in order to have sufficient mapping power. For example, a GWAS in the Framingham Heart Study for levels of circulating CD34+ cells only identified suggestive loci, none of which could be functionally validated (Cohen et al., 2013). With respect to the present results, the relevance of the identified genes and pathways to humans is still to be determined. However, it is encouraging that recent studies in the HMDP were able to directly correlate mouse association signals with those identified in human GWAS (Davis et al., 2013; Parks et al., 2013). For instance, four of the five loci identified for RBC parameters in the HMDP (Davis et al., 2013) correspond to loci recently reported for analogous phenotypes in a large human GWAS (van der Harst et al., 2012). These observations reinforce the concept that the underlying biological pathways for complex traits are likely to be conserved between mice and humans.
Experimental Procedures
Author Contributions
Acknowledgments
Introduction Precise regulation of somatic stem cell function is essential for the survival of multicellular living organisms ranging from C. elegans to humans. Somatic stem cells maintain themselves while their progeny turn over and differentiate to maintain the tissue they reside in throughout life. This process is deregulated during disease and aging; therefore, increasing attention has been dedicated to understanding somatic stem cells with an aim to improve both prevention and treatment of disease. The correct functioning of somatic stem cells depends on complex and dynamic interactions with specific cellular and molecular components of the microenvironment that surrounds them (together called “niche”) (Scadden, 2014), and in vivo imaging of stem cells is an expanding and promising field that provides a unique perspective of their behavior in situ. To date, this approach has been directly responsible for generating new hypotheses on the crucial role of the stem cell niche (Ritsma et al., 2014; Rompolas et al., 2012).