Origin and segregation of the human germline

Human germline-soma segregation occurs during weeks 2-3 in gastrulating embryos. Although direct studies are hindered, here, we investigate the dynamics of human primordial germ cell (PGCs) specification using in vitro models with temporally resolved single-cell transcriptomics and in-depth characterisation using in vivo datasets from human and nonhuman primates, including a 3D marmoset reference atlas. We elucidate the molecular signature for the transient gain of competence for germ cell fate during peri-implantation epiblast development. Furthermore, we show that both the PGCs and amnion arise from transcriptionally similar TFAP2A-positive progenitors at the posterior end of the embryo. Notably, genetic loss of function experiments shows that TFAP2A is crucial for initiating the PGC fate without detectably affecting the amnion and is subsequently replaced by TFAP2C as an essential component of the genetic network for PGC fate. Accordingly, amniotic cells continue to emerge from the progenitors in the posterior epiblast, but importantly, this is also a source of nascent PGCs.

Tracing the emergence of primordial germ cells from bilaminar disc rabbit embryos and pluripotent stem cells

Rabbit embryos develop as bilaminar discs at gastrulation as in humans and most other mammals, whereas rodents develop as egg cylinders. Primordial germ cells (PGCs) appear to originate during gastrulation according to many systematic studies on mammalian embryos. Here, we show that rabbit PGC (rbPGC) specification occurs at the posterior epiblast at the onset of gastrulation. Using newly derived rabbit pluripotent stem cells, we show robust and rapid induction of rbPGC-like cells in vitro with WNT and BMP morphogens, which reveals SOX17 as the critical regulator of rbPGC fate as in several non-rodent mammals. We posit that development as a bilaminar disc is a crucial determinant of the PGC regulators, regardless of the highly diverse development of extraembryonic tissues, including the amnion. We propose that investigations on rabbits with short gestation, large litters, and where gastrulation precedes implantation can contribute significantly to advances in early mammalian development.

Mutational synergy during leukemia induction remodels chromatin accessibility, histone modifications and three-dimensional DNA topology to alter gene expression

Altered transcription is a cardinal feature of acute myeloid leukemia (AML); however, exactly how mutations synergize to remodel the epigenetic landscape and rewire three-dimensional DNA topology is unknown. Here, we apply an integrated genomic approach to a murine allelic series that models the two most common mutations in AML: Flt3-ITD and Npm1c. We then deconvolute the contribution of each mutation to alterations of the epigenetic landscape and genome organization, and infer how mutations synergize in the induction of AML. Our studies demonstrate that Flt3-ITD signals to chromatin to alter the epigenetic environment and synergizes with mutations in Npm1c to alter gene expression and drive leukemia induction. These analyses also allow the identification of long-range cis-regulatory circuits, including a previously unknown superenhancer of Hoxa locus, as well as larger and more detailed gene-regulatory networks, driven by transcription factors including PU.1 and IRF8, whose importance we demonstrate through perturbation of network members.