MLL4 is required after implantation, whereas MLL3 becomes essential during late gestation.

Methylation of histone 3 lysine 4 (H3K4) is a major epigenetic system associated with gene expression. In mammals there are six H3K4 methyltransferases related to yeast Set1 and fly Trithorax, including two orthologs of fly Trithorax-related: MLL3 and MLL4. Exome sequencing has documented high frequencies of MLL3 and MLL4 mutations in many types of human cancer. Despite this emerging importance, the requirements of these paralogs in mammalian development have only been incompletely reported. Here, we examined the null phenotypes to establish that MLL3 is first required for lung maturation, whereas MLL4 is first required for migration of the anterior visceral endoderm that initiates gastrulation in the mouse. This collective cell migration is preceded by a columnar-to-squamous transition in visceral endoderm cells that depends on MLL4. Furthermore, Mll4 mutants display incompletely penetrant, sex-distorted, embryonic haploinsufficiency and adult heterozygous mutants show aspects of Kabuki syndrome, indicating that MLL4 action, unlike MLL3, is dosage dependent. The highly specific and discordant functions of these paralogs in mouse development argues against their action as general enhancer factors.

SETD1A protects HSCs from activation-induced functional decline in vivo.

The regenerative capacity of hematopoietic stem cells (HSCs) is limited by the accumulation of DNA damage. Conditional mutagenesis of the histone 3 lysine 4 (H3K4) methyltransferase, Setd1a, revealed that it is required for the expression of DNA damage recognition and repair pathways in HSCs. Specific deletion of Setd1a in adult long-term (LT) HSCs is compatible with adult life and has little effect on the maintenance of phenotypic LT-HSCs in the bone marrow. However, SETD1A-deficient LT-HSCs lose their transcriptional cellular identity, accompanied by loss of their proliferative capacity and stem cell function under replicative stress in situ and after transplantation. In response to inflammatory stimulation, SETD1A protects HSCs and progenitors from activation-induced attrition in vivo. The comprehensive regulation of DNA damage responses by SETD1A in HSCs is clearly distinct from the key roles played by other epigenetic regulators, including the major leukemogenic H3K4 methyltransferase MLL1, or MLL5, indicating that HSC identity and function is supported by cooperative specificities within an epigenetic framework.

Lentiviral transduction of mammary stem cells for analysis of gene function during development and cancer.

The mouse mammary gland is the only epithelial organ capable of complete regeneration upon orthotopic transplantation, making it ideally suited for in vivo gene function studies through viral-mediated gene delivery. A hurdle that has challenged the widespread adoption of this technique has been the inability to transduce mammary stem cells effectively. We have overcome this limitation by infecting total primary mammary epithelial cells in suspension with high-titer lentiviruses. Transduced cells gave rise to all major cell types of the mammary gland and were capable of clonal outgrowth and functional differentiation in serial transplants. To demonstrate that this method is a valuable alternative to developing transgenic animals, we used lentiviral-mediated Wnt-1 overexpression to replicate MMTV-Wnt-1 mammary phenotypes and used a dominant-negative Xenopus Suppressor of Hairless to reveal a requirement for Notch signaling during ductal morphogenesis. Importantly, this method is also applicable to transduction of cells from other tissues.