Faculty Bibliography

MicroRNAs (miRNAs) are important regulators of diverse biologic processes. In the hematopoietic system, miRNAs have been shown to regulate lineage fate decisions, mature immune effector cell function, apoptosis, and cell cycling, and a more limited number of miRNAs has been shown to regulate hematopoietic stem cell (HSC) self-renewal. Many of these miRNAs were initially identified as candidate regulators of HSC function by comparing miRNA expression in hematopoietic stem and progenitors cells (HSPCs) to their mature progeny. While the measurement of miRNA expression in rare cell populations such as HSCs poses practical challenges due to the low amount of RNA present, a number of techniques have been developed to measure miRNAs in small numbers of cells. Here, we describe our protocol for measuring miRNAs in purified mouse HSCs using a highly sensitive real-time quantitative PCR strategy that utilizes microfluidic array cards containing pre-spotted TaqMan probes that allows the detection of mature miRNAs in small reaction volumes. We also describe a simple data analysis method to evaluate miRNA expression profiling data using an open-source software package (HTqPCR) using mouse HSC miRNA profiling data generated in our lab.

Somatic Addition of Sex Combs Like 1 (ASXL1) mutations occur in 10-30% of patients with myeloid malignancies, most commonly in myelodysplastic syndromes (MDSs), and are associated with adverse outcome. Germline ASXL1 mutations occur in patients with Bohring-Opitz syndrome. Here, we show that constitutive loss of Asxl1 results in developmental abnormalities, including anophthalmia, microcephaly, cleft palates, and mandibular malformations. In contrast, hematopoietic-specific deletion of Asxl1 results in progressive, multilineage cytopenias and dysplasia in the context of increased numbers of hematopoietic stem/progenitor cells, characteristic features of human MDS. Serial transplantation of Asxl1-null hematopoietic cells results in a lethal myeloid disorder at a shorter latency than primary Asxl1 knockout (KO) mice. Asxl1 deletion reduces hematopoietic stem cell self-renewal, which is restored by concomitant deletion of Tet2, a gene commonly co-mutated with ASXL1 in MDS patients. Moreover, compound Asxl1/Tet2 deletion results in an MDS phenotype with hastened death compared with single-gene KO mice. Asxl1 loss results in a global reduction of H3K27 trimethylation and dysregulated expression of known regulators of hematopoiesis. RNA-Seq/ChIP-Seq analyses of Asxl1 in hematopoietic cells identify a subset of differentially expressed genes as direct targets of Asxl1. These findings underscore the importance of Asxl1 in Polycomb group function, development, and hematopoiesis.