Heinrichs Lab

Molecular Mechanisms of Stem Cell Expansion &
Molecular Pathogenesis of Myelodysplastic Syndromes (MDS)


Myelodysplastic syndromes (MDS) are a heterogeneous group of hematopoietic disorders driven by the clonal expansion of stem cells with limited differentiation potential. As a consequence, normal hematopoiesis is outcompeted leading to severe disease symptoms in patients. Frequently, erythropoiesis is affected, and the lead clinical finding is an anemia. MDS occurs with an incidence of 4-5/100,000 individuals per year that increases significantly with age (30/100,000/year at ages of 70 years or more). Genetically and clinically, MDS is related to acute myeloid leukemia (AML), and 20-25% of the MDS patients progress to AML. The therapeutic options are limited to supportive care for low risk patients, and regimens for high risk patients are modeled according to the therapy of AML mostly with a dismal prognosis. The main goal of our lab is to understand the molecular and genetic basis of MDS. We are convinced that the knowledge of the molecular pathogenesis will ultimately lead to the development of targeted therapies that strongly improve patient care. Furthermore, the understanding of mechanisms required for the malignant clonal expansion will provide leverage points to develop strategies for a transient expansion of healthy stem and progenitor cells in vitro in a controlled manner. The identification of small molecules that mimic the effect of genetic aberrations will improve cell-based therapies.


MYBL2

We recently identified MYBL2 as a tumor suppressor in MDS. Interestingly, MYBL2 is an essential gene, and its complete loss severely diminishes the proliferation potential of a cell. However, the reduction of its expression to sub-haploinsufficient levels, i.e. below 50% of the wildtype level, confers a competitive advantage to hematopoietic stem or progenitor cells. Indeed, we found reduced levels of MYBL2 expression in CD34+ cells in about 2/3 of MDS patients. Strikingly, MYBL2 is located within a chromosomal region that is recurrently affected in MDS patients, the chromosome 20q commonly deleted region (CDR). Although described more than a decade ago, a discovery of a mutated allele located within the second copy of this gene in patients with a 20q deletion has not been reported. These findings were indicative of a gene within the 20q CDR that is essential for cell viability, but whose tumor suppressor function is strongly dose-dependent and does not follow the classical two hit model according to Knudson, which predicts biallelic gene inactivation. Instead, monoallelic loss may sufficiently reduce gene expression levels to promote cell transformation. Indeed, MYBL2 fulfilled all these predictions and our discovery uncovered MYBL2 as a 20q CDR tumor suppressor. MYBL2 is a transcription factor that drives the transition from the G2- to M-phase of the cell cycle. Currently, we investigate the MYBL2 target genes and the downstream pathways that contribute to its tumor suppressor function and lead to a clonal advantage of MDS stem cells.


Other genetic aberrations

Besides MYBL2, we are interested in additional genetic aberrations in MDS that confer advantages for clonal expansion to stem and progenitor cells. The most commonly mutated genes in MDS are TET2 (33%), SF3B1 (32%), ASXL1 (24%), SRSF2 (17%), DNMT3A (13%) and RUNX1 (10%). RUNX1 is of particular interest to us as it encodes a transcription factor involved in key hematopoietic differentiation processes. Mutations in MDS lead to C-terminally truncated RUNX1 proteins that appear to act in a dominant-negative fashion. Indeed, ectopic expression of such alleles immortalizes hematopoietic stem and progenitor cells in vitro and confers a clonal advantage in vivo. We are interested to identify pathways downstream of these mutant alleles and develop models recapitulating their genetic interaction.


Methods

The key techniques that we use to achieve our goals include genetic modeling using lentiviral vectors, RNA interference, Crispr/Cas9-mediated genome engineering, gene expression analysis by QRT-PCR and microarray, immortalization of primary stem and progenitor cells, immunophenotyping and cell sorting (FACS), in vivo competitive reconstitution experiments (hematopoiesis)