Shared roles for Scl and Lyl1 in murine platelet production and function

Abstract

The stem cell leukemia (Scl or Tal1) protein forms part of a multimeric transcription factor complex required for normal megakaryopoiesis. However, unlike other members of this complex such as Gata1, Fli1, and Runx1, mutations of <i>Scl</i> have not been observed as a cause of inherited thrombocytopenia. We postulated that functional redundancy with its closely related family member, lymphoblastic leukemia 1 (Lyl1) might explain this observation. To determine whether Lyl1 can substitute for Scl in megakaryopoiesis, we examined the platelet phenotype of mice lacking 1 or both factors in megakaryocytes. Conditional <i>Scl</i> knockout (KO) mice crossed with transgenic mice expressing Cre recombinase under the control of the mouse platelet factor 4 (<i>Pf4</i>) promoter generated megakaryocytes with markedly reduced but not absent <i>Scl</i> These <i>Pf4Sclc</i>-KO mice had mild thrombocytopenia and subtle defects in platelet aggregation. However, <i>Pf4Sclc</i>-KO mice generated on an <i>Lyl1</i>-null background (double knockout [DKO] mice) had severe macrothrombocytopenia, abnormal megakaryocyte morphology, defective pro-platelet formation, and markedly impaired platelet aggregation. DKO megakaryocytes, but not single-knockout megakaryocytes, had reduced expression of <i>Gata1</i>, <i>Fli1</i>, <i>Nfe2</i>, and many other genes that cause inherited thrombocytopenia. These gene expression changes were significantly associated with shared <i>Scl</i> and <i>Lyl1 E-box</i> binding sites that were also enriched for <i>Gata1</i>, <i>Ets</i>, and <i>Runx1</i> motifs. Thus, Scl and Lyl1 share functional roles in platelet production by regulating expression of partner proteins including Gata1. We propose that this functional redundancy provides one explanation for the absence of <i>Scl</i> and <i>Lyl1</i> mutations in inherited thrombocytopenia.