DEFECTIVE HEMATOPOIESIS IN FETAL LIVER RESULTING FROM INACTIVATION OFTHE EKLF GENE

ERYTHROID Kruppel-like factor (EKLF) was originally isolated from eryt hroid cell RNA by differential screening and shown to be erythroid-spe cific, although a low level of EKLF was found in mast cell lines(1,2). EKLF contains three zinc-fingers homologous to those found in the Kru ppel family of transcription factors, Because it binds the sequence CC ACACCCT, EKLF may affect erythroid development as a result of its abil ity to bind to the CAC box in the promoter of the beta-globin gene(1,2 ). Mutation of this element leads to reduced beta-globin expression(3- 5) and it appears to mediate the effect of the globin locus control re gion on the promoter(6). Here we inactivate the EKLF gene through inse rtion of a lacZ reporter gene by homologous recombination in embryonic stem (ES) cells. Heterozygous EKLF(+/-) mice show that the reporter g ene is expressed in a developmentally specific manner in all types of erythroblasts in the fetal liver and adult bone marrow. Homozygous EKL F(-/-) mice appear normal during the embryonic stage of haematopoiesis in the yolk sac, but develop a fatal anaemia during early fetal life when haematopoiesis has switched to the fetal liver. Enucleated erythr ocytes are formed but these do not contain the proper amount of haemog lobin. We conclude that the transcription factor EKLF is essential for the final steps of definitive erythropoiesis in fetal liver.