Several reports have shown that KLF4 expression is regulated by CDX2 (caudal-type homeobox 2) in colon cancer and leukemia. Notably, the vast majority of patients with AML were shown to aberrantly express CDX2 gene in bone marrow stem and progenitor cells, resulting in down-regulation of KLF4 expression. In gene expression profiling of 136 human AML cases, KLF4 showed markedly decreased expression in patients with high CDX2 mRNA levels. Furthermore, KLF4 was one of only two genes repressed by >2-fold in gene signatures from AML patients with high CDX2 expression and murine hematopoietic stem and progenitor cells (HSPCs) transduced with murine CDX2. CDX2 was subsequently shown to act as a transcriptional repressor of KLF4 expression in leukemia by binding to CDX2 consensus sequences within the KLF4 promoter. Once bound, CDX2 recruits the H3K4 demethylase KDM5B and silences KLF4 transcription by an epigenetic mechanism. These observations suggest that CDX2 promotes leukemogenesis by suppression of KLF4.
CDX2 is a transcription factor that functions in embryonic organogenesis and early hematopoietic development in vertebrates. In adults, CDX2 expression is confined to the intestinal epithelium and is highest in the proximal colon. Cdx2-deficient embryonic stem cells show impaired production of blood progenitor colonies in vitro, consistent with an essential role of CDX2 in early embryonic hematopoiesis. CDX2 is not normally expressed in adult murine and human normal hematopoiesis, and CDX2 mRNA is not detectable in normal human bone marrow (BM), CD34+ human BM, or human cord blood cells. Recently, CDX2 was shown to be expressed in bone marrow or peripheral blood mononuclear cells of 90 percent (n = 153/170) of acute myeloid leukemia patients and in eight out of 14 AML cell lines. Similar results were observed in a separate study of 71 AML patients, in which 63 (89 percent) showed CDX2 expression.
In addition to AML, CDX2 transcripts were detectable in BM mononuclear cells (BMMCs) in 40% of patients with MDS. Transition of one of these patients to secondary AML one month later was accompanied by an increase in the number of CDX2 transcripts. CDX2 is also aberrantly expressed in 81 percent (n = 46/57) of adult patients with acute lymphocytic leukemia (ALL), and high CDX2 expression levels are significantly associated with inferior overall survival in ALL patients. The mechanism by which CDX2 expression is reactivated in leukemia is not well understood. No differences in CDX2 promoter methylation were found in CDX2-expressing leukemic cells from AML patients compared to CDX2-negative AML patients and monocytes/granulocytes from healthy individuals. Amplifications at the CDX2 gene locus were observed in only four out of 50 AML patients with detectable CDX2 expression. As with AML, no differences in promoter methylation were found between CDX2-positive and CDX2-negative ALL cases.
Several studies have demonstrated a role for CDX2 in leukemic cell growth and proliferation. In CDX2-positive AML cell lines, silencing of CDX2 expression by shRNA inhibited cell proliferation and colony-forming ability as compared with a nonsilencing control construct. CDX2 is highly leukemogenic in experimental models of AML and mice transplanted with bone marrow cells expressing CDX2 rapidly develop fatal AML. Taken together, there is significant evidence to show a causal relationship between aberrant CDX2 expression and repression of KLF4 in leukemia.