The fetal specific gene LIN28B is essential for human fetal B-lymphopoiesis and initiation of KMT2A::AFF1 infant leukemia

Infant ALL (iALL) is initiated in utero, most often by rearrangement of the KMT2A gene (KMT2Ar). It carries a very poor prognosis despite a lack of additional oncogenic driver mutations common in childhood ALL. Here, we aimed to identify specific properties of human fetal hematopoietic stem/progenitor cells (HSPC) that promote leukemic transformation in KMT2Ar iALL using molecular, functional and in vivo assays. Through comparison of human fetal HSPC with adult HSPC transcriptomes we derived a fetal-specific gene signature and identified the fetal oncogene LIN28B and its downstream effectors among the top hits. These genes were also expressed in iALL. Functional assays revealed that LIN28B was essential in human fetal liver (FL) CD34+ cells to maintain proliferation and stem-like properties, and support B- and NK-lymphopoiesis. To interrogate the role of LIN28B in iALL, we utilized a human FL-derived CRISPR-Cas9 KMT2A::AFF1 model. In this CRISPRKMT2A::AFF1 model, human FL CD34+ cells fail to transform upon induction of KMT2A::AFF1 translocation in the absence of LIN28B. Furthermore, LIN28B-expressing CRISPRKMT2A::AFF1 leukemias were more proliferative in vitro and in vivo, with this advantage being lost upon LIN28B knockdown. Mechanistic studies showed that LIN28B acts by stabilizing key early B-lymphoid genes, epigenetic regulators, and cell cycle and anti-apoptotic genes. Thus, LIN28B has an essential role in normal human fetal B-lymphopoiesis, and is necessary for the initiation of KMT2A::AFF1 iALL in human fetal cells in the absence of co-operating mutations. LIN28B activity may help explain why KMT2A::AFF1 leukemias are so aggressive, making it a potential target in LIN28B-expressing leukemias.