High-penetrance mouse model of acute promyelocytic leukemia with very low levels of PML-RARα expression

Abstract

Transgenic mice expressing PML-RAR␣ in early myeloid cells under control of human cathepsin G regulatory sequences all develop a myeloproliferative syndrome, but only 15% to 20% develop acute promyelocytic leukemia (APL) after a latent period of 6 to 14 months. However, this transgene is expressed at very low levels in the bone marrow cells of transgenic mice. Because the transgene includes only 6 kb of regulatory sequences from the human cathepsin G locus, we hypothesized that sequences required for high-level expression of the transgene might be located elsewhere in the cathepsin G locus and that a knock-in model might yield much higher expression levels and higher penetrance of disease. We, therefore, targeted a human PML-RAR␣ cDNA to the 5 untranslated region of the murine cathepsin G gene, using homologous recombination in embryonic stem cells. This model produced a high-penetrance APL phenotype, with more than 90% of knock-in mice developing APL between 6 and 16 months of age. The latent period and phenotype of APL (including a low frequency of an interstitial deletion of chromosome 2) was similar to that of the previous transgenic model. Remarkably, however, the expression level of PML-RAR␣ in bone marrow cells or APL cells was less than 3% of that measured in the low-penetrance transgenic model. Although the explanation for this result is not yet clear, one hypothesis suggests that very low levels of PML-RAR␣ expression in early myeloid cells may be optimal for the development

PML-RARA can increase hematopoietic self-renewal without causing a myeloproliferative disease in mice

“…3,4). This leukemia is typically associated with antecedent myeloproliferation (increased promyelocytes and CFUs), requires cooperating progression mutations, and generally presents with high white blood cell counts (7,8,24,25). Because human APL is characterized by low white blood cell counts and an acute onset without antecedent myeloproliferation (1), we sought to more accurately model human APL by placing PML-RARA under the regulatory control of the Pml locus, in the setting of somatic acquisition of expression and Pml haploinsufficiency.…”

“…We compared this expression to our previous model of APL, in which the same PML-RARA cDNA is expressed from the Ctsg locus using a knockin strategy (herein referred to as mCG-PR mice), which results in constitutive PML-RARA expression in hematopoietic progenitors, promyelocytes, and PMNs (peak expression is in promyelocytes) (7). The mPML-fPR allele yielded lower PML-RARA mRNA expression, consistent with the fact that the Pml gene is expressed at much lower levels than the Ctsg gene during normal hematopoiesis (Welch, et al, unpublished observations).…”

Redundant function of retinoic acid receptor isoforms in leukemogenesis unravels a prominent function of genome topology and architecture in the selection of mutagenic events in cancer

“…However, the tight correlation between cyclin A expression and BrdU incorporation favors the idea that the in vivo kinase activity in cyclin A-expressing cells was functional, as cells lacking this activity could not progress through the S phase. 7 According to the results of this study, there was no evidence of an accumulation of cyclin A-positive cells that are unable to progress through the S phase, and therefore lack BrdU incorporation, or of an accumulation of BrdU-positive cells that could not progress through the S phase because of deficient cyclin A expression.…”

“…Notably, in our previous analysis of PML-RARa driven leukaemia, we have observed a shorter median latency (173 days), and a higher penetrance of the disease (75%). The disease phenotype is highly dependent on the level of expression of the transgene, and PML-RARa in some cases behaves as a weaker oncogene: 7 we tend therefore to interpret the differences observed as mainly to be ascribed to slight differences in expression among PML-RARa and PML-RARg, rather than authentic biological differences among the fusion proteins. APL is characterized by its unique responsiveness to retinoic acid (RA).…”

Review: genetic models of acute myeloid leukaemia

“…Employing this methodology a knock-in model of PML/RARa t(15;17) using the endogenous murine cathepsin G-promoter was generated with 90% incidence of APL in comparison to only 15-20% prevalence observed in the relative hCG-PML/RARa, with similar pathology (Westervelt et al, 2003). The tetracycline (tet-) inducible (Gossen and Bujard, 1992;Gossen et al, 1995;Jaisser, 2000;Ryding et al, 2001) and Cre/loxPmediated (Muller, 1999;Jaisser, 2000;Testa and Stewart, 2000;Ryding et al, 2001;Yu and Bradley, 2001) interchromosomal translocation recombination systems have been utilized in generation of inducible models of AML1/ETO t(8;21) (Buchholz et al, 2000;Higuchi et al, 2002;Rhoades et al, 2000), MLL-CBP t(11;16) , MN1-TEL t(12 ;22) (Kawagoe and Grosveld, 2005) and MLL/AF9 t(9;11) (Collins et al, 2000).…”