Recent Advances in the Biology and Treatment of Acute Promyelocytic Leukemia, страница 2

The defining molecular event in APL is disruption of the retinoic acid receptor alpha (RARa) gene at 17q21, and its fusion with one of four partner genes (Table 1). In 99.9% of cases, the RARa fusion partner is the PML (for ProMyelo-cytic Lenkemia) gene, located at 15q22. The PML-RAKtt fusion gene (Fig 1) is the molecular counterpart of the classic t(15;17), originally described by Rowley et al¹⁰ in 1977. The structure/ftmction of PML and RARa, and the role of PML-RARa in APL pathogenesia, have been reviewed recently.¹¹"¹⁶ Several of the most notable, and clinically relevant, advances in our understanding of the molecular pathogenesis of APL will be briefly presented here.

RARa Gene Disruption is Central to the Pathogenesis of APL

Three novel RARa fusion partners have recently been described in cases of leukemia thatmorphologically resemble APL (Table 1); PLZF (for promyelocytic leukemia zinc finger),¹⁶'¹⁷ NPM (nucleophosmin),¹⁸'¹⁹ and NuMA (nuclear mitotic apparatus protein),²⁰-²¹ These cases are extremely rare: only one case of NuMA-RARa and three cases ofNPM-RARce are known to exist (S. Kamel-Reid and R. Redner, personal communication, January 1998). PLZF-RARa is slightly more common, with approximately 12 cases worldwide (J. Licht, personal communication, January 1998). Structurally, PLZF, NPM, and NuMA (and PML) are quite different proteins; however, all are predominantly nuclear in location and contain N-terminal protein-protein interaction domains. As is true with PML-RARa (Fig 1), ammo terminal sequences from either PLZF, NPM, or NuMA are fused with the B through F domains of RARa to form the oncogenic fusion protein. This fact argues persuasively that breakage of the RARa gene between the A and B domains is necessary for thedevelopment of APL, and that disruption of the retinoid signaling pathway is the key pathoge-netic feature of APL. Clinically, APL patients (and cells) with PLZF-RARa do not respond to ATRA,¹⁷ while the one published case each ofNPM-RARa and NuMA-RARot APL appeared to be ATRA-responsive.¹⁹'²⁰

Both PML-RARa and PLZF-RARa Can Cause an APL-Like Disease in Transgenic Mice

PML-RARot can cause anAPL-like disease when expressed under the control of myeloid-specific promoters in transgenic mice.²²-²⁵ However, it is worth noting that only a fraction of mice develop leukemia, and then only after a prolonged latency period. This result suggests either that PML-RARn alone is insufficient to cause APL (ie, additional genetic changes are necessary), or that the expression vectors used do not target PML-RAR« to the appropriate cell type in vivo. Additional genetic changes that may contribute to the development of APL are unknown, but possible candidates are the reciprocal fusion protein, RARa-PML, or the truncated PML protein(s) that have been described in some APL patients.²⁶ Recently, transgenic mice have been generated that express both PML-RARa and RARa-PML, and preliminary results suggest that such mice have an increased penetrance of the APL phenotype.²⁷ PLZF-RARci has also been shown to cause my-eloid leukemia in transgenic mice.²⁸ The leukemia that develops in PLZF-RARa transgenic mice is relatively resistant to ATRA, while that which develops in PML-RARot mice is ATRA-respon-sive,²⁹ a finding which reproduces in large measure the human phenotype. These studies conclusively demonstrate the potential of PML-RARa and PLZF-RARa to disrupt hematopoiesis in vivo, and offer strong support for the premise that both can create conditions conducive to the development of APL in humans.

PML-RARa Is Degraded by ATRA, but Is Also an ATRA-Dependent Transcriptional Enhancer

The biologic fact of APL suggests that a signal mediated through RARa or one of its heterodi-meric partners is required for normal myeloid maturation. Thus, while normal promyelocytes may undergo differentiation in response to physiologic levels of retinoids, far higher retinoic add concentrations are necessary to overcome thedifferentiation block imposed by PML-RARa. A number of different hypotheses could account for this observation, but two recent reports have suggested a rather simple and elegant explanation: that PML-RARa, but not endogenous RARa, is degraded in response to ATRA.³⁰'³¹ This result offers a unifying, if not yet validated, hypothesis to explain the sensitivity of leukemic promyelo-cytes to ATRA, ie, ATRA-induced loss of PML-RARa restores a normal balance of endogenous retinoid receptors (RAR/RXR heterodimers). Para-doxically, PLZF-RARci, the fusion protein associated with ATRA-resistant t<ll;17)-positive APL, also appears to be degraded by ATRA.³² This surprising finding, along with several contradictory reports regarding PML-RARct and RAR« regulation by ATRA in vitro,³³'³⁴ suggest that some caution is necessary ia accepting the premise that PML-RARa degradation is solely responsible for the sensitivity of APL cells to ATRA. For example, there is evidence that PML-RARcx itself can act as a retinoic acid-dependent transcriptional enhancer,³⁶-³⁷ suggesting that it is the presence of the PML-RARa molecule itself that is responsible for the sensitivity of 15;17-positive APL cells to ATRA. Further work is necessary to differentiate between these two different, though perhaps not mutually exclusive, hypotheses.