Retinoids in hematology: a timely revival?

The retinoic acid receptors (RARA, RARB, and RARG) are ligand-regulated nuclear receptors that act as transcriptional switches. These master genes drew significant interest in the 1990s because of their key roles in embryogenesis and involvement in a rare malignancy, acute promyelocytic leukemia (APL), in which the RARA (and very rarely, RARG or RARB) genes are rearranged, underscoring the central role of deregulated retinoid signaling in leukemogenesis. Several recent provocative observations have revived interest in the roles of retinoids in non-APL acute myeloid leukemia (AML), as well as in normal hematopoietic differentiation.

Interconversion between Tumorigenic and Differentiated States in Acute Myeloid Leukemia.

Tumors are composed of phenotypically heterogeneous cancer cells that often resemble various differentiation states of their lineage of origin. Within this hierarchy, it is thought that an immature subpopulation of tumor-propagating cancer stem cells (CSCs) differentiates into non-tumorigenic progeny, providing a rationale for therapeutic strategies that specifically eradicate CSCs or induce their differentiation. The clinical success of these approaches depends on CSC differentiation being unidirectional rather than reversible, yet this question remains unresolved even in prototypically hierarchical malignancies, such as acute myeloid leukemia (AML).

-ITD impedes retinoic acid, but not arsenic, responses in murine acute promyelocytic leukemias.

Acute promyelocytic leukemia (APL) is often associated with activating FLT3 signaling mutations. These are highly related to hyperleukocytosis, a major adverse risk factor with chemotherapy-based regimens. APL is a model for oncogene-targeted therapies: retinoic acid (ATRA) and arsenic both target and degrade its ProMyelocytic Leukemia/Retinoic Acid Receptor α (PML/RARA) driver.

p53 as an Effector or Inhibitor of Therapy Response.

Although integrity of the p53 signaling pathway in a given tumor was expected to be a critical determinant of response to therapies, most clinical studies failed to link p53 status and treatment outcome. Here, we present two opposite situations: one in which p53 is an essential effector of cure by targeted leukemia therapies and another one in advanced breast cancers in which p53 inactivation is required for the clinical efficacy of dose-dense chemotherapy. If p53 promotes or blocks therapy response, therapies must be tailored on its status in individual tumors.

Paleovirology of 'syncytins', retroviral env genes exapted for a role in placentation.

The development of the emerging field of 'paleovirology' allows biologists to reconstruct the evolutionary history of fossil endogenous retroviral sequences integrated within the genome of living organisms and has led to the retrieval of conserved, ancient retroviral genes 'exapted' by ancestral hosts to fulfil essential physiological roles, syncytin genes being undoubtedly among the most remarkable examples of such a phenomenon. Indeed, syncytins are 'new' genes encoding proteins derived from the envelope protein of endogenous retroviral elements that have been captured and domesticated on multiple occasions and independently in diverse mammalian species, through a process of convergent evolution. Knockout of syncytin genes in mice provided evidence for their absolute requirement for placenta development and embryo survival, via formation by cell-cell fusion of syncytial cell layers at the fetal-maternal interface.

Differential evolutionary fate of an ancestral primate endogenous retrovirus envelope gene, the EnvV syncytin, captured for a function in placentation.

Syncytins are envelope genes of retroviral origin that have been co-opted for a role in placentation. They promote cell-cell fusion and are involved in the formation of a syncytium layer--the syncytiotrophoblast--at the materno-fetal interface. They were captured independently in eutherian mammals, and knockout mice demonstrated that they are absolutely required for placenta formation and embryo survival.

A placenta-specific receptor for the fusogenic, endogenous retrovirus-derived, human syncytin-2.

Syncytin-2 is an envelope gene from the human endogenous retrovirus FRD (HERV-FRD) co-opted by an ancestral primate host, conserved in evolution over >40 Myr, specifically expressed in the placenta, and with a cell-cell fusogenic activity likely contributing to placenta morphogenesis. Here, using the GeneBridge4 human/Chinese hamster radiation hybrid panel, we mapped and identified the human receptor for syncytin-2. This receptor-namely Major Facilitator Superfamily Domain Containing 2 (MFSD2)-belongs to a large family of presumptive carbohydrate transporters with 10-12 membrane-spanning domains, is located at chromosomal position 1p34.

Restriction by APOBEC3 proteins of endogenous retroviruses with an extracellular life cycle: ex vivo effects and in vivo "traces" on the murine IAPE and human HERV-K elements.

Background: APOBEC3 cytosine deaminases have been demonstrated to restrict infectivity of a series of retroviruses, with different efficiencies depending on the retrovirus. In addition, APOBEC3 proteins can severely restrict the intracellular transposition of a series of retroelements with a strictly intracellular life cycle, including the murine IAP and MusD LTR-retrotransposons.

Results: Here we show that the IAPE element, which is the infectious progenitor of the strictly intracellular IAP elements, and the infectious human endogenous retrovirus HERV-K are restricted by both murine and human APOBEC3 proteins in an ex vivo assay for infectivity, with evidence in most cases of strand-specific G-to-A editing of the proviruses, with the expected signatures.

Uracil within DNA: an actor of antiviral immunity.

Uracil is a natural base of RNA but may appear in DNA through two different pathways including cytosine deamination or misincorporation of deoxyuridine 5'-triphosphate nucleotide (dUTP) during DNA replication and constitutes one of the most frequent DNA lesions. In cellular organisms, such lesions are faithfully cleared out through several universal DNA repair mechanisms, thus preventing genome injury. However, several recent studies have brought some pieces of evidence that introduction of uracil bases in viral genomic DNA intermediates during genome replication might be a way of innate immune defence against some viruses.

The GLN family of murine endogenous retroviruses contains an element competent for infectious viral particle formation.

Several families of endogenous retroviruses (ERVs) have been identified in the mouse genome, in several instances by in silico searches, but for many of them it remains to be determined whether there are elements that can still encode functional retroviral particles. Here, we identify, within the GLN family of highly reiterated ERVs, one, and only one, copy that encodes retroviral particles prone to infection of mouse cells. We show that its envelope protein confers an ecotropic host range and recognizes a receptor different from mCAT1 and mSMIT1, the two previously identified receptors for other ecotropic mouse retroviruses.

High-molecular-mass APOBEC3G complexes restrict Alu retrotransposition.

APOBEC3G (A3G) and related deoxycytidine deaminases are potent intrinsic antiretroviral factors. A3G is expressed either as an enzymatically active low-molecular-mass (LMM) form or as an enzymatically inactive high-molecular-mass (HMM) ribonucleoprotein complex. Resting CD4 T cells exclusively express LMM A3G, where it functions as a powerful postentry restriction factor for HIV-1.

Dual inhibitory effects of APOBEC family proteins on retrotransposition of mammalian endogenous retroviruses.

We demonstrated previously that the cytosine deaminase APOBEC3G inhibits retrotransposition of two active murine endogenous retroviruses, namely intracisternal A-particles (IAP) and MusD, in an ex vivo assay where retrotransposition was monitored by selection of neo-marked elements. Sequencing of the transposed copies further disclosed extensive editing, resulting in a high load of G-to-A mutations. Here, we asked whether this G-to-A editing was associated with an impact of APOBEC3G on viral cDNA yields.

APOBEC3G cytidine deaminase inhibits retrotransposition of endogenous retroviruses.

Endogenous retroviruses are multicopy retroelements accounting for nearly 10% of murine or human genomes. These retroelements spread into our ancestral genome millions of years ago and have acted as a driving force for genome evolution. Endogenous retroviruses may also be deleterious for their host, and have been implicated in cancers and autoimmune diseases.

The effects of HIV-1 Nef on CD4 surface expression and viral infectivity in lymphoid cells are independent of rafts.

The HIV-1 Nef protein is a critical virulence factor that exerts multiple effects during viral replication. Nef modulates surface expression of various cellular proteins including CD4 and MHC-I, enhances viral infectivity, and affects signal transduction pathways. Nef has been shown to partially associate with rafts, where it can prime T cells for activation.

LINE-mediated retrotransposition of marked Alu sequences.

Alu elements are the most successful transposons in humans. They are 300-bp non-coding sequences transcribed by RNA polymerase III (Pol III) and are expected to retrotranspose with the aid of reverse transcriptases of cellular origin. We previously showed that human LINEs can generate cDNA copies of any mRNA transcript by means of a retroposition process involving reverse transcription and integration by the LINE-encoded endonuclease and reverse transcriptase.

A Tetrahymena thermophila ribozyme-based indicator gene to detect transposition of marked retroelements in mammalian cells.

We devised an indicator gene for retrotransposition based on an autocatalytic ribozyme element--the Tetrahymena thermophila 23S rRNA group I intron--which can self-splice in vitro and does not require--at variance with nuclear mRNA introns--any specific pathway and cellular component for the completion of the splicing process. Several constructs, with the Tetrahymena intron adequately modified so as to be inserted at various positions within a neomycin-containing cassette under conditions that restore the neomycin-coding sequence after splicing out of the intron, were assayed for splicing efficiency in mammalian cells in culture. We show, both by northern blot analysis and by the recovery of neomycin activity upon retroviral transduction of the cassettes, that splicing efficiency depends on both the local base pairing and the global position of the intron within the neomycin transcript, and that some constructs are functional.