Normalizing complementary DNA by quantitative reverse transcriptase-polymerase chain reaction of beta2-microglobulin: molecular monitoring of minimal residual disease in acute promyelocytic leukemia.

Reverse transcription (RT)-polymerase chain reaction (PCR) raises unique methodological matters that may hamper the reliability of the procedure, especially when results should direct therapeutic decisions. One of these matters is represented by the RT step. The present study shows that differences in complementary DNA (cDNA) preparations purposely containing increasing amounts of retrotranscribed RNA were not disclosed by nonquantitative RT-PCR by two different housekeeping genes, leading to fictitious results when the expression of a given gene was quantitatively assessed. To overcome this problem, the following are proposed: 1) to evaluate the efficiency of RT step through the quantification, by competitive RT-PCR, of the expression levels of the housekeeping gene beta2-microglobulin (beta2M); 2) to normalize each cDNA preparation to be comprised within 1 standard deviation of the mean value of beta2M absolute level (3.14 +/- 1.14 attomoles/microg RNA) found by analyzing 33 cell lines of hematopoietic origin. To validate this strategy in a clinical setting, serial cDNA samples from patients were checked by conventional and quantitative RT-PCR for beta2M. Again, only a quantitative evaluation of beta2M levels was allowed to unveil significant differences, otherwise undetected, in the efficiency of RT reactions among these cDNA samples. Normalization of samples to obtain cDNA preparations containing comparable beta2M levels, eventually led to an increased sensitivity in the detection of PML-RARalpha fusion transcripts. This approach seems of great value for the monitoring of minimal residual disease in serial patient samples when a tumor-specific marker is available.

CD30 ligand (CD30L)-expressing acute myeloid leukemias: a new model of paracrine interactions for the regulation of blast cells proliferation.

CD30 ligand (CD30L) is a type-II membrane glycoprotein capable of transducing signals through its specific counterstructure CD30. Even though there are indications that CD30L plays a key role as a paracrine-acting surface molecule in the deregulated cytokine cascade of Hodgkin's disease, little is known about its biological functions in other human hemopoietic malignancies, despite the demonstration of the frequent expression of CD30L in hemopoietic neoplasms of both myeloid and lymphoid origin. The present review summarises structural and biological properties of CD30L, and focuses on CD30L+ acute myeloid leukemias (AMLs) by recapitulating some phenotypic and clinical features of this subset of acute leukemias. We also discuss some mechanisms by which CD30L-expressing leukemic blasts may gain a proliferative advantage through direct interaction with specific cells, in turn expressing its specific counterreceptor CD30. In particular, data has been provided suggesting that CD30L+ AMLs may evoke a sort of polarized T-cell response with the preferential production of Th2-like cytokines, mainly IL-4, by specific CD30-expressing T cell subsets. On the other hand, leukemic blasts presenting surface CD30L, have been shown to express a peculiar cytokine-receptors pattern that makes them an ideal target for T cells-produced Th2-like cytokines. Furthermore, some Th2-like cytokines, such as IL-4, are able to enhance blast cells proliferation, as well as to up-regulate the surface expression of specific adhesion molecules that have been shown to be associated with the presence of CD30L on AML blasts.

The RET receptor tyrosine kinase, but not its specific ligand, GDNF, is preferentially expressed by acute leukaemias of monocytic phenotype and is up-regulated upon differentiation.

The RET gene product represents the signal-transducing molecule of a surface receptor complex for the glial cell line-derived neurotrophic factor (GDNF), which includes GDNFR-alpha as a ligand-binding component. By a semi-quantitative competitive RT-PCR approach, we have analysed the relative abundances of RET transcripts in blasts purified from 40 acute myeloid leukaemia (AML) cases, revealing significant amounts of RET transcripts in 60% of AML cases (24/40). RT-PCR data was confirmed by immunocytochemical detection of RET protein in leukaemic blasts. The highest RET mRNA levels, almost exclusively confined to FAB M4/M5 AMLs, directly correlated with the presence on leukaemic cells of adhesion molecules and surface structures typically expressed by blasts of monocytic lineage and were inversely associated with the expression of the stem cell antigen CD34. Consistently, differentiation of the monoblastic cell line U937 resulted in an up-regulated expression of RET proto-oncogene, which was maximal upon exposure to agents inducing a more complete monocytic differentiation. Finally, while transcripts specific for GDNF and GDNFR-alpha were never found in leukaemic blasts, stromal cells of the haemopoietic microenvironment expressed, in the absence of RET, significant amounts of both GDNF and GDNFR-alpha. Our results suggest a role for RET in the functional regulation of AMLs through interactions with GDNF- and GDNFR-alpha-producing stromal cells.

Differential expression of the RET gene in human acute myeloid leukemia.

The RET proto-oncogene product is a receptor tyrosine kinase representing the signal-transducing molecule of a multi-subunit membrane receptor complex for at least two different types of transforming growth factor (TGF)-beta-related neurotrophic factors. We have previously shown that RET gene expression in acute myeloid leukemia (AML) occurs more frequently in AMLs displaying either a monocytic (FAB M4/M5) or intermediate-mature myeloid phenotype (FAB M2/M3) than in leukemias reflecting an earlier stage of myeloid differentiation (FAB M0/M1). To further verify the association between RET expression and the relative maturation stage of AML cells, we have performed a quantitative estimation of relative abundances of RET transcripts among various FAB subtypes of AMLs. By analyzing 13 AML samples and normal hematopoietic cells through a competitive-quantitative RT-PCR approach, we were able to show that the relative levels of RET-specific mRNAs continuously increase with blast cell maturation in human AML, i.e., the amounts of RET gene-specific transcripts differ among RET-expressing AMLs, being higher in the more differentiated FAB phenotypes. In addition, we provide evidence that the relative amounts of RET transcripts increase upon in vitro and in vivo differentiation of leukemic promyelocytes from FAB M3 AML patients, becoming overall comparable to those found in normal granulocytes. These results indicate that RET expression in human AMLs is maturation-associated, probably mirroring the developmental regulation of this gene during differentiation of normal hematopoietic cells.