Corrigendum to: Frequent microsatellite instability and BAX mutations in T cell acute lymphoblastic leukemia cell lines Leukemia Research 24 (2000), 255-262

The permeability transition pore complex in cancer cell death

The permeability transition pore (PTP) is a multi-protein complex at contact sites of the inner with the outer mitochondrial membrane. Research over the past years has led to the concept that the PTP occupies a central role in cell death induction. Numerous apoptosis signals convert this protein aggregate into an unspecific pore, thus activating mitochondria for the cellular self-destruction process. Here, we describe the evidence for this and the various approaches being undertaken to elucidate its subunit composition and mode of regulation. In particular, we review data that indicate a role of specific PTP subunits for apoptosis inhibition during tumorigenesis.

Methylation status of cyclin-dependent kinase inhibitor genes within the transforming growth factor beta pathway in human T-cell lymphoblastic lymphoma/leukemia

Epigenetic silencing of downstream components of the transforming growth factor beta pathway including the cyclin-dependent kinase inhibitors (CDKIs) p15INK4B, p27KIP1 and p21CIP1 is implicated in the pathogenesis of some hematological malignancies. Loss of p15INK4B expression due to promoter methylation occurs frequently in human T-cell acute lymphoblastic leukemia (T-ALL) but the expression and methylation status of p27KIP1 remains to be characterized in T-ALL or T-cell lymphoblastic lymphoma (T-LBL). As well, while some have reported a high frequency of p21CIP1 methylation in ALL patient samples others have found the gene to be unmethylated in this disease and the relationship between p21CIP1 expression and promoter methylation has not been examined in T-LBL. Using RNase protection assays (RPA) and methylation specific PCR (MSP), we found p27KIP1 to be expressed and its promoter unmethylated in 20 of 20 (100%) and 28 of 28 (100%) T-LBL/ALL samples, respectively. In contrast, p21CIP1 mRNA was absent in 7 of 14 (50%) T-LBL biopsies and 5 of 6 (83%) T-ALL cell lines. However, like p27KIP1 there was no evidence of p21CIP1 promoter methylation by MSP or temporal temperature gradient electrophoresis (TTGE) analysis of 35 CpG sites in any of the 28 T-LBL/ALLs analyzed. Similar to T-ALL, we found p15INK4B mRNA was absent in 13 of 14 (93%) T-LBL biopsies and its promoter methylated in 6 of 10 (64%) cases. Our results indicate that p21CIP1 mRNA is absent in human T-LBL biopsies and T-ALL cell lines at a high frequency. However, unlike p15INK4B, reduced p21CIP1 expression in T-LBL/ALL is independent of dense promoter-associated CpG methylation. In contrast to some hematological malignancies p27KIP1 methylation does not appear to contribute significantly to T-LBL/ALL pathogenesis.

Microsatellite mutations of transforming growth factor-beta receptor type II and caspase-5 occur in human precursor T-cell lymphoblastic lymphomas/leukemias in vivo but are not associated with hMSH2 or hMLH1 promoter methylation

In solid cancers, defective DNA mismatch repair (MMR) is most commonly caused by hMSH2 or hMLH1 mutations, or epigenetic silencing of hMLH1 by promoter hypermethylation, and results in the acquisition of characteristic frameshift microsatellite mutations of mononucleotide repeats located within the coding regions of defined target genes. We previously identified hMSH2 mutations in T-cell lymphoblastic lymphoma (T-LBL) patient tumor samples and others have reported coding region microsatellite mutations in T-cell acute lymphoblastic leukemia (T-ALL) cell lines. Thus, while MMR gene mutations are known to occur in some human T-lymphoblastic tumors in vivo, it is still unknown if the coding region microsatellite mutations detected in human cell lines also occur in vivo or if hMLH1 or hMSH2 promoter hypermethylation contributes to defective MMR in these tumors. We analyzed the TGFbetaRII (A)10 and caspase-5 (A)10 coding region repeats in 16 human T-LBL/ALL patient tumor samples and identified six with microsatellite mutations in one or both repeats. There was no evidence of hMSH2 or hMLH1 promoter methylation as assessed by standard methylation specific PCR or by a novel temporal temperature gradient electrophoresis (TTGE) method that analyzed 25 and 30 CpG sites in the hMLH1 and hMSH2 promoters, respectively. Our results indicate that coding region microsatellite mutations characteristic of defective MMR occur in some human T-LBL/ALL in vivo but not as a consequence of hMLH1 or hMSH2 promoter hypermethylation. Furthermore, the identification of TGFbetaRII and caspase-5 coding region mutations in vivo implicates these genes in the pathogenesis of human T-LBL/ALL.