Distinct mechanisms lead to HPRT gene mutations in leukemic cells

Collaboration of Hprt/K-RAS/c-Myc mutation in the oncogenesis of T-lymphocytic leukemia: a comparative study

Aim: Leukemia is a malignant clonal illness stem from the mutations of hematopoietic cells. Acute lymphoblastic leukemia is one of the utmost prevalent kinds of leukemia, is brought on by atypical lymphoid progenitor cell division in the bone marrow. Materials & methods: A comparative study between, titanium Nanoparticle-loaded doxorubicin or cisplatin and lactoferrin-loaded doxorubicin or cisplatin, on 7,12-dimethylbenz[a]-anthracene (DMBA)-induced leukemia was investigated and confirming the hypothesis that messenger RNA of Hprt/K-RAS/c-Myc/SAT-2/P53/JAK-2 is a forthcoming signaling pathways in leukemia. Results: A significant alteration in Hprt, K-RAS, C-Myc, P53, JAK-2 and SAT-2 genes was observed post DMBA intoxication the aforementioned Nanodrugs modulated these signaling pathways. Conclusion: The carrier-loaded drugs triggered cytotoxicity of cancer cells via enhancing drug efficacy and bio-availability.

DNA mismatch repair protein Mlh1 is required for tetravalent chromium intermediate-induced DNA damage

Hexavalent chromium (Cr[VI]) is associated with occupational lung cancer and poses a significant public health concern. When exposed to Cr[VI], cells rapidly internalize this compound and metabolize it to Cr[III]. Byproducts of Cr[VI] metabolism include unstable Cr[V] and Cr[IV] intermediates that are believed to be directly responsible for the genotoxicity and carcinogenicity caused by Cr[VI] exposure; however, the carcinogenic potential of the Cr intermediates and the mechanisms of Cr-induced carcinogenesis remain to be further defined. Utilizing synthetic Cr[IV] and Cr[V] compounds, we demonstrate here that Cr[IV] or Cr[V] exposure induces DNA double-strand breaks; however, of the two compounds, mammalian cells only respond to Cr[V]-induced DNA damage. Exposure to Cr[V], but not Cr[IV], results in initiation of cell cycle checkpoints and activates the ATM kinase, a critical regulator of the DNA damage response. Furthermore, cells exposed to Cr[IV] have significantly increased mutation frequencies in the HPRT gene compared to cells exposed to Cr[V], indicating that Cr[IV] possesses a higher mutagenic potential than Cr[V]. We also find that MLH1, a critical mismatch repair (MMR) protein, is required for activation of the G2/M cell cycle checkpoint in response to Cr[VI] exposure and to limit Cr-induced mutagenesis. Our results provide evidence for Cr[IV] as the ultimate mutagenic intermediate produced during Cr[VI] metabolism and indicate that functional MMR is crucial in the cellular response to chromium exposure.

Mutator activity induced by microRNA-155 (miR-155) links inflammation and cancer

Infection-driven inflammation has been implicated in the pathogenesis of ~15-20% of human tumors. Expression of microRNA-155 (miR-155) is elevated during innate immune response and autoimmune disorders as well as in various malignancies. However, the molecular mechanisms providing miR-155 with its oncogenic properties remain unclear. We examined the effects of miR-155 overexpression and proinflammatory environment on the frequency of spontaneous hypoxanthine phosphoribosyltransferase (HPRT) mutations that can be detected based on the resistance to 6-thioguanine. Both miR-155 overexpression and inflammatory environment increased the frequency of HPRT mutations and down-regulated WEE1 (WEE1 homolog-S. pombe), a kinase that blocks cell-cycle progression. The increased frequency of HPRT mutation was only modestly attributable to defects in mismatch repair machinery. This result suggests that miR-155 enhances the mutation rate by simultaneously targeting different genes that suppress mutations and decreasing the efficiency of DNA safeguard mechanisms by targeting of cell-cycle regulators such as WEE1. By simultaneously targeting tumor suppressor genes and inducing a mutator phenotype, miR-155 may allow the selection of gene alterations required for tumor development and progression. Hence, we anticipate that the development of drugs reducing endogenous miR-155 levels might be key in the treatment of inflammation-related cancers.

A quantitative analysis of genomic instability in lymphoid and plasma cell neoplasms based on the PIG-A gene

It has been proposed that hypermutability is necessary to account for the high frequency of mutations in cancer. However, historically, the mutation rate (mu) has been difficult to measure directly, and increased cell turnover or selection could provide an alternative explanation. We recently developed an assay for mu using PIG-A as a sentinel gene and estimated that its average value is 10.6 x 10(-7) mutations per cell division in B-lymphoblastoid cell lines (BLCLs) from normal donors. Here we have measured mu in human malignancies and found that it was elevated in cell lines derived from T cell acute lymphoblastic leukemia, mantle cell lymphoma, follicular lymphoma in transformed phase, and 2 plasma cell neoplasms. In contrast, mu was much lower in a marginal zone lymphoma cell line and 5 other plasma cell neoplasms. The highest mu value that we measured, 3286 x 10(-7), is 2 orders of magnitude above the range we have observed in non-malignant human cells. We conclude that the type of genomic instability detected in this assay is a common but not universal feature of hematologic malignancies.

Biased exon/intron distribution of cryptic and de novo 3' splice sites

We compiled sequences of previously published aberrant 3′ splice sites (3′ss) that were generated by mutations in human disease genes. Cryptic 3′ss, defined here as those resulting from a mutation of the 3′YAG consensus, were more frequent in exons than in introns. They clustered in ∼20 nt region adjacent to authentic 3′ss, suggesting that their under-representation in introns is due to a depletion of AG dinucleotides in the polypyrimidine tract (PPT). In contrast, most aberrant 3′ss that were induced by mutations outside the 3′YAG consensus (designated ‘de novo’) were in introns. The activation of intronic de novo 3′ss was largely due to AG-creating mutations in the PPT. In contrast, exonic de novo 3′ss were more often induced by mutations improving the PPT, branchpoint sequence (BPS) or distant auxiliary signals, rather than by direct AG creation. The Shapiro–Senapathy matrix scores had a good prognostic value for cryptic, but not de novo 3′ss. Finally, AG-creating mutations in the PPT that produced aberrant 3′ss upstream of the predicted BPS in vivo shared a similar ‘BPS-new AG’ distance. Reduction of this distance and/or the strength of the new AG PPT in splicing reporter pre-mRNAs improved utilization of authentic 3′ss, suggesting that AG-creating mutations that are located closer to the BPS and are preceded by weaker PPT may result in less severe splicing defects.

Mutator phenotype in cancer: timing and perspectives

Normal human cells replicate their DNA with exceptional accuracy. During every division cycle, each daughter cell receives a full and accurate complement of genetic information. It has been estimated that approximately one error occurs during DNA replication for each 10(9) to 10(10) nucleotides polymerized. Stem cells, the cells that are progenitors of cancer, may replicate their genes even more accurately. In contrast, the malignant cells that constitute a tumor are markedly heterogeneous and exhibit multiple chromosomal abnormalities and alterations in the nucleotide sequence of DNA. To account for the disparity between the rarity of mutations in normal cells and the large numbers of mutations present in cancer, we initially hypothesized that during tumor progression, cancer cells must exhibit a mutator phenotype. In this perspective, we summarize the evidence supporting a mutator phenotype in human cancer, analyze recent measurements of mutations in human cancer, consider the timing for the expression of a mutator phenotype, and focus on the important consequences of large numbers of random mutations in human tumors.