MicroRNAs, stem cells and cancer stem cells

This review discusses the various regulatory characteristics of microRNAs that are capable of generating widespread changes in gene expression via post translational repression of many mRNA targets and control self-renewal, differentiation and division of cells. It controls the stem cell functions by controlling a wide range of pathological and physiological processes, including development, differentiation, cellular proliferation, programmed cell death, oncogenesis and metastasis. Through either mRNA cleavage or translational repression, miRNAs alter the expression of their cognate target genes; thereby modulating cellular pathways that affect the normal functions of stem cells, turning them into cancer stem cells, a likely cause of relapse in cancer patients. This present review further emphasizes the recent discoveries on the functional analysis of miRNAs in cancer metastasis and implications on miRNA based therapy using miRNA replacement or anti-miRNA technologies in specific cancer stem cells that are required to establish their efficacy in controlling tumorigenic potential and safe therapeutics.

Potential therapeutic applications of microRNAs in response to DNA damage in cancer stem cells

Following genomic insults, inefficient DNA repair mechanisms may transform stem cell to cancer stem cells that have extensive self-renewal and aberrant differentiation properties rendering them highly resistant to chemotherapeutic and ionizing radiations and thus may cause tumor relapse in patients. Mature microRNAs (miRNAs), new class of modulators are single-stranded RNA molecules of 20-23 nucleotide (nt) length can form specific base interactions with the mRNAs of the genes and mediate the biological processes ranging from tumor development, maintenance of stem cell phenotype and stress responses. Due to differential functional interactions of miRNAs in cancer stem cells versus normal and differentiated cells, various miRNA therapeutic methods may provide a promising tool to address aberrant miRNA expression and specifically targeting cancer stem cells to cure cancer. Manipulating miRNAs activities by using either antagomirs for knock down the oncogenic properties of miRNAs or miRNA mimics to restore the tumor suppressor functions in order to correct their aberrant transcript levels that can regulate cellular response to damage by modulating apoptotic, cell cycle proliferation, DNA repair, invasion and differentiation functions, are also being examined. The current article also focuses some of the potential innovative strategies to overcome the major concerns for the clinical RNAi translation, the delivery and safety. Future studies should be aimed to establish multiple clinical trials with the novel gene delivery methods with high transfection efficiency, minimum toxicity and tumor targeted ability for miRNA based therapy.

Gain of antitumor functions and induction of differentiation in cancer stem cells contribute to complete cure and no relapse

The progeny of the mutated cell is maintained in a self-renewing tissue stem cell and its immediate progeny or due to cellular components that display stem cell like properties known as cancer stem cells (CSCs) that grow into cancer. Failure to achieve complete and safe eradication of cancer is due to the presence of quiescent population of cancer stem cells. Understanding and interfering with the processes of transformation of normal stem cells into their malignant counterparts, their micro-environmental niche and the mechanisms responsible for therapeutic resistance in CSCs enhance the curative efficacy of clinical elimination of all tumor cells. Functional identification of the molecular targets of CSCs provides useful insights for characterization of their response to particular therapy protocols. This review also focuses on the potential implications of promising novel therapies in suppression of oncogenic pathways specific to CSCs including their self-renewal capacity, property of drug transportation, seeding and oxidative stress, activation of apoptosis through the gain of antitumor molecules, restoration of DNA repair mechanisms and differentiation therapy that allows the terminal differentiation of CSCs thereby depleting their pool, all contribute to long-term disease control and improved patient survival. Significant developments in the optimization of combinational CSC-targeted therapies would lead to the improved outcome with great difference thus substantiating its profound impact in disease free survival with no relapse.

Mismatch repair deficiencies transforming stem cells into cancer stem cells and therapeutic implications

For the exceptional self-renewal capacity, regulated cell proliferation and differential potential to a wide variety of cell types, the stem cells must maintain the intact genome. The cells under continuous exogenous and endogenous genotoxic stress accumulate DNA errors, drive proliferative expansion and transform into cancer stem cells with a heterogeneous population of tumor cells. These cells are a common phenomenon for the hematological malignancies and solid tumors. In response to DNA damage, the complex cellular mechanisms including cell cycle arrest, transcription induction and DNA repair are activated. The cells when exposed to cytotoxic agents, the apoptosis lead to cell death. However, the absence of repair machinery makes the cells resistant to tumor sensitizing agents and result in malignant transformation. Mismatch repair gene defects are recently identified in hematopoietic malignancies, leukemia and lymphoma cell lines. This review emphasizes the importance of MMR systems in maintaining the stem cell functioning and its therapeutic implications in the eradication of cancer stem cells and differentiated tumor cells as well. The understanding of the biological functions of mismatch repair in the stem cells and its malignant counterparts could help in developing an effective novel therapies leaving residual non-tumorigenic population of cells resulting in potential cancer cures.

Microsatellite instability and its correlation with clinicopathological features in a series of thyroid tumors prevalent in iodine deficient areas

Thyroid tumors display diverse spectrum of histopathological groups with geographic variation in its prevalence. Influence of iodine deficiency (a major causative factor) in its etiology, prevalence, or aggressiveness is debatable which reflects the existence of various genetic events in pathogenesis. The present study was undertaken to study the role of Microsatellite instability (MSI) or LOH (loss of heterozygosity), an indicator of defective mismatch repair system as a genetic change and to explore it as a prognostic marker in thyroid tumors. Tumor tissues from total thyroidectomy surgical specimens and blood (matched control) of 36 patients from iodine deficient areas (10 benign; 26 malignant) were obtained after their consent. Urinary iodine analysis was done by alkali ash method for which 10 ml of urine was collected from 18 patients before surgery. Genomic DNA, isolated from tumor tissue and blood was amplified by polymerase chain reaction (PCR) using mono and dinucleotide markers - BAT-26, BAT-40, TGF(RII, IGFIIR, hMSH3, BAX, D2S123, D9S283, D9S851 and D18S58. PCR products were analysed on 8% denaturing polyacrylamide gel followed by autoradiography. Of total, 66.6% of tumors [70% (7/10) benign and 65.4% malignant cases (17/26)] showed MSI/LOH. Strong association of MSI/LOH with low iodine (P = 0.01) and with AMES risk groups i.e. age (P = 0.02), tumor size (P = 0.04) and metastases (P = 0.002) in thyroid tumors was observed. This may help in predicting the biological behaviour and strengthening the hypothesis that iodine deficiency has influence on MSI in thyroid tumors. Our results further substantiate the risk group classification and help in deciding the treatment modality in particular patient.

Microsatellite instability as prognostic marker in bladder tumors: a clinical significance

BACKGROUND

Carcinoma of urinary bladder is one of the leading causes of death in India. Successful treatment of bladder cancer depends on the early detection & specific diagnostic approaches. In the present study, microsatellite instability (MSI) has been evaluated as a prognostic marker in patients with superficial urinary bladder cancer in lower urinary tract for determining risk of recurrence.

METHODS

A total of 44 patients with bladder tumors diagnosed with Transitional Cell Carcinomas [TCC] from lower urinary tract were selected for the study. Tumors were staged and graded according to AJCC-UICC (1997) classification and patients were followed with cystoscopy as per the protocol. Polymerase chain reaction (PCR) was done to amplify microsatellite sequences at mononucleotide BAT - 26, BAT - 40, TGFbeta RII, IGFIIR, hMSH3, BAX and dinucleotide D2S123, D9S283, D9S1851 and D18S58 loci in blood (control) and tumor DNA. PCR products were separated on 8% denaturing polyacrylamide gel and visualized by autoradiography.

RESULTS

MSI was observed in 72.7% of tumors at BAT - 26, BAT - 40, D2S123, D9S283, D9S1851 and D18S58 loci. Good association of MSI was seen with tumor stage and grade. MSI - High (instability at > 30% of loci) was frequently observed in high stage (40.6%) and high grade (59.4%) tumors. Of 24 tumors of Ta-T1 stage with different grades, 11 (9/18 high grade and 2/6 low grade tumors) recurred in the mean duration of 36 months. MSI positivity was significantly high in patients who had one or more recurrences (p = 0.02 for high grade and 0.04 for low grade tumors).

CONCLUSIONS

MSI may be an independent prognostic marker for assessing risk of recurrence in superficial tumors irrespective of the grade. Further studies on progression would help in stratifying the patients of T1G3 for early cystectomy vs bladder preservation protocol.

Evaluation of microsatellite instability in tumors of central nervous system: A pilot study

Microsatellite instability (MSI) characterized by alterations at simple repetitive genomic sequences is a distinct mechanism in tumorogenesis. Central nervous system (CNS) tumors have been reported to exhibit MSI, indicator of defective mismatch repair system with controversies. The present study was undertaken to examine sixteen primary brain and two spinal tumors for MSI at six mono: BAT-26, BAT-40, BAX, TGFßRII, IGFIIR and hMSH3 and four dinucleotide loci: D2S123, D9S1851, D9S283 and D18S58. Polymerase chain reaction (PCR) was done to amplify tumour and blood DNA, analyzed on 8% denaturing Polyacrylamide gel followed by autoradiography. Out of 18 CNS tumors examined, 39% exhibited MSI at BAT-26, BAT-40, D9S1851, D9S283 and D18S58 in tumoral DNA. However, no alteration was observed at BAX, TGFßRII, IGFIIR, hMSH3 and D2S123 loci. Low incidence of MS1-high hypothesizes role of MSI in evolution of CNS tumors but not in cancer initiation or progression.

Assessment of microsatellite instability in bladder and thyroid malignancies

Microsatellite instability (MSI) is an indicator of a defective DNA mismatch repair system (MMR) that results from somatic mutations. The present work has been planned to investigate MSI and its clinical significance in human urinary bladder and thyroid cancers in Indian patients. Tumor tissues of histologically confirmed cases of urinary bladder and thyroid cancers, respectively, were obtained. Clinical data on tumor stage and histopathological grades were recorded. Corresponding matched peripheral blood was taken as a control. Genomic DNA was isolated from the tumor tissues and blood using a standard phenol-chloroform extraction method. Polymerase chain reaction was done to amplify mononucleotide microsatellite markers, BAT-26, BAT-40, TGFbetaRII, IGFIIR, hMSH3, and Bax by using specific primer sequences. For analysis of allelic patterns, the PCR products were run on 8% denaturing Polyacrylamide gel and sizing was done using a pUC18 sequencing ladder. The instability with BAT-26 and BAT-40 was found to be 20% and 45% in urinary bladder and 33% and 19% in thyroid cancers, respectively. However, no instability was observed with the other four-mononucleotide markers in either of the cancers studied. Eighty-three percent of the unstable urinary bladder cancers were found to have a high grade in a superficial group, whereas only 27% MSI+ve were muscle invasive cancers. Forty percent of unstable thyroid lesions were found to be at high risk of developing metastasis. Association of BAT-26 and BAT-40 instabilities with high grade tumors as well as risk tumors may help in choosing a more definite therapy at the outset.

DNA mismatch repair, microsatellite instability and cancer

Mismatch (MMR) repair system plays a significant role in restoration of stability in the genome. Mutations in mismatch repair genes hamper their activity thus bring about a defect in mismatch repair (MMR) mechanism thereby conferring instability in the microsatellite sequences of both the coding and non-coding regions of the genome. Mutated mismatch repair genes result in the expansion or contraction of microsatellite sequence and confer microsatellite unstable or replication error positive phenotype. Hypermethylation of promoter regions of some of the MMR genes also causes inactivation of these genes and thus contribute to MSI. Microsatellite instability is an indicator of MMR deficiency and is a prime cause of varied tumorogenesis.

Allelic variation of BAT-26 and BAT-40 poly-adenine repeat loci in North Indians

Analysis of mononucleotide repeats BAT-26 and BAT-40 in North Indians revealed that there were germline polymorphisms at both the loci. We evaluated BAT-26 and BAT-40 in 100 normal healthy individuals from North India. The DNA from normal blood was PCR amplified using primers flanking the BAT-26 and BAT-40 loci. The allelic variation of BAT-26 and BAT-40 ranged between 117-130 and 94-112 bp respectively. The most frequent BAT-26 allele was 122 bp, which corresponded to 26 repeats and had a frequency of 32% while that of BAT-40 was 109 bp corresponding to 39 repeats with a frequency of 26%. These results suggest that polymorphisms in these poly-adenine repeat loci limit their applicability in studying the microsatellite instability in cancers.