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Biomedical subjects

Harith Rajagopalan

Publications and source records attributed to Harith Rajagopalan.

8 recordsLinked to original sources

Aneuploidy and cancer.

In contrast to normal cells, aneuploidy--alterations in the number of chromosomes--is consistently observed in virtually all cancers. A growing body of evidence suggests that aneuploidy is often caused by a particular type of genetic instability, called chromosomal instability, which may reflect defects in mitotic segregation in cancer cells. A better understanding of the molecular mechanisms leading to aneuploidy holds promise for the development of cancer drugs that target this process.

Aneuploidy↗

hCDC4 and genetic instability in cancer.

About twenty years ago, scientists began to discover that colorectal cancers are caused by the sequential acquisition of genetic alterations in specific genes. To this day, we are still dissecting the genome of colorectal cancers to identify specific "culprit" genes that play a role in tumorigenesis. At the same time, we have more recently begun to turn our attention to the features of cancer cells that distinguish them from normal cells and that may be targeted therapeutically. Aneuploidy is one such hallmark of cancers, but its role in tumorigenesis is heretofore undetermined. Our efforts have focused on elucidating the fundamental mechanisms underlying aneuploidy. The assertion that a genetic basis for aneuploidy would imply its importance in tumorigenesis, and consequently make it a potential therapeutic target, represents the rationale for our pursuit of this line of research. For the last few years, we have been trying to determine whether there is a genetic cause underlying this attribute of cancers. Our recently published work entitled, "Inactivation of hCDC4 can cause chromosomal instability," attempts to address this issue and raises more questions about the cause, mechanism, timing, and therapeutic potential of genetic instability.

Cell Cycle Proteins↗

Inactivation of hCDC4 can cause chromosomal instability.

Aneuploidy, an abnormal chromosome number, has been recognized as a hallmark of human cancer for nearly a century; however, the mechanisms responsible for this abnormality have remained elusive. Here we report the identification of mutations in hCDC4 (also known as Fbw7 or Archipelago) in both human colorectal cancers and their precursor lesions. We show that genetic inactivation of hCDC4, by means of targeted disruption of the gene in karyotypically stable colorectal cancer cells, results in a striking phenotype associated with micronuclei and chromosomal instability. This phenotype can be traced to a defect in the execution of metaphase and subsequent transmission of chromosomes, and is dependent on cyclin E--a protein that is regulated by hCDC4 (refs 2-4). Our data suggest that chromosomal instability is caused by specific genetic alterations in a large fraction of human cancers and can occur before malignant conversion.

Aneuploidy↗

Linear model of colon cancer initiation.

Cancer results if regulatory mechanisms of cell birth and death are disrupted. Colorectal tumorigenesis is initiated by somatic or inherited mutations in the APC tumor suppressor gene pathway. Several additional genetic hits in other tumor suppressor genes and oncogenes drive the progression from polyps to malignant, invasive cancer. The majority of colorectal cancers present chromosomal instability, CIN, which is caused by mutations in genes that are required to maintain chromosomal stability. A major question in cancer genetics is whether CIN is an early event and thus a driving force of tumor progression. We present a new mathematical model of colon cancer initiation assuming a linear flow from stem cells to differentiated cells to apoptosis. We study the consequences of mutations in different cell types and calculate the conditions for CIN to precede APC inactivation. We find that early emergence of CIN is very likely in colorectal tumorigenesis.

Cell Transformation, Neoplastic↗

CIN-ful cancers.

Aneuploidy has long been recognized to be a cardinal feature of many neoplasias. However, the role of aneuploidy in tumorigenesis continues to be a matter of debate. We believe that aneuploidy in cancers is the result of chromosomal instability, a process in which dividing cancer cells segregate their chromosomes with decreased fidelity. Here we discuss our definition of chromosomal instability, evidence for its causal role in tumor development, and suggestions regarding the mechanisms that initiate chromosomal instability in cancer cells.

Aneuploidy↗

Histone modifications and silencing prior to DNA methylation of a tumor suppressor gene.

We attempted to answer two central questions about epigenetic silencing of the tumor suppressor gene p16(INK4a) in this study: (1) whether the maintenance of associated histone modifications is dependent on DNA methylation and (2) whether such histone modifications can occur prior to DNA methylation. By coupling chromatin immunoprecipitation with gene targeting and the analysis of specific alleles, we found that elimination of DNA methylation from a p16(INK4a) allele resulted in profound changes in surrounding histones. After continued passage of such cells, methylation of histone H3 lysine-9 occurred in conjunction with re-silencing in the absence of DNA methylation. These results have important implications for understanding the biochemical events underlying the silencing of tumor suppressor genes and the resultant growth suppression.

Cell Division↗

The significance of unstable chromosomes in colorectal cancer.

A very large fraction of cancers have an abnormal genetic content, called aneuploidy, which is characterized by changes in chromosome structure and number. One explanation for this aneuploidy is chromosomal instability, in which cancer cells gain or lose whole chromosomes or large fractions of chromosomes at a greatly increased rate compared with normal cells. Here, we explore experimental and theoretical evidence for the initiation of chromosomal instability in very early colorectal cancers, and reflect on the role that chromosomal instability could have in colorectal tumorigenesis.

Cell Transformation, Neoplastic↗

Tumorigenesis: RAF/RAS oncogenes and mismatch-repair status.

Genes of the RAF family encode kinases that are regulated by Ras and mediate cellular responses to growth signals. Activating mutations in one RAF gene, BRAF, have been found in a high proportion of melanomas and in a small fraction of other cancers. Here we show that BRAF mutations in colorectal cancers occur only in tumours that do not carry mutations in a RAS gene known as KRAS, and that BRAF mutation is linked to the proficiency of these tumours in repairing mismatched bases in DNA. Our results not only provide genetic support for the idea that mutations in BRAF and KRAS exert equivalent effects in tumorigenesis, but also emphasize the role of repair processes in establishing the mutation spectra that underpin human cancer.

Base Pair Mismatch↗