Lessons from Hereditary Colorectal Cancer

Whether and how human tumours are genetically unstable has been debated for decades. There is now evidence that most cancers may indeed be genetically unstable, but that the instability exists at two distinct levels. In a small subset of tumours, the instability is observed at the nucleotide level and results in base substitutions or deletions or insertions of a few nucleotides. In most other cancers, the instability is observed at the chromosome level, resulting in losses and gains of whole chromosomes or large portions thereof. Recognition and comparison of these instabilities are leading to new insights into tumour pathogenesis.

Genetic instabilities in human cancers

https://www.cell.com/cell/pdf/0092-8674(93)90546-3.pdf

The human mutator gene homolog MSH2 and its association with hereditary nonpolyposis colon cancer.

It is increasingly proposed that reactive oxygen species (ROS) and reactive nitrogen species (RNS) play a key role in human cancer development [1–6], especially as evidence is growing that antioxidants may prevent or delay the onset of some types of cancer (reviewed in [7,8]). ROS is a collective term often used by biologists to include oxygen radicals [superoxide (O # J−), hydroxyl (OHJ), peroxyl (RO # J) and alkoxyl (ROJ)] and certain nonradicals that are either oxidizing agents and}or are easily converted into radicals, such as HOCl, ozone (O $ ), peroxynitrite (ONOO−), singlet oxygen ("O # ) and H # O # . RNS is a similar collective term that includes nitric oxide radical (NOJ), ONOO−, nitrogen dioxide radical (NO # J), other oxides of nitrogen and products arising when NOJ reacts with O # J−, ROJ and RO # J. ‘Reactive ’ is not always an appropriate term; H # O # , NOJ and O # J− react quickly with very few molecules, whereas OHJ reacts quickly with almost anything. RO # J, ROJ, HOCl, NO # J, ONOO− and O $ have intermediate reactivities. ROS and RNS have been shown to possess many characteristics of carcinogens [4] (Figure 1). Mutagenesis by ROS}RNS could contribute to the initiation of cancer, in addition to being important in the promotion and progression phases. For example, ROS}RNS can have the following effects. (1) Cause structural alterations in DNA, e.g. base pair mutations, rearrangements, deletions, insertions and sequence amplification. OHJ is especially damaging, but "O # , RO # J, ROJ, HNO # , O $ , ONOO− and the decomposition products of ONOO− are also effective [9–13]. ROS can produce gross chromosomal alterations in addition to point mutations and thus could be involved in the inactivation or loss of the second wild-type allele of a mutated proto-oncogene or tumour-suppressor gene that can occur during tumour promotion and progression, allowing expression of the mutated phenotype [4]. (2) Affect cytoplasmic and nuclear signal transduction pathways [14,15]. For example, H # O # (which crosses cell and organelle membranes easily) can lead to displacement of the inhibitory subunit from the cytoplasmic transcription factor nuclear factor κB, allowing the activated factor to migrate to the nucleus [14]. Nitration of tyrosine residues by ONOO− may block phosphorylation. (3) Modulate the activity of the proteins and genes that respond to stress and which act to regulate the genes that are related to cell proliferation, differentiation and apoptosis [4,14–17]. For example, H # O # can stimulate transcription of c-jun

/pdf/damage-to-dna-by-reactive-oxygen-and-nitrogen-species-role-2988jdgpnd.pdf

Damage to DNA by reactive oxygen and nitrogen species: role in inflammatory disease and progression to cancer.

https://www.cell.com/cell/pdf/0092-8674(93)90330-S.pdf

Mutations of a mutS homolog in hereditary nonpolyposis colorectal cancer

The genomes of all eukaryotes contain tracts of DNA in which a single base or a small number of bases is repeated. Expansions of such tracts have been associated with several human disorders including the fragile X syndrome. In addition, simple repeats are unstable in certain forms of colorectal cancer, suggesting a defect in DNA replication or repair. We show here that mutations in any three yeast genes involved in DNA mismatch repair (PMS1, MLH1 and MSH2) lead to 100- to 700-fold increases in tract instability, whereas mutations that eliminate the proof-reading function of DNA polymerases have little effect. The meiotic stability of the tracts is similar to the mitotic stability. These results suggest that tract instability is associated with DNA polymerases slipping during replication, and that some types of colorectal cancer may reflect mutations in genes involved in DNA mismatch repair.

Destabilization of tracts of simple repetitive DNA in yeast by mutations affecting DNA mismatch repair

Nature 365, 274-276 (1993) THE penultimate sentence in the legend to Table 1 of this letter should read: "The plasmid pSH91 contains an in-frame insertion of poly(GT) (33 bp) in URA3; this plasmid is identical to pSH44 (ref. 8) except for the orientation of the tract", and not that the plasmid is identical to pSH36 apart from the size of the tract, as originally stated.

/pdf/erratum-destabilization-of-tracts-of-simple-repetitive-dna-4dzrzv5p1m.pdf

Erratum: Destabilization of tracts of simple repetitive DNA in yeast by mutations affecting DNA mismatch repair (Nature (1993) 365 (274-276))

Available carbon (C) is defined as soil organic C that heterotrophic microorganisms can readily utilize as an energy and C source. Several techniques for assessing available C have been described in the literature, but none has become standard. Four of these methods, (i) mlneralizable C (Min‐C), (ii) cold water soluble C (CWS‐C), (iii) boiling water extractable C (BWE‐C), (iv) and total C (Tot‐C), were compared to each other and to a denitrification potential (DP) bioassay of C availability. These comparisons were made across selected forest soils which exhibited wide ranges of textural composition and organic C content and which ranged in acidity from pH 3.1 to 5.4. Carbon mineralized to CO2 during a 7 day aerobic incubation of field‐moist soil (Min‐C) provided the best prediction of DP. Min‐C also appeared to distinguish between available C and refractory C in a soil with high Tot‐C. Air drying increased CWS‐C levels 2‐ to 10‐fold. The C made soluble by drying and rewetting was strongly correla...

Assessing available carbon: Comparison of techniques across selected forest soils

Eukaryotic genomes contain tracts of DNA in which a single base or a small number of bases are repeated (microsatellites). Mutations in the yeast DNA mismatch repair genes MSH2, PMS1, and MLH1 increase the frequency of mutations for normal DNA sequences and destabilize microsatellites. Mutations of human homologs of MSH2, PMS1, and MLH1 also cause microsatellite instability and result in certain types of cancer. We find that a mutation in the yeast gene MSH3 that does not substantially affect the rate of spontaneous mutations at several loci increases microsatellite instability about 40-fold, preferentially causing deletions. We suggest that MSH3 has different substrate specificities than the other mismatch repair proteins and that the human MSH3 homolog (MRP1) may be mutated in some tumors with microsatellite instability.

Mutations in the MSH3 gene preferentially lead to deletions within tracts of simple repetitive DNA in Saccharomyces cerevisiae

Antisense nopaline synthase (nos) (D-nopaline synthase; EC 1.5.1.19) RNA is stably expressed from the cauliflower mosaic virus 35S promoter in transformed tobacco plants. The expression of a previously introduced wild-type nos gene is inhibited by the antisense RNA, with less nos enzyme activity detected (by a factor of 8-50) depending on the tissue analyzed. The steady-state levels of nos mRNA are reduced in the presence of the antisense RNA, implying that mRNA degradation is probably the main mode of action for the decrease in expression in this system. The antisense RNA-expressing gene and its inhibition of nos expression are shown to be heritable, demonstrating that it is a potentially useful method for the modification of phenotype.

Micheline K. Strand

Papers

Destabilization of tracts of simple repetitive DNA in yeast by mutations affecting DNA mismatch repair

Erratum: Destabilization of tracts of simple repetitive DNA in yeast by mutations affecting DNA mismatch repair (Nature (1993) 365 (274-276))

Assessing available carbon: Comparison of techniques across selected forest soils

Mutations in the MSH3 gene preferentially lead to deletions within tracts of simple repetitive DNA in Saccharomyces cerevisiae

Stable and heritable inhibition of the expression of nopaline synthase in tobacco expressing antisense RNA