Berg J.M., Tymoczko J.L., Stryer L. Biochemistry

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Defects in other repair systems can increase the frequency of other tumors. For example, hereditary nonpolyposis

colorectal cancer (HNPCC, or Lynch syndrome) results from defective DNA mismatch repair. HNPCC is not rare as

many as 1 in 200 people will develop this form of cancer. Mutations in two genes, called hMSH2 and hMLH1, account

for most cases of this hereditary predisposition to cancer. The striking finding is that these genes encode the human

counterparts of MutS and MutL of E. coli. The MutS protein binds to mismatched base pairs (e.g., G-T) in DNA. An

MutH protein, together with MutL, participates in cleaving one of the DNA strands in the vicinity of this mismatch to

initiate the repair process (Figure 27.51). It seems likely that mutations in hMSH2 and hMLH1 lead to the accumulation

of mutations throughout the genome. In time, genes important in controlling cell proliferation become altered, resulting

in the onset of cancer.

27.6.6. Some Genetic Diseases Are Caused by the Expansion of Repeats of Three

Nucleotides

Some genetic diseases are caused by the presence of DNA sequences that are inherently prone to errors in the

course of replication. A particularly important class of such diseases are characterized by the presence of long

tandem arrays of repeats of three nucleotides. An example is Hunt-ington disease, an autosomal dominant neurological

disorder with a variable age of onset. The mutated gene in this disease expresses a protein called huntingtin, which is

expressed in the brain and contains a stretch of consecutive glutamine residues. These glutamine residues are encoded by

a tandem array of CAG sequences within the gene. In unaffected persons, this array is between 6 and 31 repeats,

whereas, in those with the disease, the array is between 36 and 82 repeats or longer. Moreover, the array tends to become

longer from one generation to the next. The consequence is a phenomenon called anticipation: the children of an

affected parent tend to show symptoms of the disease at an earlier age than did the parent.

The tendency of these trinucleotide repeats to expand is explained by the formation of alternative structures in DNA

replication (Figure 27.52). Part of the array within the daughter strand can loop out without disrupting base-pairing

outside this region. DNA polymerase extends this strand through the remainder of the array, leading to an increase in the

number of copies of the trinucleotide sequence.

A number of other neurological diseases are characterized by expanding arrays of trinucleotide repeats. How do these

long stretches of repeated amino acids cause disease? For huntingtin, it appears that the polyglutamine stretches become

increasingly prone to aggregate as their length increases; the additional consequences of such aggregation are still under

active investigation.

27.6.7. Many Potential Carcinogens Can Be Detected by Their Mutagenic Action on

Bacteria

Many human cancers are caused by exposure to chemicals. These chemical carcinogens usually cause mutations, which

suggests that damage to DNA is a fundamental event in the origin of mutations and cancer. It is important to identify

these compounds and ascertain their potency so that human exposure to them can be minimized. Bruce Ames devised a

simple and sensitive test for detecting chemical mutagens. In the Ames test, a thin layer of agar containing about 10

bacteria of a specially constructed tester strain of Salmonella is placed on a petri dish. These bacteria are unable to grow

in the absence of histidine, because a mutation is present in one of the genes for the biosynthesis of this amino acid. The

addition of a chemical mutagen to the center of the plate results in many new mutations. A small proportion of them

reverse the original mutation, and histidine can be synthesized. These revertants multiply in the absence of an external

source of histidine and appear as discrete colonies after the plate has been incubated at 37°C for 2 days (Figure 27.53).

For example, 0.5 µg of 2-aminoanthracene gives 11,000 revertant colonies, compared with only 30 spontaneous

revertants in its absence. A series of concentrations of a chemical can be readily tested to generate a dose-response curve.

These curves are usually linear, which suggests that there is no threshold concentration for mutagenesis.

Some of the tester strains are responsive to base-pair substitutions, whereas others detect deletions or additions of base

pairs (frameshifts). The sensitivity of these specially designed strains has been enhanced by the genetic deletion of their

excision-repair systems. Potential mutagens enter the tester strains easily because the lipopolysaccharide barrier that

normally coats the surface of Salmonella is incomplete in these strains.

A key feature of this detection system is the inclusion of a mammalian liver homogenate (Section 4.1.2). Recall that

some potential carcinogens such as aflatoxin are converted into their active forms by enzyme systems in the liver or

other mammalian tissues (Section 27.6.1). Bacteria lack these enzymes, and so the test plate requires a few milligrams of

a liver homogenate to activate this group of mutagens.

The Salmonella test is extensively used to help evaluate the mutagenic and carcinogenic risks of a large number of

chemicals. This rapid and inexpensive bacterial assay for mutagenicity complements epidemiological surveys and animal

tests that are necessarily slower, more laborious, and far more expensive. The Salmonella test for mutagenicity is an

outgrowth of studies of gene-protein relations in bacteria. It is a striking example of how fundamental research in

molecular biology can lead directly to important advances in public health.