Structure of the human MLH1 locus and analysis of a large hereditary nonpolyposis colorectal carcinoma kindred for mlh1 mutations.

Hereditary nonpolyposis colorectal carcinoma is a major cancer susceptibility syndrome known to be caused by inheritance of mutations in at least four genes such as hMSH2, hMLH1, hPMS1, and hPMS2 which encode components of a DNA mismatch repair system. The hMLH1 genomic locus on chromosome 3p has been cloned and shown to cover approximately 58 kilobases of genomic DNA and contain 19 exons. The sequence of all of the intron-exon junctions has been determined and used to develop methods for analyzing each hMLH1 exon for mutations. Using these methods to analyze a 3p-linked hereditary nonpolyposis colorectal carcinoma kindred, we have demonstrated that cancer susceptibility in this family is due to the inheritance of a frame shift mutation in the hMLH1 gene.

Genetic recombination of bacterial plasmid DNA. Physical and genetic analysis of the products of plasmid recombination in Escherichia coli.

Derivatives of plasmid pBR322 DNA containing tet mutations were constructed by inserting XhoI linkers at various sites in the tetracycline resistance gene. Monomer plasmids containing either the tet-10 allele located at nucleotide position 23 or the tet-14 allele located at nucleotide position 1267 were used to construct a circular dimer containing one copy of each allele and a circular trimer containing one copy of the tet-10 allele and two copies of the tet-14 allele. Genetic recombination of these plasmid DNAs to produce a functional tetracycline resistance gene could be detected as the production of tetracycline-resistant progeny during the growth of transformants or using a restriction mapping assay which detected the rearrangement of the mutant alleles. The structure of individual tetracycline-resistant recombination products was determined by restriction mapping. This analysis suggested that as many as 70% of the plasmid recombination events in Escherichia coli AB1157 could have involved gene conversion events. The formation of these recombination products was most easily predicted by a model involving figure 8 recombination intermediates and the formation of symmetric regions of heteroduplex. Recombination in JC10287 delta(srlR-recA)304 occurred at 5% of the wild-type frequency and appeared to occur by a similar mechanism. Recombination in JC9604 recA56 recB21 recC22 sbcA23 occurred at 20 times the wild-type frequency and appeared to involve multiple independent recombination events.

Purification and characterization of an activity from Saccharomyces cerevisiae that catalyzes homologous pairing and strand exchange.

An activity that catalyzes the formation of joint molecules from linear M13mp19 replicative form DNA and circular M13mp19 viral DNA was purified 1000- to 2000-fold from mitotic Saccharomyces cerevisiae cells. The activity appeared to reside in a Mr 132,000 polypeptide. The reaction required that the substrates be homologous and also required Mg2+. There was no requirement for ATP. The reaction required stoichiometric amounts of protein and showed a cooperative dependence on protein concentration. Electron microscopic analysis of the joint molecules indicated they were formed by displacement of one strand of the linear duplex by the single-stranded circular molecule. This analysis also showed that heteroduplex formation started at the 3'-homologous end of the linear duplex strand followed by extension of the hybrid region toward the 5'-homologous end of the linear duplex strand (3'-to-5' direction).

Isolation of genetic elements that increase frequencies of plasmid recombinants.

Some plasmid DNAs, when maintained in wild-type Escherichia coli strains, form high levels of oligomeric species while others remain primarily monomers. One explanation of this observation is that the plasmids that do not form circular oligomers lack a DNA sequence necessary for the formation or maintenance of circular oligomeric species. Here we describe the isolation of segments of DNA from the E. coli genome and other sources that through a recA+ -dependent process: (1) stimulate the conversion of monomeric plasmids to different oligomeric forms, (2) stimulate the conversion of an oligomeric plasmid to a mixture of monomeric and different oligomeric forms, and (3) increase the frequency of recovery of figure-8 molecules. Both cis-acting and trans-acting elements were found. These elements seen to act by stimulating either the frequency of the recombination events that lead to the interconversion of different oligomeric plasmid DNA molecules or some process involved in the maintenance of newly-formed recombinant molecules.

Role of the cytoskeletal protein paxillin in oncogenesis.

The focal adhesion is an important cellular structure that is involved in cell signaling, cell motility, and oncogenic transformation. Paxillin is a unique adapter protein that is localized to the focal adhesion and is involved in regulating various functions of the focal adhesion. The predicted amino acid structure for paxillin shows at the amino-terminus five LD motifs, a proline-rich domain, several potential phosphorylation sites and four carboxy-terminal LIM domains. Paxillin interacts with cell surface receptors and the actin cytoskeleton and activates several signal transduction pathways that are known to regulate normal cell physiology. Because paxillin is a central protein within the focal adhesion, it is a common target of many different oncoproteins, such as BCR/ABL, v-Src, and E6. In this review we summarize the current knowledge about the structure/function of paxillin and its family members, its role in integrin, cytokine signaling, and oncogenic transformation.

Surgical prophylaxis of malignant melanoma.

A review of a 14-year experience with prophylactic pigmented skin lesion removal is presented. Data obtained during a 4-year interval of this 14-year experience is analyzed specifically. During this 4-year interval, 250 patients with melanoma were seen. Of these patients, 75 with a history of stage I (localized) melanoma and three patients with stage II (history of controlled regionally metastatic melanoma) underwent removal of multiple skin lesions on a prophylactic basis. Of the removed lesions, 28% showed hyperplasia, atypia, dysplasia, or melanoma. Nine unsuspected in situ, or level I melanomas, and three unsuspected invasive melanomas were removed from these 75 melanoma patients while excising lesions prophylactically during the 4-year interval. It is estimated that four to six additional melanomas were prevented by excision of precursor lesions. During the same 4-year interval, an additional 112 of approximately 1000 patients without a previous history of melanoma underwent prophylactic lesion removals. In 31% of the 112 patients, there was a history of melanoma in a first-degree relative. In 22% of the removed lesions there was hyperplasia, atypia, or dysplasia. Three cases of melanoma in situ were detected and it is estimated that an additional three to five cases of melanoma were prevented. Atypical findings occurred in 71, or 63%, of the patients biopsied, which represented 7% of the approximately 1000 patients screened. During the 4-year interval, an average of 17.7 lesions were removed from each of the 190 melanoma and nonmelanoma patients undergoing prophylactic skin lesion excision. This was accomplished in one to four sessions per patient. This average reflects only those patients who underwent one excision or more and does not include those patients treated without operation. When including the nonoperated patients screened during this interval, the average number of lesions removed was 2.7 per patient. Death from new melanomas was prevented during the 14-year period of this study as evidenced by the fact that no patient died or developed metastatic disease from a cutaneous melanoma that was not apparent or known about at the time of first examination.

Molecular analysis of the Escherichia coli recO gene.

The plasmid pLC7-47, which contains lep, rnc, and era, was found to complement the UV-sensitive and recombination-deficient phenotypes caused by the recO1504::Tn5 mutation. Southern blotting analysis demonstrated that pLC7-47 contained a segment of Escherichia coli DNA that covered the region of the E. coli chromosome containing the recO1504::Tn5 mutation. A combination of deletion mapping and insertional mutagenesis localized the recO-complementing region to an approximately 1-kilobase region of a 1.6-kilobase BamHI fragment. The DNA sequence of the 1.6-kilobase BamHI fragment was determined and contained part of era and a 726-base-pair recO open reading frame. The recO open reading frame contained three possible translation start codons and could potentially encode a polypeptide of Mr 26,000. Computer analysis indicated that the putative RecO protein had suboptimal codon usage and did not show significant homology with previously identified proteins whose sequences were present in protein data bases. A combination of primary sequence analysis and secondary structure predictions suggested that recO contains a mononucleotide-binding fold.

Mutation in the DNA mismatch repair gene homologue hMLH1 is associated with hereditary non-polyposis colon cancer.

The human DNA mismatch repair gene homologue hMSH2, on chromosome 2p is involved in hereditary non-polyposis colon cancer (HNPCC). On the basis of linkage data, a second HNPCC locus was assigned to chromosome 3p21-23 (ref. 3). Here we report that a human gene encoding a protein, hMLH1 (human MutL homologue), homologous to the bacterial DNA mismatch repair protein MutL, is located on human chromosome 3p21.3-23. We propose that hMLH1 is the HNPCC gene located on 3p because of the similarity of the hMLH1 gene product to the yeast DNA mismatch repair protein, MLH1, the coincident location of the hMLH1 gene and the HNPCC locus on chromosome 3, and hMLH1 missense mutations in affected individuals from a chromosome 3-linked HNPCC family.