Humanizing mismatch repair in yeast: towards effective identification of hereditary non-polyposis colorectal cancer alleles.

The correction of replication errors is an essential component of genetic stability. This is clearly demonstrated in humans by the observation that mutations in mismatch repair genes lead to HNPCC (hereditary non-polyposis colorectal cancer). This disease accounts for as many as 2-3% of colon cancers. Of these, most of them are in the two central components of mismatch repair, MLH1 (mutL homologue 1) and MSH2 (mutS homologue 2). MLH1 and MSH2 function as a complex with two other genes PMS2 and MSH6. Mismatch repair genes, and the mechanism that ensures that incorrectly paired bases are removed, are conserved from prokaryotes to human. Thus yeast can serve as a model organism for analysing mutations/polymorphisms found in human mismatch repair genes for their effect on post-replicative repair. To date, this has predominantly been accomplished by making the analogous mutations in yeast genes. However, this approach is only useful for the most highly conserved regions. Here, we discuss some of the benefits and technical difficulties involved in expressing human genes in yeast. Modelling human mismatch repair in yeast will allow the assessment of any functional effect of novel polymorphisms found in patients diagnosed with colon cancers.

Human secreted carbonic anhydrase: cDNA cloning, nucleotide sequence, and hybridization histochemistry.

Complementary DNA clones coding for the human secreted carbonic anhydrase isozyme (CA VI) have been isolated and their nucleotide sequences determined. These clones identify a 1.45-kb mRNA that is present in high levels in parotid submandibular salivary glands but absent in other tissues such as the sublingual gland, kidney, liver, and prostate gland. Hybridization histochemistry of human salivary glands shows mRNA for CA VI located in the acinar cells of these glands. The cDNA clones encode a protein of 308 amino acids that includes a 17 amino acid leader sequence typical of secreted proteins. The mature protein has 291 amino acids compared to 259 or 260 for the cytoplasmic isozymes, with most of the extra amino acids present as a carboxyl terminal extension. In comparison, sheep CA VI has a 45 amino acid extension [Fernley, R. T., Wright, R. D., & Coghlan, J. P. (1988b) Biochemistry 27, 2815]. Overall the human CA VI protein has a sequence identity of 35% with human CA II, while residues involved in the active site of the enzymes have been conserved. The human sheep secreted carbonic anhydrases have a sequence identity of 72%. This includes the two cysteine residues that are known to be involved in an intramolecular disulfide bond in the sheep CA VI. The enzyme is known to be glycosylated and three potential N-glycosylation sites (Asn-X-Thr/Ser) have been identified. Two of these are known to be glycosylated in sheep CA VI. Southern analysis of human DNA indicates that there is only one gene coding for CA VI.

Dyspnea in aging rats due to disseminated intravascular coagulation (DIC).

During an 18-month oncogenicity study using rats, approximately 10% of the animals developed a form of respiratory distress very similar to that seen in the terminal stages of chronic respiratory disease, commonly associated with Mycoplasma pulmonis infection. Investigation of the lungs of the affected rats revealed not only that they did not have the consolidation usually associated with chronic respiratory disease, but they also appeared macroscopically normal. Further investigation of a number of cases revealed systemic intravascular thrombus formation of the type usually referred to as disseminated intravascular coagulation. Using an antiserum to fibrin we have demonstrated the presence of intravascular fibrin deposits in the lungs of the affected rats and have shown them to be the same as experimentally induced intravascular fibrin deposits induced in rat lungs by the administration of thrombin after blocking the fibrinolytic system. This is the first example of such a phenomenon being recorded in aging rats.

Use of a DNA probe in the diagnosis of adult polycystic kidney disease.

Adult polycystic kidney disease (APKD) is one of the most common inherited diseases in man. A diagnosis based on the demonstration of renal cysts with ultrasonography or computerised tomography may be inconclusive in early adulthood, the crucial years before child-bearing is complete. Here we describe the improved diagnostic probability that is possible using genetic linkage studies. A 24-year-old woman, whose father and younger sister were affected by APKD, was demonstrated to have a single cyst in each kidney. These findings were insufficient for a diagnosis of APKD and for this reason genetic linkage studies were undertaken. DNA was extracted from peripheral blood leukocytes from the presenting individual, and her immediate and extended family; the DNA was cut with the restriction enzyme PvuII, electrophoresed in a 0.7% agarose gel and blotted onto nitrocellulose before probing with a 32P-labelled 4 kb fragment. This contained DNA from the hypervariable region (3' hypervariable region, 3'HVR) that is linked to the gene for APKD on the short arm of chromosome 16 and has been used in other family studies by Reeders et al. We correlated the findings on Southern blotting with ultrasound evidence of APKD and found that the disease segregated with a 7.0 kb fragment in the presenting individual's father and sister. She was shown to have inherited this allele also; the use of this technique thus increased the probability of her having APKD from 50% to 96.5%.

Hybridization histochemistry.

In this review we have used our own recent work as a flagship to illustrate the recent renaissance of interest in hybridization histochemistry. A trickle of papers followed the initial key excursion into the in situ labeling of tissue sections (48-50). Our own entry into this field started in 1978 and since then a confluence of important questions and technical advances has served to make hybridization histochemistry much more attractive as a research tool. Hybridization histochemistry is able to solve some problems for which there is no other suitable technique at this time. Hybridization histochemistry provides the location of anatomical sites of gene expression, and viral replication, with uniquely high specificity. We have taken 32P-labeled probes to what appears to be their limit of resolution, which is single cells in thin sections. While 32P has clear disadvantages, exposure time is relatively short and the use of fast-X-ray film to preview the results and estimate exposure time for emulsion has been turned to advantage. Our introduction (27) of the use of whole-mouse sections in hybridization histochemistry has great potential in hormonal, enzymatic, and growth factor gene expression and will no doubt prove of great use in developmental studies and examination of viral infection. The use of synthetic DNA (synthetic oligonucleotides) unshackles the technique from the need for an associated molecular biology laboratory and at once widens the horizon of application of the technique. Although hybridization histochemistry is a valuable research tool which will soon find a niche in many fields, in a short time it should become a key diagnostic aid. It may well become the method of preference for detection of the expression of oncogenes and other cancer-related genes and for viruses which for other reasons are difficult to detect.