Localization of a candidate colon tumor-suppressor gene (DRA) to 7q22-q31.1 by fluorescence in situ hybridization

The colorectal adenoma-carcinoma sequence

BACKGROUND

It is widely accepted that the adenoma-carcinoma sequence represents the process by which most, if not all, colorectal cancers arise. The evidence supporting this hypothesis has increased rapidly in recent years and the purpose of this article is to review this evidence critically and highlight its clinical significance.

METHODS

Medline searches were used to identify recent key articles relating to the adenoma-carcinoma sequence. Further pertinent articles were obtained by manual scanning of the reference lists of identified papers.

RESULTS

The evidence supporting the adenoma-carcinoma sequence can be classified as epidemiological, clinicopathological and genetic. The most recent and largest body of data relates to molecular genetic events and their cellular effects; however, many other approaches, such as cytogenetics, molecular cytogenetics and cytometry, have also yielded valuable information.

CONCLUSION

Recent work continues to support the adenoma-carcinoma sequence, but there is a paucity of data on the interrelationship between different genetic mutations and on the relationship between molecular and other types of genetic abnormalities. The clinical utility of the observations described has yet to be fully realized and global genetic analysis of colorectal tumours may prove to be central in rational adenoma management.

Physiological roles and regulation of mammalian sulfate transporters

All cells require inorganic sulfate for normal function. Sulfate is among the most important macronutrients in cells and is the fourth most abundant anion in human plasma (300 microM). Sulfate is the major sulfur source in many organisms, and because it is a hydrophilic anion that cannot passively cross the lipid bilayer of cell membranes, all cells require a mechanism for sulfate influx and efflux to ensure an optimal supply of sulfate in the body. The class of proteins involved in moving sulfate into or out of cells is called sulfate transporters. To date, numerous sulfate transporters have been identified in tissues and cells from many origins. These include the renal sulfate transporters NaSi-1 and sat-1, the ubiquitously expressed diastrophic dysplasia sulfate transporter DTDST, the intestinal sulfate transporter DRA that is linked to congenital chloride diarrhea, and the erythrocyte anion exchanger AE1. These transporters have only been isolated in the last 10-15 years, and their physiological roles and contributions to body sulfate homeostasis are just now beginning to be determined. This review focuses on the structural and functional properties of mammalian sulfate transporters and highlights some of regulatory mechanisms that control their expression in vivo, under normal physiological and pathophysiological states.

Assessments of clonal composition of colorectal adenomas by FISH analysis of chromosomes 1, 7, 13 and 20

Chromosome banding analysis has shown that numerical aberrations, in particular gains of chromosomes 7, 13 and 20, are common in colorectal adenomas but cannot provide reliable information on the size of the abnormal clones in vivo. We examined interphase nuclei from 70 colorectal adenomas, of which 64 had been previously karyotyped, using fluorescence in situ hybridization (FISH) with probes for the pericentromeric regions of chromosomes 1, 7, 13 and 20. Gain of chromosome 7 was seen in 34% of the analyzed adenomas, +13 was seen in 44% and trisomy 20 was found in 32% of the adenomas, verifying that the trisomies are in vivo phenomena. The median proportion of cells with trisomy was larger than 50%. A comparison with the G-banding analysis showed a good correlation between the results yielded by the 2 methods. Based on the clonal size and karyotypic findings, a likely order of events during clonal evolution could be ascribed to each case. More than 1 numerical aberration was detected by FISH analysis in 16 adenomas. In 6 adenomas, a clone with only trisomy 7 was present alongside a clone with additional gain(s) of chromosomes 13 and/or 20. Seven cases had gain of chromosome 13 and/or gain of chromosome 20 in the largest clone, suggesting that a clone with either of these changes was present before the changes in chromosome 7 copy number took place. On the basis of the results of this combined meta- and interphase cytogenetic study, we conclude that gains of chromosomes 7, 13 and 20 are common in colorectal adenomas and that the trisomies usually are present in a large proportion of the cells. They seem to be primary chromosome aberrations in some adenomas, whereas in others they arise secondarily as part of the clonal evolution. Although the first gain usually is of chromosome 7, it is evident that it is the end result of the chromosomal aberrations, not the exact sequence in which they occur, that determines the pathogenetic consequences.

Molecular analysis of the PDS gene in Pendred syndrome

Pendred syndrome is an autosomal recessive disorder characterized by the association between sensorineural hearing loss and thyroid swelling or goitre and is likely to be the most common form of syndromic deafness. Within the thyroid gland of affected individuals, iodide is incompletely organified with variable effects upon thyroid hormone biosynthesis, whilst the molecular basis of the hearing loss is unknown. The PDS gene has been identified by positional cloning of chromosome 7q31, within the Pendred syndrome critical linkage interval and encodes for a putative ion transporter called pendrin. We have investigated a cohort of 56 kindreds, all with features suggestive of a diagnosis of Pendred syndrome. Molecular analysis of the PDS gene identified 47 of the 60 (78%) mutant alleles in 31 families (includes three homozygous consanguineous kindreds and one extended family segregating three mutant alleles). Moreover, four recurrent mutations accounted for 35 (74%) of PDS disease chromosomes detected and haplotype analysis would favour common founders rather than mutational hotspots within the PDS gene. Whilst these findings demonstrate molecular heterogeneity for PDS mutations associated with Pendred syndrome, this study would support the use of molecular analysis of the PDS gene in the assessment of families with congenital hearing loss.

Human DRA functions as a sulfate transporter in Sf9 insect cells

DRA is a gene that is down-regulated in colon adenomas and adenocarcinomas in humans. We have previously shown that DRA proteins are found as various forms in tissue due to differential glycosylation. This study has focused on the function of DRA related to its subcellular localization. We used the baculovirus expression system and overexpressed a nearly full-length DRA driven by a polyhedrin promoter in Sf9 insect cells. DRA protein expressed in this cell was underglycosylated relative to normal colon mucosa, but uniformly targeted to the cell membrane. It also appears to undergo posttranslational cleavage, removing about 100 amino acids from its amino terminus. This membrane localization is similar to what we observed in the colon mucosa. An ion transport assay demonstrated that DRA functions as a sulfate transporter. When DRA was expressed, sulfate import was increased more than threefold compared to the control. Sulfate import was inhibitable by the anion transporter inhibitor, DIDS, in a dose-dependent fashion. Given that (1) DRA has high similarity to other identified sulfate transporters and the proposed structure of DRA polypeptide is characteristic of those transporters, (2) DRA localization is limited to the cell membrane, and (3) DRA expression correlates with intestinal differentiation in mouse, we suggest that DRA represents a tissue-specific member of the sulfate transporter family.