Genetics. Evolution of the X chromosome

Lower incidence of urothelial cell carcinoma due to the concept of a clonal origin

Synchronous and metachronous tumours are frequently observed in the urinary tract and may be explained by the concept of field cancerisation, i.e. exposure to carcinogens leads to independent transformation of many urothelial cells resulting in genetically unrelated tumours. However, increasing evidence supports the concept of clonality, i.e. the progeny of a single transformed cell spreads through the urinary system resulting in genetically related tumours. The aim of this study was to review the molecular biological evidence for both concepts and to assess the consequences of a clonality assumption on the incidence of urothelial cell carcinoma (UCC). In total 1198 non-invasive and 1113 invasive (> or = T1) UCCs of the bladder were registered as incident tumours in 1996-1997 by three Dutch cancer registries following the current registration rules of the International Association of Cancer Registries (IACR). Assuming clonality, the number of non-invasive and invasive bladder UCCs decreased by 10.9% and 11.5% respectively. A decline of 8.5% and 9.5% was found for UCCs of the ureter and renal pelvis, respectively. Current registration rules have substantial impact on the incidence estimates of UCC. New insights into the molecular biology of UCC should be translated into registration rules.

Epigenetics: regulation through repression

Epigenetics is the study of heritable changes in gene expression that occur without a change in DNA sequence. Epigenetic phenomena have major economic and medical relevance, and several, such as imprinting and paramutation, violate Mendelian principles. Recent discoveries link the recognition of nucleic acid sequence homology to the targeting of DNA methylation, chromosome remodeling, and RNA turnover. Although epigenetic mechanisms help to protect cells from parasitic elements, this defense can complicate the genetic manipulation of plants and animals. Essential for normal development, epigenetic controls become misdirected in cancer cells and other human disease syndromes.

First steps in eukaryogenesis: physical phenomena in the origin and evolution of chromosome structure

Our present understanding of the origin and evolution of chromosomes differs considerably from current understanding of the origin and evolution of the cell itself. Chromosome origins have been less prominent in research, as the emphasis has not shifted so far appreciably from the phenomenon of primeval nucleic acid encapsulation to that of the origin of gene organization, expression, and regulation. In this work we discuss some reasons why preliminary steps in this direction are being taken. We have been led to examine properties that have contributed to raise the ancestral prokaryotic programmes to a level where we can appreciate in eukaryotes a clear departure from earlier themes in the evolution of the cell from the last common ancestor. We shift our point of view from evolution of cell morphology to the point of view of the genes. In particular, we focus attention on possible physical bases for the way transmission of information has evolved in eukaryotes, namely, the inactivation of whole chromosomes. The special case of the inactivation of the X chromosome in mammals is discussed, paying particular attention to the physical process of the spread of X inactivation in monotremes (platypus and echidna). When experimental data is unavailable some theoretical analysis is possible based on the idea that in certain cases collective phenomena in genetics, rather than chemical detail, are better correlates of complex chemical processes.

Evidence for clonal spread in the development of multiple meningiomas

Meningiomas are common intracranial tumors that arise from the arachnoid cells of the meninges. Occasionally patients develop multiple meningiomas. Because the underlying mechanism of multiple meningioma formation is unknown, the authors examined the pattern of X chromosome inactivation in multiple meningiomas. Fifteen intracranial meningiomas were resected in four patients with multiple meningiomas to determine whether the tumors in patients with multiple meningiomas originate from a common progenitor cell or arise independently. Specimens were examined using polymerase chain reaction assays to detect the pattern of X chromosome inactivation. In each patient, all tumors showed inactivation of the same X chromosome, suggesting that tumors arose from the same clone of cells (p < 0.0005). The authors conclude that multiple meningiomas arise from the uncontrolled spread of a single progenitor cell.

Genomic reorganization and disrupted chromosomal synteny in the siamang (Hylobates syndactylus) revealed by fluorescence in situ hybridization

We employed in situ hybridization ("chromosome painting") of chromosome-specific DNA libraries of all human chromosomes to establish homologies between the human and siamang karyotypes (Hylobates syndactylus, 2n = 50). Numerous intra- and interchromosomal rearrangements have led to a massive reorganization of the siamang karyotype. There have been a minimum of 33 translocations. The 24 siamang autosomes are composed of 60 recognizable segments that show DNA homology to regions of the 22 human autosomes. Only two autosomes have not been involved in translocations. The siamang presents a case, in a primate closely related to humans, in which chromosome morphology and synteny are highly disturbed in a manner similar to that encountered among rodents.

Clonal origin of bladder cancer

BACKGROUND

Patients with cancer of the urinary bladder often present with metachronous tumors, appearing at different times and at different sites in the bladder. This observation has been attributed to a "field defect" in the bladder that allows the independent transformation of epithelial cells at a number of sites. We tested this hypothesis using molecular genetic techniques.

METHODS

We examined 13 tumors from cystectomy specimens from four women, using a method that analyzes the pattern of X-chromosome inactivation to determine whether the tumors were derived from the same precursor cell. In addition, we analyzed allelic loss on autosomes to determine whether different tumors had the same genetic alterations. The alterations evaluated included the loss of chromosome 9q sequences (commonly found in superficial bladder tumors) and the loss of 17p and 18q sequences (usually found only in advanced tumors).

RESULTS

For each patient studied, all the tumors had inactivation of the same X chromosome, whereas normal bladder mucosa cells had random patterns of inactivation. Moreover, each tumor that could be evaluated from a given patient had lost the same allele on chromosome 9q, suggesting that the loss of this allele preceded the spread of neoplastic cells elsewhere in the bladder. The losses of chromosome 17p and 18q alleles, which are late events in tumor progression, were not common to different tumors from the same patient.

CONCLUSIONS

A number of bladder tumors can arise from the uncontrolled spread of a single transformed cell. These tumors can then grow independently with variable subsequent genetic alterations.

Duchenne's muscular dystrophy: review and recent scientific findings

In this brief review, we describe the clinical manifestations of Duchenne's muscular dystrophy (DMD) and other similar syndromes, outline the history of the dystrophin gene's identification and its relationship to these muscular dystrophies, and relate the importance of the gene's discovery to clinical neurology. We do not discuss treatment.