C Tyler-Smith — Page 5

PubMed1979

The development of resistance to methotrexate in a mouse melanoma cell line. I. Characterisation of the dihydrofolate reductases and chromosomes in sensitive and resistant cells.

PG19T3 mouse melanoma cells were selected for resistance to methotrexate. Nine sub-lines that are resistant to concentrations of methotrexate ranging from 1.27 x 10(-7) M, to 1 x 10(-4) M methotrexate were selected and characterised in terms of their content of dihydrofolate reductase activity and their chromosomes. The intracellular level of dihydrofolate reductase activity increases with increasing resistance such that at the highest level of resistance PG19T3:MTXR10(-4)M cells contain approximately 1,000 fold more enzyme activity than the parental PG19T3 cells. It is shown that the enhanced activity is due to an increase in the amount of the enzyme rather than any structural change to the enzyme in resistant cells. Comparisons of pH activity profiles, profiles under different activating conditions and titrations with methotrexate suggest that the sensitive and resistant cells contain identical dihydrofolate reductases. Analysis of the chromosomes of resistant cells shows the presence of up to 5 large marker chromosomes which contain homogeneously staining regions after G-banding. These same regions stain intensely after C-banding and fluoresce brightly after staining with Hoechst 33258. The size of homogeneously staining regions increases throughout the process of selection. For one marker chromosome this increase may have been mediated via a ring chromosome.

Animals↗

PubMedDate not supplied

A selective difference between human Y-chromosomal DNA haplotypes.

DNA analysis is making a valuable contribution to the understanding of human evolution [1]. Much attention has focused on mitochondrial DNA (mtDNA) [2] and the Y chromosome [3] [4], both of which escape recombination and so provide information on maternal and paternal lineages, respectively. It is often assumed that the polymorphisms observed at loci on mtDNA and the Y chromosome are selectively neutral and, therefore, that existing patterns of molecular variation can be used to deduce the histories of populations in terms of drift, population movements, and cultural practices. The coalescence of the molecular phylogenies of mtDNA and the Y chromosome to recent common ancestors in Africa [5] [6], for example, has been taken to reflect a recent origin of modern human populations in Africa. An alternative explanation, though, could be the recent selective spread of mtDNA and Y chromosome haplotypes from Africa in a population with a more complex history [7]. It is therefore important to establish whether there are selective differences between classes (haplotypes) of mtDNA and Y chromosomes and, if so, whether these differences could have been sufficient to influence the distributions of haplotypes in existing populations. A precedent for this hypothesis has been established for mtDNA in that one mtDNA background increases susceptibility to Leber hereditary optic neuropathy [8]. Although studies of nucleotide diversity in global samples of Y chromosomes have suggested an absence of recent selective sweeps or bottlenecks [9], selection may, in principle, be very important for the Y chromosome because it carries several loci affecting male fertility [10] [11] and as many as 5% of males are infertile [11] [12]. Here, we show that one class of infertile males, PRKX/PRKY translocation XX males, arises predominantly on a particular Y haplotypic background. Selection is, therefore, acting on Y haplotype distributions in the population.

Alleles↗