Fragile chromosome, expression

In order to systematically investigate the mechanism(s) of fragile chromosome expression in folate deficient media we developed a basic culture medium. The medium is deficient in folic acid and a number of metabolites produced by folate mediated one-carbon transfer reactions (Fig. 1). These include the amino acids, serine, glycine and methionine the purine source hypoxanthine and the thymidylate source thymidine. Previous studies of fragile site expression have used leucocytes isolated in autologous plasma and cultured in medium supplemented with undialyzed serum. In order to eliminate serum sources of folate, amino acids and nucleotide precursors, we have used washed lymphocytes isolated from ficoll-hypaque banding and medium containing dialyzed human AB serum. 

The set of restrictive conditions outlined in Table 1 was used to examine the specificity of folate deficiency producing fragile chromosome expression. Chromosomes were prepared by adding colcemid, 0.1 yg/ml, to lymphocyte cultures 1,5 h before harvesting. Cells were treated with a hypotonic solution, 0.075 M KCl, for 20 min and... 

These studies indicate a specific depletion of thymidine nucleotides appear to be involved in fragile chromosome expression, whereas the restriction of amino acids or purines have no apparent affect. 

PURINE, THYMIDYLATE AND AMINO ACID REQUIREMENTS FOR HUMAN LYMPHOCYTE TRANSFORMATION AND FRAGILE CHROMOSOME SITE EXPRESSION... 

The expression may require bromodeoxyuridine as in the case of fragile site 10q25 or may also involve replacement of thymidine with deoxyuridine as indicated with studies on fragile site Xq27. Further studies of pyrimidine nucleotide metabolism and incorporation into DNA may clarify the molecular and hereditary bases of fragile chromosome site expression. 

Sutherland GR (1979a) Heritable fragile sites on human chromosomes. I. Factors affecting expression in lymphocyte culture. Am J Hum Genet 31 125-135... 

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