Skin Fibroblasts from Spermine Synthase-deficient Hemizygous Gyro Male (Gy/Y) Mice Overproduce Spermidine and Exhibit Increased Resistance to Oxidative Stress but Decreased Resistance to UV Irradiation

Hemizygous gyro male (Gy/Y) mice are a model for X-linked hypophosphataemic rickets. As in humans, the disease is caused by deletions in the Phex gene, a phosphate-regulating gene having homologies with endopeptidases on the X chromosome. Some phenotypic abnormalities in Gy/Y mice have recently been attributed to the fact that the Gy deletion also includes the neighbouring spermine synthase gene, resulting in spermine deficiency. Spermine and its precursors spermidine and putrescine are essential for cell growth and differentiation. As a novel method for studying the function of spermine, we established primary cultures of skin fibroblasts from hemizygous Gy/Y mice. The Gy/Y cells contained no detectable spermine. In view of the fact that spermine is a free-radical scavenger in vitro, we were surprised to find that Gy/Y cells were more resistant to oxidative stress than their normal (X/Y) counterparts. However, our finding that spermidine accumulates markedly in the spermine-deficient Gy/Y cells can probably explain this increased resistance. It is the first indication that spermidine can serve as a free-radical scavenger in vivo and not only in vitro. When subjecting the Gy/Y cells to UV-C irradiation we made another interesting finding: the mutant cells were more sensitive than the normal X/Y cells. This finding indicates that spermine, probably because of its high-affinity binding to DNA, is important in protection against chromatin damage.