Tissue and Age Specific Expression of the Myc Proto-oncogene Family Throughout the Life Span of the C57BL/6J Mouse Strain

The expression of the proto-oncogene myc family (c, L and N) in terms of steady-state mRNA levels was determined in seven different normal non-cancerous tissues throughout the life span (seven different ages) of the C57BL/6J male mouse strain. C-myc oncogene expression was highest in prenatal and newborn ages and then decreased to its lowest levels at about 6 months of age. With further increase of age, a progressive pattern of increase in expression of c-myc was found in brain, liver, skin, and small intestine. However, for kidney, spleen and heart, little or no significant change was evident. Significant differential expression of c-myc was found in most tissues in animals of the same age, with highest expression consistently being found in spleen and liver at all ages. For the N-myc and L-myc oncogenes, expression was also highest in prenatal and newborn tissue as compared to the 6-month young adult, but little or no further change was found at older ages. However, substantial tissue-dependent differences in expression were also found, and no expression at all was detected at any age for N-myc in liver and for L-myc in heart, small intestine and liver. Taken together, these results indicate that the expression of the proto-oncogenes c-, L- and N-myc is dependent not only on tissue and embryonic development, as previously shown by other workers, but also on age past the young adult stage of life span. The age-dependent increase in expression of c-myc oncogene found in normal-appearing non-cancerous tissues is of particular interest as possibly reflecting tissue alterations related to both the aging process and the age-dependent increase in cancer incidence.