Homozygous Deficiency at Autosomal Locus aprt in Human Somatic Cells in Vivo Induced by Two Different Mechanisms¹

Associations between germinal and somatic mutations at autosomal loci play an important role in the development of some tumors, including retinoblastoma. In an attempt to determine whether equivalent events occur in vivo at other loci, we cloned and enumerated somatic T-cells with mutations at the aprt locus, by taking advantage of both the presence of a human disease caused by genetic defects at this locus and an effective selection procedure for the deficient mutants. T-cells homozygously deficient at this locus (aprt^-/-) were found in all four heterozygotes (aprt^+/-) studied, at an average frequency of 1.3 × 10^-4. From 310 normal individuals, we identified only one aprt^-/- clone, and the calculated frequency of aprt^-/- t-cells in aprt^+/+ individuals was 5.0 × 10^-9. These results confirm that a two-step process (aprt^+/+ → aprt^+/- → aprt^-/-) is functional through two different mechanisms (germinal-somatic and somatic-somatic) in vivo. Our data suggest that the two-step mutations leading to homozygous deficiencies at the somatic cell level, as proposed for the carcinogenic mechanisms for retinoblastomas and other human tumors, generally occur at rather high frequencies at various autosomal loci in humans.