In vivo human somatic mutation: frequency and spectrum with age

The number and molecular nature of in vivo mutations in relation to age was studied at the autosomal HLA-A locus in human lymphocytes. Mutant lymphocytes were isolated by immunoselection, cloned at limiting dilution and enumerated, and the HLA-A gene and other polymorphic gene loci on chromosome 6 were studied by Southern blotting to determine gene dosage and loss of heterozygosity. Results of 167 assays in 73 individuals showed that the total number of mutant lymphocytes increased significantly with age from a geometric mean frequency of 0.71 × 10^-5 in neonates to 6.53 × 10^-5 in elderly individuals. Analysis of rearrangement of T lymphocyte receptor β or γ chain genes gave a best estimate of 3.3% for the proportion of mutant lymphocytes detected which are clonally related. Molecular study of 434 mutants from 31 individuals showed no change on Southern blotting in 64.7%, gene deletion in 2.8% and mitotic recombination in 32.5%. Two mutants due to gene conversion but no mutants due to non-disjunction were detected. The number of 'no change' and recombination mutants increased significantly with age. There was a significant difference between individuals in the proportion of mutants which resulted from mitotic recombination and the data suggested that the proportion was bimodally distributed. The point of crossing-over in recombination mutants was predominantly randomly distributed between the HLA-A locus and the centromere.