Mice lacking DNA topoisomerase IIIβ develop to maturity but show a reduced mean lifespan

Abstract

Targeted gene disruption in the murine TOP3␤ gene-encoding DNA topoisomerase III␤ was carried out. In contrast to the embryonic lethality of mutant mice lacking DNA topoisomerase III␣, top3␤ ؊͞؊ nulls are viable and grow to maturity with no apparent defects. Mice lacking DNA topoisomerase III␤ have a shorter life expectancy than their wild-type littermates, however. The mean lifespan of the top3␤ ؊͞؊ mice is about 15 months, whereas that of their wild-type littermates is longer than 2 years. Mortality of the top3␤ ؊͞؊ nulls appears to correlate with lesions in multiple organs, including hypertrophy of the spleen and submandibular lymph nodes, glomerulonephritis, and perivascular infiltrates in various organs. Because the DNA topoisomerase III isozymes are likely to interact with helicases of the RecQ family, enzymes that include the determinants of human Bloom, Werner, and Rothmund-Thomson syndromes, the shortened lifespan of top3␤ ؊͞؊ mice points to the possibility that the DNA topoisomerase III isozymes might be involved in the pathogenesis of progeroid syndromes caused by defective RecQ helicases.RecQ helicases ͉ human progeroid syndromes I n human as well as in mouse, DNA topoisomerase III␤ is the newest member of the DNA topoisomerase family (1, 2). It belongs to the type IA subfamily of DNA topoisomerases that are characterized by their transient breakage of a DNA strand by transesterification between an enzyme tyrosyl group and a DNA 5Ј phosphoryl group (3, 4). The type IA enzymes also display a strong specificity for negatively supercoiled DNA and DNA containing single-stranded regions (3, 4). Biochemically, mammalian DNA topoisomerase III␤ resembles mammalian DNA topoisomerase III␣ as well as bacterial and yeast DNA topoisomerase III (2).Information on the cellular functions of the type IA enzymes mostly is derived from studies of bacteria and yeasts. In bacteria, there are two type IA enzymes, DNA topoisomerases I and III. The major role of bacterial DNA topoisomerase I, encoded by the topA gene, appears to be the prevention of excessive negative supercoiling of intracellular DNA (3-5). The lethality resulting from inactivating Escherichia coli DNA topoisomerase I, for example, can be alleviated by mutations that reduce the cellular level of DNA gyrase (6-8), an enzyme that catalyzes DNAnegative supercoiling, or by the expression of eukaryotic or vaccinia virus DNA topoisomerase I (9, 10), a type IB enzyme that relaxes positively or negatively supercoiled DNA but is structurally and mechanistically very different from the type IA enzymes (3). The prominent role of E. coli DNA topoisomerase I in the removal of negative supercoils indicates that the other type IA enzyme in E. coli, the topB-encoded DNA topoisomerase III, is ineffective in this role. Purified E. coli DNA topoisomerase III is proficient in linking or unlinking DNA single strands, but is inefficient in the relaxation of negatively supercoiled DNAs (11). The two type IA E. coli enzymes may share, however, an essential cellular function unrel...

A Journey in the World of DNA Rings and Beyond

“…As they age, these mutant mice exhibit progressively worsening inflammatory responses in multiple organs, leading to a much shortened life span. Kelvin also found, in collaboration with Peter Moens at York University in Toronto, a progressive reduction in the fecundity of the top3β mutant mice over time and through successive generations (89). We attributed the complex physiological consequences of inactivating the enzyme to its role in the resolution of chromosomes with entangled strands, which is consistent with a high incidence of chromosome loss observed in spermatocytes, splenocytes, and bone marrow cells of the mutant mice (89).…”

Functional and physical interaction between Sgs1 and Top3 and Sgs1-independent function of Top3 in DNA recombination repair.

“…Knockout of the Top3a gene in mice resulted in embryonic lethality, suggesting that Top3 α is essential for cell viability (Li et al, 1998). In contrast, Top3 β knockout mice develop normally, but their life span is reduced compared to that of wild-type mice (Kwan et al, 2001). A mutant allele of SGS1 of S. cerevisiae was identified as a suppressor of the slow-growth phenotype of the top3 mutant (Gangloff et al, 1994).…”

The Absence of Top3 Reveals an Interaction Between the Sgs1 and Pif1 DNA Helicases inSaccharomyces cerevisiae

“…There are two mammalian isoforms. In mice, Topo IIIa deficiency is embryonic lethal while Topo IIIb deficiency causes multiple organ defects and reduced life span (Li and Wang 1998;Kwan and Wang 2001). Topo 1 III belongs to the type 1A subfamily of topoisomerases whose members catalyze ssDNA passage events and are characterized by the ability to relax negatively supercoiled DNA (Champoux 2001).…”