The metabolic syndrome resulting from a knockout of the NEIL1 DNA glycosylase

Abstract

Endogenously formed reactive oxygen species continuously damage cellular constituents including DNA. These challenges, coupled with exogenous exposure to agents that generate reactive oxygen species, are both associated with normal aging processes and linked to cardiovascular disease, cancer, cataract formation, and fatty liver disease. Although not all of these diseases have been definitively shown to originate from mutations in nuclear DNA or mitochondrial DNA, repair of oxidized, saturated, and ring-fragmented bases via the base excision repair pathway is known to be critical for maintaining genomic stability. One enzyme that initiates base excision repair of ring-fragmented purines and some saturated pyrimidines is NEIL1, a mammalian homolog to Escherichia coli endonuclease VIII. To investigate the organismal consequences of a deficiency in NEIL1, a knockout mouse model was created. In the absence of exogenous oxidative stress, neil1 knockout (neil1-/-) and heterozygotic (neil1+/-) mice develop severe obesity, dyslipidemia, and fatty liver disease and also have a tendency to develop hyperinsulinemia. In humans, this combination of clinical manifestations, including hypertension, is known as the metabolic syndrome and is estimated to affect >40 million people in the United States. Additionally, mitochondrial DNA from neil1-/- mice show increased levels of steady-state DNA damage and deletions relative to wild-type controls. These data suggest an important role for NEIL1 in the prevention of the diseases associated with the metabolic syndrome.

Fig. 1.

Pedigree of neil1^−/− mice. Open symbols are wild-type mice, half-filled symbols are heterozygous mice, and filled symbols are homozygous KO mice. Yellow symbols represent obese mice, black symbols represent mice that were killed before disease manifestation, and blue symbols represent mice that did not or have not yet shown an obesity phenotype.

Fig. 2.

Images of neil1 KO mice. (A) Age-matched comparison of a morbidly obese KO male with a wild-type male at ≈7 months of age. Necropsies revealed extensive fat deposits in the abdominal cavity, completely encasing many organs for male (B) and female (C) KO mice.

Fig. 3.

Alterations in liver and kidney tissues from neil1 KO mice. (A) Photograph of enlarged, pale yellow-colored liver from an obese KO male. (B and C) Comparison of hematoxylin and eosin-stained sections of liver from wild-type and male KO mice, respectively. Micro- and macrovesicular steatosis of centrilobular hepatocytes is prominent in the KO. (Original magnifications: ×400.) (D) Oil red O staining of sections of flash-frozen liver for an obese KO male. (Original magnification: ×400.) (E) Hematoxylin and eosin-stained section of kidney from an obese KO male showing vacuoles in proximal tubule epithelial cells.

Fig. 4.

Leptin levels are elevated in neil1 KO mice. Circulating levels of leptin were measured in male wild-type and neil1^−/− mice that ranged in age from 3 to 12 months. Error bars indicate SDs.

Fig. 5.

mtDNA damage and deletions are increased in neil1^−/− mice. (A) QPCR products from neil1^−/− and control mice; less product indicates increased mtDNA damage. (B) Southern blot of amplification products from a modified QPCR designed to enrich for mtDNA molecules containing deletions, probed with radiolabeled mtDNA.