Synergistic heterozygosity in mice with inherited enzyme deficiencies of mitochondrial fatty acid β-oxidation
Introduction
Inherited disorders of mitochondrial fatty acid β oxidation represent a challenging group of diseases to diagnose. Children affected by disorders of fatty acid oxidation often present clinically with a stereotypical phenotype. However, at times these patients have no specific diagnostic metabolite patterns, such as urinary organic acids or acylcarnitine profiles, or an unequivocal enzyme deficiency. Vockley and colleagues [1] postulated that these unclear cases for diagnosis may have a genotype representing a condition designated “synergistic heterozygosity.” They proposed that although inborn errors of metabolism are frequently autosomal recessive disorders, heterozygous mutations in two or more related pathways or steps in a single pathway could result in development of a disease phenotype [1]. To illustrate this point, several clinically relevant cases were discussed. Notably, “patient 8” [1] was an infant who died at 3 months of age after a mild bronchopneumonia. Postmortem studies on this infant’s cultured fibroblasts revealed a decreased ability to oxidize myristate (41% of control) and palmitate (71% of control). Molecular studies revealed heterozygous point mutations in both the carnitine transporter (OCTN2) and very long-chain acyl-CoA dehydrogenase genes (ACADVL). Since this is a potentially important disease mechanism that appears difficult to diagnose, we believed it was important to test the genetic/metabolic concept in the available mouse models of mitochondrial fatty acid oxidation enzyme deficiencies. We hypothesized that mice that are double or triple heterozygous for mutations in genes involved in mitochondrial fatty acid β-oxidation would display impaired fatty acid oxidation and energy generation, as evidenced by cold intolerance, and hypoglycemia.
Section snippets
Mice
Male and female BALB/cByJ [short-chain acyl CoA dehydrogenase (Acads) deficient-SCAD−/−] [2], C57BL/6NTTacfBR, 129S6/SvEvTac (B6, 129) (long-chain acyl CoA dehydrogenase (Acadl) deficient) (LCAD−/−) [3], B6,129- (very-long-chain acyl CoA dehydrogenase (Acadvl) deficient) (VLCAD−/−) [4], and wild-type controls were maintained at the University of Alabama at Birmingham. Double and triple heterozygous mice included the following genotypes: VLCAD+/−//LCAD+/−, LCAD+/−//SCAD+/−, or
Results
Within 5 h of starting the cold challenge, 36% of the LCAD+/−//SCAD+/− mice and 33% of the VLCAD+/−//LCAD+/− mice became lethally hypothermic (Fig. 1). Of the VLCAD+/−//LCAD+/−//SCAD+/−mice, 33% became lethally hypothermic (data not shown). These three groups were significantly different (P < 0.001) than the control groups of wild-type and the single heterozygous groups of mice. None of the wild-type mice became hypothermic (Fig. 1). Single heterozygous mice of each individual enzyme deficiency
Discussion
Cold challenge produces a reproducible phenotype in mice homozygous for any of the acyl-CoA dehydrogenase deficiencies [6]. Homozygous mice (LCAD−/−, SCAD−/−, and VLCAD−/−) uniformly experience an inability to thermoregulate when placed in the cold (4 °C) for more than 1 h. This result can be exacerbated by a fast prior to cold challenge. They also become hypoglycemic and presumably hypoketotic when exposed to the cold [6]. This abnormal phenotype makes these mice an excellent model for disorders
Acknowledgments
This work was supported by NIH grants RO1-RR02599, T-32 RR07003, and P30 DK56336.
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