Baz1b Dosage Influences Cardiovascular Function, Predisposing to Dilated Cardiomyopathy

Abstract

BAZ1B is one of several genes deleted in Williams-Beuren Syndrome (WBS), a complex, multisystem genetic condition that occurs in ~1 in 8000 live births. Also known as Williams Syndrome Transcription Factor (WSTF), BAZ1B is thought to be essential for neural crest migration. To evaluate the impact of Baz1b loss of function, we evaluated the "knockout first" allele of Baz1b^{tm2a(KOMP)Wtsi} . Quantitative PCR revealed markedly reduced, but not absent, expression of Baz1b, suggesting that Baz1b^{tm2a(KOMP)Wtsi} mutants are knockdowns rather than knockouts. Homozygous Baz1b^{tm2a(KOMP)Wtsi} mutant mice die just hours after birth, and both homozygous mutants and heterozygotes are smaller than age-matched wildtype littermates. Survival analyses conducted on 388 Baz1b^{tm2a(KOMP)Wtsi} mice revealed that heterozygotes and homozygous mutants are approximately three and sixteen times more likely to die than wildtype mice, respectively [hazard ratio for death in Baz1b^+/- : 3.04 (95% CI, 1.83-5.06), p<0.0001; hazard ratio for death in Baz1b^-/- : 15.83 (95% CI, 8.54-29.37); p<0.0001]. Furthermore, a linear mixed effects model for the weights of wildtype and heterozygous mice over a 29-day period showed a significant difference in size based on genotype (mean: WT 7.97 g, Baz1b^+/- 6.56 g, p<0.0001). Because neural crest lineages contribute to cardiac development, structure, and function, we hypothesized that early sudden death and failure to thrive in mutant mice may be at least partially attributable to cardiac abnormalities. To evaluate any morphologic and functional abnormalities, we performed microCT and echocardiography. MicroCT analysis of the hearts from P0 pups did not reveal congenital heart disease typical of neural crest defects (e.g. tetralogy of Fallot, truncus arteriosus, double outlet right ventricle, or interrupted aortic arch). Echocardiograms, performed at 1-month to align with the growth analysis timeline, revealed mildly decreased ejection fraction (EF, median: WT 64%, Baz1b^+/- 56%, p<0.01) and fractional shortening (FS, median: WT 34%, Baz1b^+/- 29%, p<0.01), increased left ventricular internal dimension at diastole (LViDd) normalized to animal size (median: WT 0.22 mm/g, Baz1b^+/- 0.27 mm/g, p<0.05), and unchanged left ventricular posterior wall dimension at diastole (LVPWd) normalized to body size (median: WT 0.041 mm/g, Baz1b^+/- 0.048 mm/g, p=0.19) in Baz1b^+/- when compared to wildtype. However, Baz1b^+/- LVPWd is significantly smaller than WT when body size is not considered (median: WT 0.63 mm, Baz1b^+/- 0.62 mm, p<0.01), suggesting a relationship between cardiac function and mutant animal growth (all tests for genotype in n=14 WT and n=14 Baz1b^+/- by Mann-Whitney U Test). Taken together, our data suggest that Baz1b^+/- mice exhibit a dilated cardiomyopathy and that dosage for this gene may contribute to early death, decreased somatic growth, and cardiac abnormalities in Baz1b mutant mice. Additional analyses in older mice and with mutants generated using the conditional Baz1b^{tm2a(KOMP)Wtsi} allele will allow us to better explore the mechanisms of both the growth failure and cardiomyopathy phenotypes in this model.