Dose Fractionation During Puberty Is More Detrimental to Mammary Gland Development Than an Equivalent Acute Dose of Radiation Exposure.

PURPOSE: Computed tomographic (CT) scans in adolescents have increased dramatically in recent years. However, the effects of cumulative low-dose exposures on the development of radiation sensitive organs, such as the mammary gland, is unknown. The purpose of this work was to define the effects of dose rate on mammary organ formation during puberty, an especially sensitive window in mammary development. We used a fractionated low-dose x-ray exposure to mimic multiple higher dose CT scans, and we hypothesized that fractionated exposure would have less of an effect on the number of mammary gland defects compared with an acute exposure. METHODS AND MATERIALS: Female mice were subjected to fractionated low-dose x-ray exposure (10 cGy/d for 5 days), acute x-ray exposure (1 × 50 cGy), or sham exposure. As the wide genetic diversity in humans can play a role in a person's response to irradiation, 2 genetically diverse mouse strains differing in radiation sensitivity (BALB/c-sensitive; C57BL/6-resistant) were used to investigate the role of genetic background on the magnitude of the effect. RESULTS: Unexpectedly, our data reveal that multiple low-dose exposures produce greater immune and mammary defects for weeks after exposure compared with controls. The most pronounced defects being increased ductal branching in both strains and a greater percentage of terminal end buds in the BALB/c strain of mice exposed to fractionated radiation compared with sham. Radiation-induced defects near the terminal end bud were also increased in both strains. CONCLUSIONS: The findings suggest that fractionated low-dose exposures are potentially more damaging to organ development compared with an equivalent, single acute exposure and that genetic background is an important parameter modifying the severity of these effects.

Transgenic overexpression of the SUR2A-55 splice variant in mouse heart reduces infract size and promotes protective mitochondrial function.

ATP-sensitive potassium channels found in both the sarcolemma (sarcKATP) and mitochondria (mitoKATP) of cardiomyocytes are important mediators of cardioprotection during ischemic heart disease. Sulfonylurea receptor isoforms (SUR2), encoded by Abcc9, an ATP-binding cassette family member, form regulatory subunits of the sarcKATP channel and are also thought to regulate mitoKATP channel activity. A short-form splice variant of SUR2 (SUR2A-55) was previously shown to target mitochondria and display diaxoxide and ATP insensitive KATP activity when co-expressed with the inward rectifier channels Kir6.2 and Kir6.1. We hypothesized that mice with cardiac specific overexpression of SUR2A-55 would mediate cardioprotection from ischemia by altering mitoKATP properties. Mice overexpressing SUR2A-55 (TGSUR2A-55) in cardiomyocytes were generated and showed no significant difference in echocardiographic measured chamber dimension, percent fractional shortening, heart to body weight ratio, or gross histologic features compared to normal mice at 11-14 weeks of age. TGSUR2A-55 had improved hemodynamic functional recovery and smaller infarct size after ischemia reperfusion injury compared to WT mice in an isolated hanging heart model. The mitochondrial membrane potential of TGSUR2A-55 mice was less sensitive to ATP, diazoxide, and Ca2+ loading. These data suggest that the SUR2A-55 splice variant favorably affects mitochondrial function leading to cardioprotection. These data support a role for the regulation of mitoKATP activity by SUR2A-55.

An East Asian Common Variant Vinculin P.Asp841His Was Associated With Sudden Unexplained Nocturnal Death Syndrome in the Chinese Han Population.

BACKGROUND: We have identified the cardiomyopathy-susceptibility gene vinculin (VCL) mutation M94I may account for a sudden unexplained nocturnal death syndrome (SUNDS) case. We addressed whether VCL common variant D841H is associated with SUNDS. METHODS AND RESULTS: In 8 of 120 SUNDS cases, we detected an East Asian common VCL variant p.Asp841His (D841H). Comparing the H841 allele frequency of the general population in the local database (15 of 1818) with SUNDS victims (10 of 240) gives an odds ratio for SUNDS of 5.226 (95% CI, 2.321, 11.769). The VCL-D841H variant was engineered and either coexpressed with cardiac sodium channel (SCN5A) in HEK293 cells or overexpressed in human induced pluripotent stem-cell-derived cardiomyocytes to examine its effects on sodium channel function using the whole-cell patch-clamp method. In HEK293 cells, under physiological pH conditions (pH 7.4), D841H caused a 29% decrease in peak INa amplitude compared to wild type (WT), whereas under acidotic conditions (pH 7.0), D841H decreased further to 43% along with significant negative shift in inactivation compared to WT at pH 7.4. In induced pluripotent stem-cell-derived cardiomyocytes, similar effects of D841H on INa were observed. VCL colocalized with SCN5A at the intercalated disk in human cardiomyocytes. VCL was also confirmed to directly interact with SCN5A, and VCL-D841H did not disrupt the association of VCL and SCN5A. CONCLUSIONS: A VCL common variant was genetically and biophysically associated with Chinese SUNDS. The aggravation of loss of function of SCN5A caused by VCL-D841H under acidosis supports that nocturnal sleep respiratory disorders with acidosis may play a key role in the pathogenesis of SUNDS.

Vinculin variant M94I identified in sudden unexplained nocturnal death syndrome decreases cardiac sodium current.

Sudden unexplained nocturnal death syndrome (SUNDS) remains an autopsy negative disorder with unclear etiology. Vinculin (VCL) was linked to sudden arrhythmia death in VCL knockout mice prior to the appearance of cardiomyopathy. We hypothesized VCL mutations underlie risk for SUNDS. A rare heterozygous variant VCL-M94I was found in a SUNDS victim who suffered sudden nocturnal tachypnea and lacked pathogenic variants in known arrhythmia-causing genes. VCL was identified to interact with SCN5A in vitro/vivo. The VCL-M94I was co-expressed with the cardiac sodium channel in HEK293 cells and also overexpressed in induced pluripotent stem cells derived cardiomyocytes (iPSCs-CM). In HEK293 cells with pH 7.4, VCL-M94I caused ~30% decrease in peak sodium current (INa) amplitude compared to WT; under acidotic conditions (pH 7.0) typically found with hypoxia during sleep apnea, M94I resulted in 37% reduction in peak INa compared to WT and the combination of VCL-M94I and pH 7.0 decreased peak INa by ~56% compared to WT at pH 7.4. In iPSCs-CM, similar effects of M94I on reduction of peak INa were observed. This study initially shows both physical and functional interaction between VCL and cardiac sodium channel, and suggests an important role for respiratory acidosis in triggering the fatal arrhythmia underlying SUNDS.