Nucleotide sequence encoding the carboxyl-terminal half of apolipoprotein B from spontaneously hypercholesterolemic pigs.

Previous studies from this laboratory characterized the hypercholesterolemia of pigs with a mutant allele of apolipoprotein B (apoB), designated Lpb5. This apoB allele is associated with low density lipoprotein (LDL) particles deficient in binding to the LDL receptor. To identify potential causative mutations in Lpb5 DNA, 10.6 kb of genomic DNA, encoding the carboxyl-terminal 58% of apoB were sequenced from the Lpb5 allele and from an allele encoding phenotypically normal apoB. Comparison of the two DNA sequences revealed 33 polymorphisms, 13 of which resulted in amino acid polymorphisms. To determine whether any of the amino acids at the polymorphic positions in Lpb5-encoded apoB were unique to that isoform, those positions were sequenced in four other pig apoB alleles encoding phenotypically normal apoB. None of the amino acids were by themselves uniquely encoded by the Lpb5 allele. However, a unique haplotype consisting of Asp3164 in conjunction with Ala3447 distinguished the Lpb5-encoded apoB from all other allelic isoforms sequenced in this region. To gain insight into changes in the tertiary structure of the mutant apoB, 13C-NMR analysis of LDL reductively methylated with [13C]-formaldehyde was performed. LDL has lysine residues that titrate at pH 10.5 and others that titrate at pH 8.9. The latter residues are thought to include those involved in the interaction of LDL with the LDL receptor. LDL from Lpb5 pigs possessed a smaller proportion of lysine residues titrating at pH 8.9 than did LDL from non-Lpb5 pigs, suggesting that the Lpb5-encoded apoB is altered in a manner affecting the microenvironment of particular lysine residues

Differential regulation of DNA damage response activation between somatic and germline cells in Caenorhabditis elegans.

The germline of Caenorhabditis elegans is a well-established model for DNA damage response (DDR) studies. However, the molecular basis of the observed cell death resistance in the soma of these animals remains unknown. We established a set of techniques to study ionizing radiation-induced DNA damage generation and DDR activation in a whole intact worm. Our single-cell analyses reveal that, although germline and somatic cells show similar levels of inflicted DNA damage, somatic cells, differently from germline cells, do not activate the crucial apical DDR kinase ataxia-telengiectasia mutated (ATM). We also show that DDR signaling proteins are undetectable in all somatic cells and this is due to transcriptional repression. However, DNA repair genes are expressed and somatic cells retain the ability to efficiently repair DNA damage. Finally, we demonstrate that germline cells, when induced to transdifferentiate into somatic cells within the gonad, lose the ability to activate ATM. Overall, these observations provide a molecular mechanism for the known, but hitherto unexplained, resistance to DNA damage-induced cell death in C. elegans somatic cells. We propose that the observed lack of signaling and cell death but retention of DNA repair functions in the soma is a Caenorhabditis-specific evolutionary-selected strategy to cope with its lack of adult somatic stem cell pools and regenerative capacity.Cell Death and Differentiation advance online publication, 15 June 2012; doi:10.1038/cdd.2012.69