Telomere shortening in mTR-/- embryos is associated with failure to close the neural tube

Mice genetically deficient for the telomerase RNA (mTR) can be propagated for only a limited number of generations. In particular, mTR(-/-) mice of a mixed C57BL6/129Sv genetic background are infertile at the sixth generation and show serious hematopoietic defects. Here, we show that a percentage of mTR(-/-) embryos do not develop normally and fail to close the neural tube, preferentially at the forebrain and midbrain. The penetrance of this defect increases with the generation number, with 30% of the mTR(-/-) embryos from the fifth generation showing the phenotype. Moreover, mTR(-/-) kindreds in a pure C57BL6 background are only viable up to the fourth generation and also show defects in the closing of the neural tube. Cells derived from mTR(-/-) embryos that fail to close the neural tube have significantly shorter telomeres and decreased viability than their mTR(-/-) littermates with a closed neural tube, suggesting that the neural tube defect is a consequence of the loss of telomere function. The fact that the main defect detected in mTR(-/-) embryos is in the closing of the neural tube, suggests that this developmental process Is among the most sensitive to telomere loss and chromosomal instability.