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 f or 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 defe ct increases with the generation number, with 30% of the mTR(-/-) embryos f rom the fifth generation showing the phenotype. Moreover, mTR(-/-) kindreds in a pure C57BL6 background are only viable up to the fourth generation an d also show defects in the closing of the neural tube. Cells derived from m TR(-/-) embryos that fail to close the neural tube have significantly short er telomeres and decreased viability than their mTR(-/-) littermates with a closed neural tube, suggesting that the neural tube defect is a consequenc e 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 thi s developmental process Is among the most sensitive to telomere loss and ch romosomal instability.