HH5 double-carrier embryos fail to progress through early conceptus elongation.

Area of Science:

• Genetics
• Reproductive Biology
• Developmental Biology

Background:

• Deleterious haplotypes in cattle, such as Holstein haplotype 5 (HH5), contribute to preterm mortality.
• HH5 is associated with the ablation of the mitochondrial transcription factor B1 (TFB1M) gene in Holstein Friesian cattle.
• The precise developmental stage of embryonic loss in HH5 homozygous (double-carrier, DC) cattle is unknown, hindering economic loss assessment.

Purpose of the Study:

• To determine the developmental stage at which HH5 DC embryos or fetuses die.
• To investigate the impact of TFB1M gene ablation on early embryonic development in cattle.
• To assess the economic implications of HH5 carrier status in cattle breeding.

Main Methods:

• Flushing of embryonic day 14 (E14) conceptuses from superovulated carrier cows inseminated with carrier bulls.
• Generation and in vitro culture of TFB1M knockout (KO) embryos using CRISPR technology up to day 12 (D12).
• Assessment of embryonic and extra-embryonic membrane development, lineage differentiation, proliferation, and mitochondrial DNA (mtDNA) content.

Main Results:

• HH5 DC conceptuses were recovered at Mendelian rates at E14 but exhibited significantly reduced extra-embryonic membranes (>26-fold shorter).
• TFB1M ablation did not affect embryonic lineage differentiation, proliferation, or relative mtDNA content up to D12 in vitro.
• HH5 DC embryos can develop to the early embryonic disc stage (D12) but fail to undergo early conceptus elongation.

Conclusions:

• HH5 DC embryos arrest development due to a failure in early conceptus elongation, a critical step for pregnancy recognition.
• The ablation of the TFB1M gene is the likely cause of developmental failure in HH5 DC embryos.
• Understanding this specific developmental block is crucial for managing HH5 carrier status and mitigating economic losses in Holstein Friesian cattle.