Truncation of IFT80 causes early embryonic loss in cattle

Recessive alleles represent a risk in populations that have undergone bottleneck events. We present a comprehensive framework for identification and validation of these genetic defects, including haplotype-based detection, variant selection from sequence data, and validation using knockout embryos. Holstein haplotype 2 (HH2), which causes embryonic death, was used to demonstrate the approach. HH2 was identified using a deficiency-of-homozygotes approach and confirmed to negatively affect conception rate and stillbirths. Five carriers were present in a group of 183 sequenced Holstein bulls selected to maximize the coverage of unique haplotypes. Three variants concordant with haplotype calls were found in HH2: a high-priority frameshift mutation resulting in a deletion, and two low-priority variants (1 synonymous variant, 1 premature stop codon). The frameshift in intraflagellar protein 80 (IFT80) was confirmed in a separate group of Holsteins from the 1000 Bull Genomes Project that shared no animals with the discovery set. IFT80-null embryos were generated by truncating the IFT80 transcript at exon 2 or 11 using a CRISPR-Cas9 system. Abattoir-derived oocytes were fertilized in vitro and embryos were injected at the one-cell stage either with CRISPR-Cas9 complex (n=100) or Cas9 mRNA (control, n=100) before return to culture, and replicated 3 times. IFT80 is activated at the 8-cell stage, and IFT80-null embryos arrested at this stage of development, which is consistent with data from mouse hypomorphs and HH2 carrier-to-carrier matings. This frameshift in IFT80 on chromosome 1 at 107,172,615 bp (p.Leu381fs) disrupts WNT and hedgehog signaling, and is responsible for the death of homozygous embryos. Significance StatementHolstein haplotype 2 is an embryonic lethal present in 1.21% of the US Holstein cattle population, and unrecognized carrier-to-carrier matings are responsible for >$2 million/year in additional breeding expenses. A high-impact frameshift mutation in exon 11 of intraflagellar protein 80 (IFT80) was identified as the putative causal variant. Biallelic IFT80 knockout embryos were produced in vitro and compared to wild-type embryos. IFT80-null embryos consistently arrested at the 8-cell stage of development. The IFT80 protein expressed in knockout embryos had substantially altered protein structure, resulting in a loss of functional domains. These results validate the putative causal mutation observed in Holsteins. This system is a good model for investigating possible causal variants that affect livestock fertility early in development.