New paper: Mosaicism and complexity in #CRISPR founder mice.

Yen et al. recently published a paper in Developmental Biology (Yen at al, 2014) with some important observations about founder mice generated from injecting CRISPR tools into mouse zygotes.

They targeted Tyrosinase (Tyr), which causes albinism in the homozygous-null state and thus an easy readout of CRISPR function to mutate this gene.   Like others, they observed high rates of success with many fully albino mice being generated, indicating no surviving wild-type alleles.   However, they note many genetically mosaic animals were born too - these had patches of white fur among the pigmented fur, indicating clusters of homozygous-mutant cells, among other patches of cells that clearly still had wild type Tyr function.   This clearly suggests a high rate of CRISPR mutation that did not occur till after the first embryonic cell division.  They also did sequencing on the live born founders to determine the new mutations in the target gene.  This revealed that the mice could clearly contain more than two types of detectable new mutant alleles.  In fact, they found 57 mutant alleles in 23 total mice!    So, determining the exact mutagenic outcome in a founder animal is complex.   Furthermore, founder mice could potentially transmit more than two types of mutant alleles to their progeny, presuming their germline will also be mosaic.   Of course, F1 progeny must be carefully screened and sequence-validated to figure this out.