Zebrafish cdc6 hypomorphic mutation causes Meier-Gorlin syndrome-like phenotype

Cell Division Cycle 6 (Cdc6) is a component of pre-replicative complex (preRC) forming on DNA replication origins in eukaryotes. Recessive mutations in ORC1, ORC4, ORC6, CDT1 or CDC6 of the preRC in human cause Meier-Gorlin syndrome (MGS) that is characterized by impaired post-natal growth, short stature and microcephaly. However, vertebrate models of MGS have not been reported. Through N-ethyl-N-nitrosourea mutagenesis and Cas9 knockout, we generate several cdc6 mutant lines in zebrafish. Loss-of-function mutations of cdc6, as manifested by cdc6(tsu4305) and cdc6(tsu7cd) mutants, lead to embryonic lethality due to cell cycle arrest at the S phase and extensive apoptosis. Embryos homozygous for a cdc6 hypomorphic mutation, cdc6(tsu21cd), develop normally during embryogenesis. Later on, compared with their wild-type (WT) siblings, cdc6(tsu21cd) mutant fish show growth retardation, and their body weight and length in adulthood are greatly reduced, which resemble human MGS. Surprisingly, cdc6(tsu21cd) mutant fish become males with a short life and fail to mate with WT females, suggesting defective reproduction. Overexpression of Cdc6 mutant forms, which mimic human CDC6(T323R) mutation found in a MGS patient, in zebrafish cdc6(tsu4305) mutant embryos partially represses cell death phenotype, suggesting that the human CDC6(T323R) mutation is a hypomorph. cdc6(tsu21cd) mutant fish will be useful to detect more tissue defects and develop medical treatment strategies for MGS patients.