Backcrossing to an appropriate genetic background improves the birth rate of carbohydrate sulfotransferase 14 gene-deleted mice.

Ehlers-Danlos syndromes (EDSs) are heterogeneous group of heritable connective tissue disorders characterized by joint and skin hyperextensibility as well as fragility of various organs. Recently, we described a new type of EDS, musculocontractual EDS (mcEDS-CHST14), caused by pathogenic variants of the carbohydrate sulfotransferase 14 (CHST14) gene mutation. B6;129S5-Chst14/Mmucd (B6;129-Chst14 KO) mice are expected to be an animal model of mcEDS-CHST14. However, >90% of B6;129-Chst14 KO homozygous (B6;129-Chst14) mice show perinatal lethality. Therefore, improvement of the birth rate of Chst14 mice is needed to clarify the pathophysiology of mcEDS-CHST14 using this animal model. Some B6;129-Chst14 embryos had survived at embryonic day 18.5 in utero, suggesting that problems with delivery and/or childcare may cause perinatal lethality. However, in vitro fertilization and egg transfer did not improve the birth rate of the mice. A recent report showed that backcrossing to C57BL/6 strain induces perinatal death of all Chst14 mice, suggesting that genetic background influences the birthrate of these mice. In the present study, we performed backcrossing of B6;129-Chst14 KO mice to a BALB/c strain, an inbred strain that shows lower risks of litter loss than C57BL/6 strain. Upon backcrossing 1 to 12 times, the birth rate of Chst14 mice was improved with a birth rate of 6.12-18.64%. These results suggest that the genetic background influences the birth rate of Chst14 mice. BALB/c congenic Chst14 (BALB.Chst14) mice may facilitate investigation of mcEDS-CHST14. Furthermore, backcrossing to an appropriate strain may contribute to optimizing animal experiments.