Hairless Streaks in Cattle Implicate TSR2 in Early Hair Follicle Formation

Four related cows showed hairless streaks on various parts of the body with no correlation to the pigmentation pattern. The stripes occurred in a consistent pattern resembling the lines of Blaschko. The non-syndromic hairlessness phenotype observed occurred across three generations of a single family and was compatible with an X-linked mode of inheritance. Linkage analysis and subsequent whole genome sequencing of one affected female identified two perfectly associated non-synonymous sequence variants in the critical interval on bovine chromosome X. Both variants occurred in complete linkage disequilibrium and were absent in more than 3900 controls. An ERCC6L missense mutation was predicted to cause an amino acid substitution of a non-conserved residue. Analysis in mice showed no specific Ercc6l expression pattern related to hair follicle development and therefore ERCC6L was not considered as causative gene. A point mutation at the 5'-splice junction of exon 5 of the TSR2, 20S rRNA accumulation, homolog (S. cerevisiae), gene led to the production of two mutant transcripts, both of which contain a frameshift and generate a premature stop codon predicted to truncate approximately 25% of the protein. Interestingly, in addition to the presence of both physiological TSR2 transcripts, the two mutant transcripts were predominantly detected in the hairless skin of the affected cows. Immunohistochemistry, using an antibody against the N-terminal part of the bovine protein demonstrated the specific expression of the TSR2 protein in the skin and the hair of the affected and the control cows as well as in bovine fetal skin and hair. The RNA hybridization in situ showed that Tsr2 was expressed in pre- and post-natal phases of hair follicle development in mice. Mammalian TSR2 proteins are highly conserved and are known to be broadly expressed, but their precise in vivo functions are poorly understood. Thus, by dissecting a naturally occurring mutation in a domestic animal species, we identified TSR2 as a regulator of hair follicle development.
Author Summary The identification of causal mutations of rare monogenic disorders provides an insight into the function of single genes. We herein report an example which demonstrates that the bovine species presents an excellent system for identifying these inherited phenotypes. The individual health status of modern dairy cows is well monitored, and emerging disorders are routinely recorded. An Italian breeder of ~500 Pezzata Rossa cattle reported a case of congenital streaked hairlessness. Three additional, closely related cows, showing similar hairless pattern following Blaschko’s lines were subsequently observed. A causative mutation was discovered in a previously uncharacterized rRNA processing gene. Cows possessing a single copy of this TSR2 mutation located on the X chromosome showed a mosaic skin pattern which is very likely due to the skewed inactivation of the X-chromosome, also known as lyonization. The expression of TSR2 was shown in skin and hair of cattle and mice. This study is the first to implicate an essential role for TSR2 during hair follicle development and reflects once more the potential of using rare diseases in cows to gain additional insights into mammalian biology.