Functionally reciprocal mutations of the prolactin signalling pathway define hairy and slick cattle

Abstract

Lactation, hair development and homeothermy are characteristic evolutionary features that define mammals from other vertebrate species. Here we describe the discovery of two autosomal dominant mutations with antagonistic, pleiotropic effects on all three of these biological processes, mediated through the prolactin signalling pathway. Most conspicuously, mutations in prolactin (PRL) and its receptor (PRLR) have an impact on thermoregulation and hair morphology phenotypes, giving prominence to this pathway outside of its classical roles in lactation.

Figure 1. Phenotypic characteristics of hairy syndrome cattle.

(a) Photograph showing coat differences between wild-type and mutant half-sibs, with muddy coat due to wallowing behaviour typical of affected animals. (b) Hair morphology differences between mutant (N=12) and wild-type (N=12) half-sibs. (c,d) Heat stress response phenotypes of mutant (N=12) and wild-type (N=12) half-sibs measured at different ambient temperatures. Responses of twelve wild-type, seven mutant and five clipped mutants also indicated. (e) Sweating rate contrast between mutant (N=6) and wild-type (N=6) cows. (f) Differences in milk volumes between wild-type (N=740) and mutant (N=77) half-sibs. These differences underestimate the extent of lactation effects since at least 25% of mutant animals failed to initiate lactation. Box plots define the median, upper and lower quartiles for the various phenotypes, with whiskers representing the furthest data points within 1.5 × of the interquartile range, and outlier samples indicated beyond this range. *P<0.05, **P<0.001, ***P<0.0001 (two-sided t-tests, Bonferroni-adjusted).

Figure 2. Mapping and bioinformatic characterization of PRL and PRLR mutations.

(a) Manhattan plot showing the hairy locus on chromosome 23, with significance plotted on the y axis, and chromosome number and position indicated on the x axis. (b,e) Graphics depicting PRL and PRLR gene structures, showing locations of the respective p.Cys221Gly and PRLR p.Leu462* mutations and representative Sanger sequence traces. (c) ClustalW alignment showing conservation of the prolactin Cys221 residue in five vertebrates, and in human placental lactogen and growth hormone (residues coloured by polarity). (d) Disruption of the C-terminal disulphide bridge because of p.Cys221Gly, modelled on the 3D structure of human prolactin (1RW5.pdb). (f) 200 C-terminal amino acids of PRLR, with truncated residues because of the p.Leu462* mutation indicated in red.

Figure 3. ‘Slick’ coat type.

Photographs contrasting slick and nonslick Senepol crossbreeds. The animal pictured on the left (a) carries the PRLR p.Leu462* mutation and is a three-way cross of Tuli (0.5), Senepol (0.25) and Red Angus (0.25); the animal on the right (b) is wild-type and contains Senepol (0.375), Red Angus (0.25), Beefmaster (0.1875) and Simmental (0.1875) ancestry. Pictured animals are representative of the crossbreeds used for genetic analysis of the slick locus, representing coat scores of 1 and 4, respectively.

Figure 4. Skin histology of hairy and slick cattle.

Example haemotoxylin/eosin-stained skin sections at 100 × magnification representing wild-type (N=11), hairy (N=12) and slick (N=3) cows. The epidermis is top of field in each panel, sweat glands (SG), sebaceous glands (SbG) and hair follicles with and without fibre cross-sections (HF) are indicated. No qualitative or quantitative differences were observed between the different genotypes. Scale bar: 300 μm.

Figure 5. Prolactin secretory responses to TRH infusion.

Serum ELISA results showing mean prolactin secretory responses to TRH challenge in PRL p.Cys221Gly mutant (N=6) and wild-type (N=6) animals. The x axis denotes time relative to TRH infusion (time=0), only positive values for error bars (s.e.m.) are plotted. Peak serum prolactin response was not significantly different between groups (two-sided t-test, P=0.96).