Shedding light on canine pituitary dwarfism

Abstract

Pituitary dwarfism, associated with growth hormone deficiency, is an autosomal, recessively inherited disorder in shepherd dogs. Due to the serious nature of pituitary dwarfism and lack of efficient treatment, it is preferable to prevent dwarfs from being born by applying a correct breeding policy. However, because pituitary dwarfism is a recessively inherited disorder and carriers do not differ phenotypically from non-carriers, genetic testing is required to prevent mating of 2 carriers. But before such a DNA-test could be developed, the mutated gene had to be identified first. The results of this thesis show that a contraction of a 7-bp DNA repeat in intron 5 of canine LHX3 leads to deficient splicing and is associated with pituitary dwarfism in German shepherd dogs. Splicing of the mutant intron 5 is expected to be hampered by its reduced size. In humans, 3 different isoforms of LHX3 are known: LHX3a, LHX3b and M2-LHX3. Earlier in vitro studies concluded that LHX3a and M2-LHX3 are potent gene activators in humans and that LHX3b is not. In dogs, the predicted start codon of LHX3a is followed shortly by a stop codon, which makes LHX3a seem to be redundant in dogs. The function of exon 1 may be to circumvent exon 2 in order to direct production of isoform M2-LHX3, highlighting the significance of isoform M2-LHX3. The canine situation opens the possibility that also in other species the LHX3a isoform is redundant. Congenital dwarfism associated with growth hormone deficiency is also known in Saarloos wolfdogs and Czechoslovakian wolfdogs. Both are German shepherd dog-wolf cross-breeds. These breeds have the same 7-bp deletion in intron 5 of LHX3 as do German shepherd dwarfs. The frequency of carriers of this mutation among clinically healthy Saarloos and Czechoslovakian wolfdogs used for breeding was 31% and 21%, respectively, emphasizing the need for screening before breeding. In canine pituitary dwarfs with neurological signs indicative of a cervical problem, atlanto-axial abnormalities that resemble those encountered in human pituitary dwarfs with an LHX3 mutation, were identified. These findings suggest an association between the LHX3 mutation in dogs with pituitary dwarfism and atlanto-axial malformations. The most important endocrine differential diagnosis of pituitary dwarfism due to growth hormone deficiency is juvenile hypothyroidism. Hypothyroidism can be classified as primary or central. In central hypothyroidism the thyroids are not affected primarily but are deprived of stimulation by thyroid-stimulating hormone. Primary hypothyroidism is a common endocrinopathy in dogs. In contrast, central hypothyroidism is rare in this species. The results of this thesis show that central hypothyroidism might be an underdiagnosed disorder that could be quite common in Miniature Schnauzers. The fact that this rare disorder occurred in 7 dogs from the same breed suggests that the disorder may have a genetic background in this breed. No mutations were found in the TSHB gene and the exons of the TRHR gene that could explain the presence of central hypothyroidism in Miniature Schnauzers.