A thyroid peroxidase (TPO) mutation in dogs reveals a canid-specific gene structure

Clinical and magnetic resonance imaging findings in a French bulldog puppy with genetically confirmed congenital hypothyroidism

A 7-month-old male French bulldog was referred for abnormal mentation and gait. Physical examination revealed a dome shaped calvarium and persistent bregmatic fontanelle. Neurological examination revealed proprioceptive ataxia, pelvic limb paraparesis and strabismus with moderate ventriculomegaly, thinning of the cerebral parenchyma, and widened cerebral sulci on magnetic resonance imaging. Masses were identified in the region of the thyroid, which appeared heterogeneous and hyperintense in T1-weighted and T2-weighted compared with the adjacent muscle signal masses were identified. Radiological diagnosis was hydrocephalus "ex vacuo" and goiter. Blood test revealed abnormally low total thyroxine (TT4), free thyroxine (FT4), and normal thyrotropin concentration. A diagnosis of congenital hypothyroidism was confirmed by positive genetic test for thyroid peroxidase mutation. Thyroxine supplementation treatment rapidly improved clinical signs.

Association of recessive c.430G>A (p.(Gly144Arg)) thyroid peroxidase variant with primary congenital hypothyroidism in cats

BACKGROUND

Primary congenital hypothyroidism (CH) is a rare endocrine disorder in cats with a largely unknown genetic cause.

OBJECTIVES

Describe the clinical presentation of CH in 11 affected cats and identify the causal genetic variant.

ANIMALS

Eleven CH-cats from 10 unrelated families, 11 CH-free family members, 21 unrelated CH-free cats, and 155 unrelated nondiagnosed cats from different breeds.

METHODS

Case control study of CH-cats and their siblings (2019-2021). Diagnosis was based on low to low-normal serum thyroxine (T4) concentrations, high thyroid-stimulating hormone (TSH) concentrations and clinical signs compatible with CH. We identified the causal variant using Sanger sequencing, genotyping via PCR-RFLP and variant interpretation using ACMG/AMP guidelines.

RESULTS

All CH-cats (5 weeks-8 years) had disproportionate dwarfism. A goiter was not palpable in all. Thyroid scintigraphy with radiopertechnetate showed abnormally high uptake by thyroid glands, whereas scintigraphy with radioiodine showed abnormally low uptake, compatible with a defect in iodine organification by thyroid peroxidase (TPO). All cases were homozygous for TPO variant XM_006930524.4:c.430G>A(p.(Gly144Arg)), while none of the CH-free cats were. All sampled parents were heterozygous for this recessive variant. This variant was found in 15 cat breeds with an estimated allele frequency of 9%.

CONCLUSIONS AND CLINICAL IMPORTANCE

Disproportionate dwarfism, abnormally high TSH and abnormally low to low-normal T4 concentrations are diagnostic for CH in cats. All cases had dyshormonogenesis demonstrated by thyroid scintigraphy. This novel TPO missense variant (not described in humans) causes CH in cats and awareness of it can assist in diagnosis and breeding.

Deleterious Mutations in the TPO Gene Associated with Familial Thyroid Follicular Cell Carcinoma in Dutch German Longhaired Pointers

Familial thyroid cancer originating from follicular cells accounts for 5-15% of all the thyroid carcinoma cases in humans. Previously, we described thyroid follicular cell carcinomas in a large number of the Dutch German longhaired pointers (GLPs) with a likely autosomal recessive inheritance pattern. Here, we investigated the genetic causes of the disease using a combined approach of genome-wide association study and runs of homozygosity (ROH) analysis based on 170k SNP array genotype data and whole-genome sequences. A region 0-5 Mb on chromosome 17 was identified to be associated with the disease. Whole-genome sequencing revealed many mutations fitting the recessive inheritance pattern in this region including two deleterious mutations in the TPO gene, chr17:800788G>A (686F>V) and chr17:805276C>T (845T>M). These two SNP were subsequently genotyped in 186 GLPs (59 affected and 127 unaffected) and confirmed to be highly associated with the disease. The recessive genotypes had higher relative risks of 16.94 and 16.64 compared to homozygous genotypes for the reference alleles, respectively. This study provides novel insight into the genetic causes leading to the familial thyroid follicular cell carcinoma, and we were able to develop a genetic test to screen susceptible dogs.

Congenital dyshormonogenic hypothyroidism with goiter caused by a sodium/iodide symporter (SLC5A5) mutation in a family of Shih-Tzu dogs

An iodide transport defect (ITD) in the thyroid gland was determined to cause congenital dyshormonogenic hypothyroidism with goiter (CDHG) in 2 members of a family of Shih-Tzu dogs. Strikingly, both dogs were also diagnosed with dilated cardiomyopathy at 24 and 1.5 mo of age. The only sign of hypothyroidism was a moderate growth delay in the adult dog. The ITD was recognized by the absence of uptake of technetium-99m in the salivary glands (sg) and goiter observed by scintigraphy. In the same scan, radiopharmaceutical uptake was found in the anterior mediastinum of both dogs and in the right axillary lymph node in the oldest dog. A follicular thyroid carcinoma was diagnosed by histopathology after thyroidectomy of the older dog. An adenomatous goiter with ectopic thyroid tissue, and degenerative changes in myocardium were the findings after necropsy in the youngest dog. A homozygous mutation of the intron 9 splice acceptor site of SLC5A5 gene, encoding the sodium/iodine symporter (NIS), was found in the DNA of one of the affected dogs. The mutation was a single base transition of guanine > adenine (G > A) at position 45,024,672 of dog chromosome 20 (CFA20). Five of eight healthy dogs, including both parents of one of the dogs exhibiting CDHG, were heterozygous A/G, and the other 3 were homozygous for the wild-type allele G/G. No sequence variant was found in thyroid peroxidase of the affected dog. Congenital dyshormonogenic hypothyroidism with goiter in this family is an autosomal recessive trait. Our findings are the first evidence of an SLC5A5 mutation in dogs and establish a new genetic cause of CDHG.

Molecular Genetic Characterization of Thyroid Dyshormonogenesis in a French Bulldog

Background

A case of congenital hypothyroidism with goiter (CHG) in a juvenile French bulldog was identified and hypothesized to be caused by dyshormonogenesis of genetic etiology.

Objectives

To describe case management, unusual phenotypic aspects, and a CHG‐causing mutation in a French bulldog.

Animals

Thyroid tissue and blood from a CHG‐affected French bulldog and 4 normal control dogs and buccal brush samples of 125 French bulldogs were studied.

Methods

Standard clinical assessment and laboratory tests were applied. Thyroid peroxidase (TPO) iodide oxidation activity was measured in vitro, and TPO protein was assessed on Western blots. Thyroid peroxidase exons and flanking splice sites were amplified from genomic DNA and sequenced. Thyroid peroxidase cDNA was amplified from thyroid RNA and sequenced.

Results

At 9 months of age, the affected dog had signs of cretinism, but near‐normal skeletal maturation. The enlarged thyroid glands exhibited noninflammatory fibrosis and aberrant follicular organization. Thyroid peroxidase activity and immunocrossreactive protein were undetectable. There was a T>C mutation of the intron 12 splice donor consensus that caused abnormally spliced mRNA, consistent with absent TPO function. The mutant allele was not observed in 125 clinically normal French bulldogs.

Conclusions

Presumptive CHG in a French bulldog with unusual clinical presentation is described. Genetic etiology was confirmed by identifying the underlying TPO mutation.