Parent-of-origin allelic contributions to deiodinase-3 expression elicit localized hyperthyroid milieu in the hippocampus

Thyroid hormone deficiency and excess have long been associated with mood- and anxiety-related disorders. Adjuvant treatment with high doses of thyroid hormone has been used effectively to treat mood disorders in the absence of peripheral thyroid disease,^1,2 suggesting that aberrant central, but not necessarily peripheral, thyroid hormone levels relate to the symptoms. Levels of the active thyroid hormone, 3,5,3′ triiodothyronine (T3), are maintained differentially across tissues by thyroid hormone transporters, and the three iodothyronine selenodeio-dinases (type 1, 2, and 3) together allow the molecular actions of T3 to be locally specific.³ The adult brain contains high levels of Dio3, which serves a significant role as it is the sole inactivator of T3 and because it is subject to genetic imprinting.^4,5

Genetic imprinting (non-equivalent parental genetic contributions to gene expression) is prevalent in the brain and furthermore, imprinting patterns can differ across brain regions.^6,7 We hypothesized that the Dio3 gene is region specifically imprinted because thyroid homeostasis is maintained in individual brain regions, independently from the periphery and other regions.⁸ We tested this hypothesis by reciprocally mating Sprague–Dawley (S) and Brown Norway (B) rats, which harbor a single-nucleotide polymorphism in the Dio3 exon, to generate two hybrid rat strains. We then tracked allele-specific expression of Dio3 in the F1_BS and F1_SB hybrid offspring.

Previous studies indicate that expression of the Dio3 gene is preferentially paternal.^7,9 We confirmed this expression pattern in the fetal frontal cortex of both the F1_BS and F1_SB crosses (Figure 1a), and found no difference in the total levels of Dio3 transcript between the two crosses. In adults, Dio3 expression relaxed to biallelic in the frontal cortex of both crosses. No differences were observed in total Dio3 mRNA or T3 levels. The F1_BS fetal hippocampus also exhibited preferential paternal expression, relaxing to biallelic in the adult (Figure 1a). Surprisingly, the imprinting pattern in the F1_SB hippocampus was inverted such that the maternal Dio3 allele predominated and became even more pronounced in the adult F1_SB hippocampus (Figure 1a). This manifested in lower total Dio3 mRNA levels in both the fetus and adult, presumably due to reduced paternal transcription in this cross (Figure 1b). The finding of preferential maternal Dio3 expression in hippocampus of only one cross of a reciprocally crossed pair is unique among published reports on Dio3’s imprinted status, and demonstrates that allelic gene expression can be modulated by genetic background in a brain region-specific manner. We will follow-up these findings with analyses of imprinting mechanisms, including DNA methylation changes, histone modifications and others, by which the paternal allele is relatively silenced in the F1_SB hippocampus.

Figure 1.

Disparate allelic and total expression of Dio3 elicits local hyperthyroidism in the F1_SB hippocampus. (a) Parental allelic ratio of Dio3 expression in fetal (gestational day 21) and adult frontal cortex and hippocampus of F1_BS and F1_SB hybrids (in which maternal strain is given first) by pyrosequencing. Data were confirmed by sequencing (not shown). The F1_SB hippocampus exhibited unique preferential maternal expression (strain × region, F_1,64 = 16.27, P <0.001, post hoc test **P < 0.01). (b) Quantitative RT-PCR indicated lower total Dio3 messenger RNA levels in the fetal (t₁₈ = 3.00, **P < 0.01) and adult (t₁₆ = 2.37, *P < 0.05) hippocampus of F1_SB compared with F1_BS.(c) Levels of free T3, extracted from individual hippocampi and measured by radioimmunoassay, and of TRα1 transcript measured by quantitative RT-PCR, were higher in the F1_SB adult hippocampus than in F1_BS (T3: t₁₄ = 2.86, *P < 0.05; TRα1: t₈ = 3.62, **P < 0.01). T3 levels did not differ by strain in the frontal cortex of adult males or females (data not shown). Error bars indicate s.e.m.

Genetic background could affect psychiatric phenotypes by adjusting the dosage of imprinted genes in specific brain regions. We present the first functional example of this hypothesis, wherein lower Dio3 levels and consequently lower thyroid hormone inactivation in the F1_SB hippocampus elicit predictable physiological and behavioral differences compared with the reciprocal cross. Free T3 levels and thyroid hormone receptor alpha-1 mRNA levels were higher in the hippocampus of F1_SB animals (Figure 1c) even though plasma thyroid-stimulating hormone levels, the most sensitive indicator of peripheral thyroid state, did not differ between the crosses (F1_BS: 3.6±0.30 ng ml⁻¹; F1_SB: 3.9±0.33 ng ml⁻¹). In accordance with their hippocampal thyroid function, F1_SB animals were more anxious and exhibited a working memory deficit compared with F1_BS cross (Supplementary Figures 1a-c).

This is the first report showing that parent-specific genotype affecting Dio3 allelic expression can cause local hypo- or hyperthyroid states in the brain. A similar phenomenon could occur in subsets of patients with mood disorders who respond differentially to adjuvant treatment with thyroid hormones. Moreover, individual differences in regionally restricted parental biases of Dio3 and other genes in the brain may influence neuropsychiatric vulnerability in the general population.