Kcne2 deletion uncovers its crucial role in thyroid hormone biosynthesis

doi:10.1038/nm.2029

Nat Med. Author manuscript; available in PMC 2010 April 1.

Published in final edited form as:

Nat Med. 2009 October; 15(10): 1186–1194.

Published online 2009 September 20. doi: 10.1038/nm.2029

PMCID: PMC2790327

NIHMSID: NIHMS144501

Kcne2 deletion uncovers its crucial role in thyroid hormone biosynthesis

Torsten K. Roepke,¹^* Elizabeth C. King,¹ Andrea Reyna-Neyra,² Monika Paroder,² Kerry Purtell,¹ Wade Koba,³ Eugene Fine,³ Daniel J. Lerner,⁴ Nancy Carrasco,² and Geoffrey W. Abbott¹^†

¹Greenberg Division of Cardiology, Department of Medicine and Department of Pharmacology, Weill Medical College of Cornell University, New York, NY

²Department of Molecular Pharmacology, Albert Einstein College of Medicine, The Bronx, NY

³M. Donald Blaufox MicroPet Lab for Molecular Imaging, Department of Nuclear Medicine, Albert Einstein College of Medicine, The Bronx, NY

⁴CV Ingenuity, San Francisco, CA.

^†Address correspondence to: Dr. Geoffrey W. Abbott, Starr 463, Greenberg Division of Cardiology, Weill Medical College of Cornell University, 1300 York Avenue, New York, NY, 10021. Tel: 212-746-6275; Fax: 212-746-7984; Email: gwa2001/at/med.cornell.edu

^*present address: Clinic for Cardiology and Angiology, Charite University-medicine Berlin 10098 Berlin, Campus Mitte and Experimental and Clinical Research Center, Max Delbrueck Center for Molecular Medicine Berlin, Germany

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Abstract

Thyroid dysfunction affects 1–4% of the population worldwide, causing defects including neurodevelopmental disorders, dwarfism and cardiac arrhythmia. Here, we show that KCNQ1 and KCNE2 form a TSH-stimulated, constitutively-active, thyrocyte K⁺ channel required for normal thyroid hormone biosynthesis. Targeted disruption of Kcne2 impaired thyroid iodide accumulation up to 8-fold, impaired maternal milk ejection and halved milk T₄ content, causing hypothyroidism, 50% reduced litter size, dwarfism, alopecia, goiter, and cardiac abnormalities including hypertrophy, fibrosis, and reduced fractional shortening. The alopecia, dwarfism and cardiac abnormalities were alleviated by T₃/T₄ administration to pups, by supplementing dams with T₄ pre- and postpartum, or by pre-weaning surrogacy with Kcne2^+/+ dams; conversely these symptoms were elicited in Kcne2^+/+ pups by surrogacy with Kcne2^−/− dams. The data identify a critical thyrocyte K⁺ channel, provide a possible novel therapeutic avenue for thyroid disorders, and predict an endocrine component to some previously-identified KCNE2- and KCNQ1-linked human cardiac arrhythmias.

Keywords: cardiac hypertrophy, hypothyroidism, KCNE2, KCNQ1, MiRP1, NIS, potassium channel, thyroid

INTRODUCTION

KCNQ1 is a voltage-gated K⁺ channel α subunit noted for its key role in human ventricular repolarization because it generates the I_Ks ventricular repolarization current, by co-assembling with the KCNE1 (MinK) single transmembrane domain β subunit¹^–⁴. KCNE1 belongs to a family of five genes including KCNE2 (MiRP1), which like KCNE1 can regulate KCNQ1 and other α subunits such as hERG, often endowing unique functional properties⁵^–⁷. Inherited human gene variants in KCNQ1, hERG, KCNE1 and KCNE2 are all associated with life-threatening cardiac arrhythmias including long QT syndrome³^,⁴^,⁶^,⁸, and may play a role in atrial fibrillation (AF)⁹^–¹². These subunits are also expressed in a variety of other tissues, but the possible cardiac effects secondary to their dysfunction in these other tissues are little-studied.

KCNQ1 is unique among the voltage-gated K⁺ channel α subunits in that it can form constitutively active, K⁺ ‘leak’ channels; this is achieved by co-assembly with KCNE2 or KCNE3. This permits KCNQ1 to facilitate background K⁺ flux in some non-excitable, polarized epithelial cells. Thus, KCNQ1 and KCNE3 are thought to form a channel in the basolateral membrane of colonic crypt cells¹³. KCNQ1-KCNE2 channels support function of the H⁺/K⁺-ATPase in the apical membrane of parietal cells; disruption of Kcnq1 or Kcne2 in mice causes achlorhydria and gastric hyperplasia¹⁴^,¹⁵.

Analogous to parietal cells and colonic crypt cells in the gastrointestinal tract, thyrocytes are non-excitable, polarized epithelial cells expressing ion transporters essential for the function of the thyroid gland. The thyroid hormones (TH) triiodothyronine (T₃) and tetraiodothyronine (thyroxine, or T₄) are critical for normal growth and development of the fetus and newborn as well as for regulation of metabolism in virtually all tissues at all ages. Because of the scarcity of iodine, an essential constituent of T₃ and T₄, iodide (I⁻) deficiency disorders are still prevalent in many areas of the world and are thus at the forefront of global health initiatives. I⁻ enters thyrocytes via the basolaterally located Na⁺/I⁻ symporter (NIS)¹⁶^,¹⁷ and exits apically into the colloid, where it is covalently incorporated into thyroglobulin, the precursor of T₃ and T₄. NIS-mediated I⁻ transport uses the downhill Na⁺ gradient generated by the Na⁺/K⁺ ATPase at the basolateral membrane of the thyrocyte. The role of K⁺ channels in the thyroid is unknown.

Here, we show that potassium channel subunits KCNQ1 and KCNE2 - originally recognized for their functional roles in repolarizing cardiac myocytes - form a constitutively-active K⁺ channel in thyrocytes, and that Kcne2 is required for normal TH biosynthesis. Aging Kcne2^−/− mice, and pups from Kcne2^−/− dams, exhibit a panoply of symptoms including goiter, cardiomegaly, dwarfism and alopecia. Remarkably, these symptoms are alleviated highly efficiently by administration of T₃/T₄ to pups, by supplementing dams with T₄ pre- and postpartum, or by surrogacy with wild-type dams; conversely, symptoms are triggered in wild-type pups by surrogacy with Kcne2^−/− dams. Put in the context of existing studies, our findings raise the possibility of an endocrine component to some cardiac arrhythmias and early-onset myocardial infarction previously associated with human KCNQ1 and KCNE2 genetic variants.

RESULTS

Kcne2^−/− pups from Kcne2^−/− dams exhibit cardiac hypertrophy, fibrosis and reduced cardiac output

Mutations and polymorphisms in human KCNQ1 and KCNE2 are associated with ventricular and atrial cardiac arrhythmias, presumed to arise from dysfunction of the K⁺ channels they form in cardiac myocytes⁴^,⁶^,¹⁸^,¹⁹. We previously found that at 3 months of age, Kcne2^−/− mice from heterozygous crosses have normal echocardiographic parameters and ventricular myocyte size²⁰. In contrast, in the current study, when we bred Kcne2^−/− pups from homozygous knockout crosses, we found that they exhibited striking cardiomegaly: >2-fold increased heart mass, and >3-fold increased heart weight:bodyweight ratio, at 3 weeks (Fig. 1a). The 3-week-old Kcne2^−/− pups from Kcne2^−/− dams also exhibited >50% increases in end-diastolic left ventricular (LV) internal diameter, LV anterior and posterior wall thickness, and a 45% decrease in fractional shortening (Fig. 1b,c). The anterior and posterior wall thickening demonstrate cardiac hypertrophy, and this is probably the primary effect, with the LV dilation and reduced fractional shortening likely arising from a compensatory response (Starling mechanism) to the impaired contractility resulting from sustained hypertrophy.

Figure 1

Kcne2 disruption causes cardiac hypertrophy, fibrosis and reduced fractional shortening

Ventricular myocytes of 3-week-old Kcne2^−/− pups from homozygous crosses had a 2-fold larger membrane capacitance than those from age-matched Kcne2^+/+ pups, also indicative of hypertrophy – defined as increased organ or tissue size due to increase in size of the constituent cells (Fig. 1d). Raw amplitudes of two of the predominant mouse ventricular repolarization K⁺ currents, I_K,slow and I_to,f, were unaltered by Kcne2 disruption, but because of the doubling in capacitance this constituted a 2-fold reduction in current density for each. In contrast, the raw amplitude of the steady-state K⁺ current (I_ss) doubled, indicating that its density did not change with Kcne2 disruption (Fig. 1d–f).

Hearts from 3-week-old Kcne2^−/− mice from Kcne2^−/− dams (Fig. 1g) also exhibited marked LV fibrosis and papillary muscle degeneration, necrosis and mineralization - a feature of sustained hypertrophy (Fig. 1h). Hepatic fibrosis was also observed, suggesting right heart failure (Fig. 1i) and the liver had an unusually pale appearance (Fig. 1g) possibly indicative of fatty liver. Furthermore, 1-year-old Kcne2^−/− mice bred from Kcne2^+/− dams also exhibited cardiomegaly with LV fibrosis (Fig. 1j). In sum, it is apparent that there exists a strong influence of parental genotype on cardiac phenotype of the Kcne2^−/− offspring, and also that aging Kcne2^−/− mice from heterozygous parents show severe cardiovascular hypertrophy and fibrosis not yet manifested in their 3-month-old counterparts²⁰.

Kcne2^−/− mice exhibit dwarfism and alopecia

Aside from their cardiac pathology, Kcne2^−/− mice exhibited other gross abnormalities that were exacerbated by the maternal Kcne2^−/− genotype. Pups from Kcne2^−/− dams exhibited 50% embryonic lethality whether the sire was Kcne2^−/− or Kcne2^+/− (Fig. 2a). Maternal genotype was the determining factor in litter size: litters from Kcne2^+/− dams with Kcne2^−/− sires were of normal size, and surviving pups in litters from Kcne2^−/− × Kcne2^+/− crosses showed an approximately Mendelian distribution (Fig. 2a). Kcne2^−/− pups from Kcne2^−/− dams also exhibited severe dwarfism (Fig. 2b,c). While birth weights were similar in all cases, mean body weight of pups from Kcne2^−/− × Kcne2^−/− crosses was 40% lower than that of pups from Kcne2^+/+ × Kcne2^−/− crosses at 5 weeks. Reduced body mass correlated with the maternal Kcne2^−/− genotype, but also to pup genotype since pups from (+/−) sires were significantly larger than those from Kcne2^−/− sires (both with Kcne2^−/− dams) (Fig. 2d). By 15 weeks of age mean bodyweights were similar regardless of pup genotype (Supplementary Fig. 1a).

Figure 2

Kcne2 disruption causes embryonic lethality, dwarfism and alopecia

Radiological examination revealed retarded skeletal development in the Kcne2^−/− pups, producing dwarfism due to slow growth of both long bones and vertebrae. This was also apparent in the form of larger epiphyseal gaps and less ossification of the epiphyses in the large joints, which were irregularly shaped, fragmented, and heterogeneously sclerotic, characteristic of slow multifocal ossification (Fig. 2c).

Surprisingly, Kcne2^−/− pups from homozygous crosses also exhibited striking alopecia of the trunk which began at 1–2 weeks of age and peaked at 4–5 weeks (Fig. 2b,e–g). Alopecia was also observed in aging Kcne2^−/− mice from Kcne2^+/− × Kcne2^+/− crosses, initiating between the ears then spreading postero-dorsally with an abrupt loss of mature hair follicles at the transition zones (Fig. 2h–j).

Cardiomegaly, dwarfism and alopecia in Kcne2^−/− mice are attenuated by wild-type surrogacy, T₄ supplementation of dams, or T₃/T₄ administration to pups

Considering the combination of cardiac hypertrophy, cardiac and hepatic fibrosis, dwarfism, alopecia, and skeletal abnormalities indicative of retarded development, we investigated whether or not hypothyroidism might be the underlying cause. To test this hypothesis, we first determined serum TH concentrations. Indeed, serum T₄ was 2-fold decreased, and TSH 2-fold increased, in 3-week-old Kcne2^−/− pups from homozygous crosses compared to Kcne2^+/+ pups from homozygous crosses (Fig. 3a), confirming the former were hypothyroid. Serum T₄ and TSH were, however, normal in nubile 3–6 month old Kcne2^−/− mice from heterozygous crosses (Supplementary Fig. 2), consistent with growth, litter size, and cardiac morphology trends (see Fig. 2 and ²⁰). By age 12–15 months T₄ and TSH were trending down and up respectively in mice from heterozygous crosses (Supplementary Fig. 2), consistent with a latent hypothyroidism, and the late onset of alopecia (Fig. 2h), cardiac hypertrophy and fibrosis (Fig. 1j). Thyroid glands from these aged mice bred from Kcne2^+/− crosses showed a 40% greater mean mass post-mortem than Kcne2^+/+ thyroids, with Kcne2^+/− thyroids having intermediate mean mass, indicative of goiter formation due to Kcne2 disruption (Fig. 3 b). In contrast to nubile adult mice, pregnant Kcne2^−/− dams exhibited almost 3-fold reduced serum T₄ concentration compared to pregnant Kcne2^+/+ dams (Figure 3 c), suggesting an explanation for the exaggerated phenotype and reduced litter size of Kcne2^−/− pups bred from Kcne2^−/− dams (Fig. 2). Consistent with previous reports for mice and rats (but in contrast to humans)²¹^,²², pregnant mice in our study exhibited lower serum T₄ than age- and genotyped-matched nubile adults (Fig. 3c; Supplementary Fig. 2).

Figure 3

Kcne2^−/− mice are hypothyroid and treatable with T₃/T₄ or wild-type surrogacy

We next tested whether or not surrogacy of pre-weaning pups would alleviate any of the observed abnormalities. Significantly, normal body weight was fully restored in Kcne2^−/− pups from Kcne2^−/− × Kcne2^−/− crosses by pre-weaning surrogacy with Kcne2^+/+ dams (Fig. 3d,e). Conversely, pre-weaning surrogacy of Kcne2^+/+ pups with Kcne2^−/− dams resulted in mean pre-weaning body weight similar to Kcne2^−/− pups, and intermediate body weight in post-weaning pups (Fig. 3d,e). These data suggested the possibility that maternal TH passed through milk – perhaps at higher concentrations, or in higher volumes of milk, from wild-type dams – were compensating for the defect in pups. Confirming the role of TH in body weight differences, Kcne2^−/− pups born and raised by Kcne2^−/− dams and from Kcne2^−/− sires, showed significantly improved body weight by 3 weeks of age after T₃/T₄ administration every 48 hours (QOD) from birth (Fig. 3e, Supplementary Fig. 1b). Furthermore, T₄ supplementation of Kcne2^−/− dams, from 2 weeks pre-birth to weaning, also resulted in normal pup body weight (Fig. 3e).

Dermatologic disorders occur frequently in hypothyroidism²³. Remarkably, here we found that alopecia was completely reversed in adult Kcne2^−/− mice with 2 weeks QOD administration of T₃/T₄ (exemplars in Fig. 3f,g). Alopecia was also completely reversed in 19 of 21 Kcne2^−/− pups by 10 days QOD T₃/T₄ administration, in 19/20 Kcne2^−/− pups by surrogacy with Kcne2^+/+ dams, and in 16/16 Kcne2^−/− pups by T₄ supplementation of their mothers from 2 weeks pre-birth to weaning. Conversely, alopecia was observed in 13/23 Kcne2^+/+ pups surrogated with Kcne2^−/− dams (Fig. 3g).

Hypothyroidism is associated with dilated and hypertrophic cardiomyopathies, reduced fractional shortening and heart failure²⁴. Supporting a link between these cardiac defects and the hypothyroidism observed in Kcne2^−/− mice, surrogacy with Kcne2^+/+ dams resulted in a dramatic reduction in the relative mass of the heart compared to body weight; conversely, surrogacy of Kcne2^+/+ pups with Kcne2^−/− dams had the opposite effect (Fig. 3 h,i). As observed with non-surrogated Kcne2^−/− pups (Fig. 1g), the liver of Kcne2^+/+ pups surrogated with Kcne2^−/− dams had an unusually pale appearance possibly indicative of fatty liver (Fig. 3h). Furthermore, echocardiographic determination of the potential beneficial effects of surrogacy of Kcne2^−/− pups with Kcne2^+/+ dams showed significant reduction in ventricular wall thickness and chamber diameter, and increased fractional shortening compared to non-surrogated Kcne2^−/− pups (Fig. 3j).

KCNE2 and KCNQ1 form a TSH-stimulated thyrocyte K⁺ channel

Our data (Fig. 1–Fig. 3) suggested a potential role for KCNE2 in TH biosynthesis. Previous studies indicate that KCNE2 probably forms channel complexes with the KCNQ1 K⁺ channel α subunit in gastric epithelium¹⁴^,¹⁵^,²⁵^,²⁶. Here, we found that both KCNE2 and KCNQ1 are expressed in human (Fig. 4a) and mouse (Fig. 4b–d) thyroid glands; in both species they partially co-localized with NIS, the basolateral membrane glycoprotein that mediates active I⁻ transport, the first step in TH biosynthesis. Furthermore, thyroid follicular epithelia in Kcne2^−/− mice exhibited abnormal architecture: compared to those in Kcne2^+/+ mice, Kcne2^−/− thyrocytes were often flattened and less abundant (Fig. 4e,f).

Figure 4

KCNE2 and KCNQ1 form a TSH-stimulated thyrocyte K⁺ channel

We next sought to determine whether KCNQ1-KCNE2 K⁺ currents were expressed in thyrocytes, employing the highly functional rat thyroid-derived FRTL5 cell line. We detected endogenously-expressed KCNQ1 and KCNE2 proteins, which appeared to be upregulated by TSH or its major downstream effector, cAMP, in FRTL5 cell membrane fractions (Fig. 4g). We then measured endogenous currents from FRTL5 cells using patch-clamp recording in the whole-cell configuration. A TSH-stimulated K⁺ current in FRTL5 cells bore the signature linear current-voltage relationship of KCNQ1-KCNE2 channels and was inhibited by the KCNQ-specific antagonist XE991 (Fig. 4h,i). In sum, KCNQ1-KCNE2 channels are expressed in human and rodent thyrocytes, where they generate a TSH-stimulated, constitutively-active K⁺ current.

KCNE2 is required for normal thyroid I⁻ accumulation

TH requirements are especially high in early development: developing fetuses and neonates rely upon not only their own TH biosynthesis, but also maternal T₄ in utero and perhaps via milk. Therefore, we examined thyroid I⁻ accumulation, a critical step in TH biosynthesis, in lactating dams and their pups. We injected ¹²⁴I only into the tail vein of lactating dams, which were then placed back together with their pups to feed them. Both dams and pups were imaged by positron emission tomography (PET). Kcne2^−/− dams showed a striking defect in ¹²⁴I accumulation in the thyroid, with 4-fold less accumulation over the first hour post-injection and continuing deficiency in the following three days (Fig. 5a–c). In pups, whose sole source of ¹²⁴I was dams’ milk, Kcne2 deletion caused a 5-fold reduction of pup thyroid ¹²⁴I accumulation 24-hours post-injection of dams, and continuing deficiency for the following 2 days (Fig. 5d,e). When normalized to stomach ¹²⁴I count the thyroid ¹²⁴I count was reduced 8-fold in Kcne2^−/− pups compared to wild-type pups at 72 hours post-injection of the dam (Fig. 5f).

Figure 5

Kcne2 is required for normal thyroid I⁻ accumulation

Thus, Kcne2 deletion causes a thyroid I⁻ accumulation defect, which in turn causes a TH biosynthesis defect, the gross phenotypic effects of which are particularly striking in pre-weaning pups feeding from Kcne2^−/− dams. To examine the mechanistic basis for this, and for the beneficial effects of surrogacy by Kcne2^+/+ dams, we first performed PET on pups surrogated with dams of opposite genotype, after tail vein injection of lactating dams with ¹²⁴I followed by imaging of pups feeding from them. Strikingly, we found that Kcne2^−/− pups feeding from Kcne2^+/+ dams had higher stomach and thyroid ¹²⁴I counts (measured as peak counts/cc), and higher thyroid:stomach count ratio, than Kcne2^+/+ pups feeding from Kcne2^−/− dams (Fig. 6a–c). This suggested that the surrogating dams’ genotype was critical in determining thyroid ¹²⁴I uptake of pups, although pup genotype was also important because when pups of either genotype fed from Kcne2^+/+ dams, Kcne2^+/+ pups still had an almost 2-fold higher thyroid:stomach count ratio at 48–72 hrs compared to Kcne2^−/− pups (Fig. 5f, Fig. 6c).

Figure 6

Kcne2^−/− dams have a milk ejection defect and produce low-T₄ milk

Total thyroid and total body activity was also quantified for all surrogated and non-surrogated pups, thereby permitting comparison of thyroid radioactive iodide uptake (RAIU) as a percentage of whole body activity (Fig. 6d). Total thyroid ¹²⁴I was higher in all pups when feeding from Kcne2^+/+ dams than when feeding from Kcne2^−/− dams, whereas total body ¹²⁴I was only significantly higher than other groups for Kcne2^−/− pups feeding from Kcne2^+/+ dams (Fig. 6d left). In contrast, thyroid RAIU – a measure of the efficiency of the thyroid at accumulating ¹²⁴I from the available total body ¹²⁴I – was only significantly higher than other groups in Kcne2^+/+ pups feeding from Kcne2^+/+ dams (Fig. 6d right). These data again demonstrated that Kcne2^−/− pups’ thyroids are less efficient than those of Kcne2^+/+ pups at accumulating I⁻, but also indicated that Kcne2^−/− dams supply less I⁻ to their pups than do Kcne2^+/+ dams. These results also revealed that Kcne2^−/− pups are relatively better at accumulating total body I⁻ than Kcne2^+/+ pups.

Examining first the poor delivery of I⁻ from Kcne2^−/− dams, we compared milk ejection from Kcne2^+/+ and Kcne2^−/− dams by weighing pups before and after feeding, as previously described²⁷. This led to the discovery that Kcne2^−/− dams have a highly significant milk ejection defect, manifested as pups (of either genotype) failing to gain weight (from milk ingestion) during the first 30 min of feeding from Kcne2^−/− dams, in sharp contrast to pups feeding from Kcne2^+/+ dams (Fig. 6e). Similar results were seen over the first 60 min of feeding (Supplementary Fig. 3). Importantly, Kcne2^+/+ and Kcne2^−/− pups showed no significant difference in their feeding rate measured by weight gain (Fig. 6e, Supplementary Fig. 3). Pups were latched on to dams of either genotype for the entire period under study (30 or 60 min). Thus, the milk ejection defects were not related to behavioral differences in either pups or dams, and instead reflected a pathophysiological defect in milk ejection by Kcne2^−/− dams. Hypothyroid rats were previously shown to have impaired milk ejection due to reduced serum oxytocin compared to euthyroid rats²⁷. Accordingly, here we found that injection of Kcne2^−/− dams with oxytocin returned their milk ejection to the same level as Kcne2^+/+ dams (Fig. 6e). The milk ejection defect was probably a dominant factor in the beneficial effects of Kcne2^+/+ surrogacy, and the negative effects of Kcne2^−/− surrogacy. Additionally, however, we discovered that milk from Kcne2^−/− dams contained only half as much T₄ as that from Kcne2^+/+ dams (Fig. 6f), potentially also contributing to the observed effects of surrogacy.

Finally, addressing the superior total body accumulation of I⁻ by Kcne2^−/− pups, we found equal serum I⁻ concentrations in non-surrogated 3-week-old Kcne2^+/+ and Kcne2^−/− pups (Fig. 6g), suggesting that despite inferior milk ejection by Kcne2^−/− dams, Kcne2^−/− pups were able to maintain normal plasma I⁻ concentrations. This was not surprising given that hypothyroidism is known to result in decreased I⁻ excretion²⁸.

DISCUSSION

More than a decade ago, KCNQ1 mRNA was found to be expressed at a higher level in human thyroid than in the heart or stomach²⁹, but its role in the thyroid has not previously been reported. Furthermore, kcnq1 gene-disrupted mice were previously found, like our Kcne2^−/− mice, to have enlarged hearts and thickened ventricular walls, but the mechanistic basis for this was not described³⁰^,³¹.

T₃ and T₄ biosynthesis requires active I⁻ transport in the thyroid, where I⁻ concentrations reach 20–40 times that of the plasma. NIS, located on the basolateral side of the thyrocytes – thyroid epithelial cells which encircle the colloid – transports I⁻ into the thyrocyte; at the cell/colloid interface I⁻ ion is oxidized and covalently incorporated into thyroglobulin, for TH production¹⁷. NIS function requires a basolateral Na⁺/K⁺-ATPase for Na⁺ efflux but the necessity for other channels or transporters in this process is not known. Here, KCNQ1-KCNE2 is identified as a TSH-stimulated thyrocyte K⁺ channel critical for normal thyroid I⁻ accumulation, and probably expressed predominantly at the basolateral membrane.

The dramatic effects of surrogacy in the current study add to the debate over whether or not maternal T₄ is at high enough concentrations in milk to deliver therapeutic effects in hypothyroxinaemic newborns³². Our findings suggest mouse milk TH could be beneficial in this context, as there are significant levels of T₄ in milk, reduced by Kcne2 disruption (Fig. 6f), albeit the poor development of pups feeding from Kcne2^−/− dams probably arises from a combination of this and the impaired milk ejection of Kcne2^−/− dams (Fig. 6e). The mechanisms underlying the whole-animal and molecular effects of surrogacy appear complex, as one would expect. We speculate that the thyroid I⁻ accumulation of Kcne2^−/− pups is diminished by Kcne2 deletion but that this is partially balanced by e.g., adaptation to developing in a low maternal T₄ environment in the womb (Fig. 3c) and being initially fed with poorly-ejected, low-T₄ milk (Fig. 6e,f). The end result is that Kcne2^−/− pups are less efficient at accumulating thyroid I⁻ compared to Kcne2^+/+ pups when either are fed by Kcne2^+/+ dams, but when fed by Kcne2^−/− dams their thyroid RAIUs are similar (Fig. 6d). Part of this adaptation may involve reduced I⁻ excretion by Kcne2^−/− pups, as previously reported²⁸ and supported by our current data (Fig. 6d,e,g). Interestingly, as observed for NIS¹⁶^,³³, KCNQ1 is also expressed in mammary gland epithelium, where it may co-assemble with KCNE3 to play a role in K⁺ homeostasis³⁴. While a role for KCNE2 in mammary epithelial function should not be ruled out, our PET data demonstrate that mammary gland I⁻ uptake is not impaired in Kcne2^−/− dams (Fig. 5). Nevertheless, the phenotypes we describe herein for Kcne2^−/− pups bred from homozygous Kcne2^−/− crosses include features such as alopecia and cardiac hypertrophy, not always observed in hypothyroid mouse models³⁵. While this may at least partly be explained by the combination of both Kcne2^−/− dam and pup (heterozygous crosses are typically employed), it could possibly indicate additional pathogenesis beyond thyroid impairment but successfully treatable by TH supplementation.

Human thyroid dysfunction negatively impacts the brain, heart and GI tract; fatalities may occur from thyroid storm in hyperthyroidism, and myxedema coma in hypothyroidism³⁶. In addition, thyroid dysfunction during pregnancy increases the risk of adverse maternal and fetal outcomes³⁷^–³⁹. Subclinical human maternal hypothyroxinemia causes severe neurodevelopmental disorders⁴⁰, may include changes in blood lipid profile, myocardial function, and neuropsychiatric function⁴¹^–⁴³, and is an independent risk factor in heart failure due to structural and electrical remodeling in the heart²⁴. Importantly, a SNP near KCNE2 was recently shown to associate with early-onset myocardial infarction⁴⁴ – suggesting the possibility of a genetic link to previously-reported subclinical hypothyroidism-associated accelerated coronary artery disease and myocardial infarction⁴⁵.

Subclinical hypothyroidism is also associated with prolonged QTc⁴⁶, a hallmark of loss-off-function mutations in KCNE2 and KCNQ1³^,⁶, and with AF, an increasingly prevalent disease in the aging population⁴⁷^,⁴⁸ that is also associated with some KCNQ1 and KCNE2 gene variants ⁹^,¹². As many as 13% of patients with idiopathic AF exhibit biochemical evidence of hyperthyroidism⁴⁹; in one study, 62% of 163 patients reverted to sinus rhythm within 8–10 weeks after treatment for hyperthyroidism returned them to a euthyroid state⁵⁰. Therefore, the finding here that KCNE2-KCNQ1 channels contribute to thyroid function raises the tantalizing hypothesis that there is a thyroid component to some KCNE2- or KCNQ1-associated cardiac arrhythmias. In previous studies of sudden cardiac or unexplained death, it was often assumed that ion channel gene mutations were not causative in those cases exhibiting overt structural heart disease upon autopsy⁵¹; historically, ‘electrical’ heart diseases arising from ion channel defects have mostly been considered genetically distinct from ‘structural’ heart disease, although variants in the human SCN5A Na⁺ channel gene have been associated with dilated cardiomyopathy⁵². Our current findings suggest reconsideration of patients with structural heart disease exhibiting ventricular or atrial arrhythmias, given the possibility that mutations in KCNQ1 and KCNE2 could be arrhythmogenic due both to primary electrical defects in myocyte K⁺ channels containing these subunits, and to cardiac structural abnormalities arising secondarily from thyroid dysfunction due to defective thyroid KCNQ1-KCNE2 channels.

Identification of KCNE2-KCNQ1 as a thyrocyte channel important for I⁻ accumulation may also have therapeutic implications. Agonists and antagonists of KCNQ1-KCNE2 channels have already been developed. Because the pharmacology of KCNQ1-KCNE2 complexes is markedly different from that of homomeric KCNQ1, KCNQ1-KCNE1, or KCNQ1-KCNE3 channels⁵³, identification of the requirement of KCNQ1-KCNE2 complexes for normal thyroid function may permit semi-specific, reversible pharmacological targeting of the KCNQ1-KCNE2 complex to treat thyroid disease.

Supplementary Material

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ACKNOWLEDGEMENTS

G.W.A. is supported by the US National Heart, Lung and Blood Institute (HL079275) and the American Heart Association (0855756D). N.C. is supported by the US National Institute of Diabetes and Digestive and Kidney Diseases (DK41544) and the US National Cancer Institute (CA098390). M.P. is supported by US National Institutes of Health Medical Scientist Training Grant 5T32GM002788. We are grateful for expert technical assistance from S. Backovic, L. Cohen-Gould, G. J.-S. Abbott, K. La Perle, the Molecular Cytology Core Facility of Memorial Sloan-Kettering Cancer Center, and C. Basson and J. Chen (WCMC Center for Molecular Cardiology Small Animal Physiology Core Facility). We also thank T. Denecke for interpreting the radiographs and B. Abbott for critical reading of the manuscript.

ABBREVIATIONS


AF	atrial fibrillation

AW	anterior wall

cAMP	cyclic AMP

FS	fractional shortening

IVST	interventricular septal thickness

ESD	end-systolic dimension

EDD	end-diastolic dimension

LV	left ventricular

MiRP	MinK-related peptide

NIS	Na⁺/I⁻ symporter

PET	positron emission tomography

PW	posterior wall

T3	triiodothyronine

T4	tetraiodothyronine/thyroxine

TH	thyroid hormone(s)

TSH	thyroid-stimulating hormone.

Footnotes

AUTHOR CONTRIBUTIONS

All authors contributed to design of experiments, data analysis, figure preparation and manuscript writing.

All authors except E.F. and D.J.L. performed experiments.

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