Identification of a novel mutation in the thyroid hormone receptor β gene that causes thyroid hormone resistance syndrome: A case report.

Thyroid hormone resistance syndrome is a rare disease in which the level of thyroid hormone is elevated and the level of thyroid stimulating hormone is not suppressed. Mutations in the thyroid hormone receptor β (THRβ) gene are thought to be the primary cause of pathogenesis. In the present study, a Chinese boy of 4 years and 8 months, who had been pre‑diagnosed with resistance to thyroid hormone, was assessed for mutations. The clinical features and thyroid function of the proband and his parents were collected and gene mutations were analyzed using DNA sequencing. Gene sequencing showed that the THRβ genes in the parents of the proband were consistent with the standard sequence, however, in the proband there was a mutation in the tenth exon of the THRβ gene (c. 824 T>C). This is a newly identified mutation site and, to the best of our knowledge, there have been no previous reports of this mutation site. Therefore, it is hypothesized that this mutation is the cause of the pathology in the proband.

Introduction

Thyroid hormone resistance syndrome (RTH) is a disease characterized by a low response to thyroid hormone in target tissues, which may be caused by a thyroid hormone receptor (THR) gene mutation. RTH is a rare disease with an incidence of ~1 in 40,000 (1). This disease has a genetic predisposition, however, a few cases are sporadic (2); sporadic cases are more common in children and adolescents. The clinical features of RTH are diverse and are manifested in a number of ways, including hyperthyroidism, hypothyroidism or non-toxic goiter. However, significantly elevated blood thyroid hormone is a common clinical feature (3).

According to the different subtypes of mutant receptors, RTH is classified into thyroid hormone receptor α (THRα) gene mutation-induced RTH (RTHα) and thyroid hormone receptor β (THRβ) gene mutation-induced RTH (RTHβ) (4). In total, 85% of RTH cases are caused by mutations in the THRβ gene (2,5). RTH is an autosomal dominant heredity disease, with recessive heredity being less common (6). Gene sequencing is the gold standard for diagnosing RTH. The present study identified a rare mutation (p. I275T) in the THRβ gene in a patient with RTH. This is a novel gene mutation site and, to the best of our knowledge, there have been no reports of this mutation before.

Case report

The Ethics Committee of The First Hospital of Lanzhou University approved the present work. Informed written consent was obtained from the patient for publication of this case report and the accompanying images.

The proband (II:1), a Chinese boy of four years and eight months, visited the hospital due to growth retardation. The parents of the proband said that their son was born 2 weeks prematurely by a caesarean section. At 18 months of age, the proband was often dull and his height was 3–4 cm shorter than his peers. He was treated at a local hospital. There is no relevant medical history in the family. The previous medical records of the proband showed that his nutrition and skin condition was good, that he had no eye symptoms, his intelligence quotient (IQ) was normal, that his height and weight were lower than his peers (related data is shown in Table I), his thyroid was not large, and his thyroid function showed elevated levels of thyroid hormone and thyroid stimulating hormone (TSH). A physician prescribed levothyroxine sodium tablets for the proband. The proband had visited the doctor many times and the dose of the drug had been adjusted, however, the thyroid function did not recover (thyroid function and therapy process are shown in Table II). At present, the proband is prescribed with oral levothyroxine sodium tablets at a dose of 100 µg/day. The family tree of the proband and his parents are shown in Fig. 1. Among the two generations, only the proband had thyroid dysfunction and his parents had no relevant medical history.

Table I.

Height and weight of the proband.

	Age, years
Variable	2	2.5	3	3.5	4	4.5
Height, cm	77.2	80.1	86.3	90.5	95.2	99.6
Weight, kg	9.5	10.8	11.9	12.5	15.0	19.4

Table II.

Thyroid function and treatment program for the proband.

	Thyroid function
Date	FT3 (3.1–6.8 pmol/l)	FT4 (9.8–22 pmol/l)	TSH (0.31–5.3 mIU/l)	Treatment program (Levothy-roxine, µg/day)
12/22/2015	6.85	14.10	>150	25.0
01/22/2016	5.97	12.63	>150	37.5
03/22/2016	5.93	18.77	>150	50.0
07/19/2016	4.63	13.52	>150	50.0
10/05/2016	6.56	18.31	>100	62.5
11/12/2016	7.78	26.40	25.72	75.0
12/26/2016	9.22	36.94	0.90	87.5
04/24/2017	7.63	25.59	6.64	87.5
06/17/2017	9.16	29.31	6.82	87.5
04/02/2018	9.07	39.08	7.20	100.0
07/30/2018	7.90	36.80	9.44	100.0
08/11/2018	7.84	32.8	35.06	100.0

The values in brackets indicate reference values. FT3, free triiodothyronine; FT4, free thyroxine; TSH, thyroid stimulating hormone.

Figure 1.

Pedigree of thyroid hormone resistance syndrome. Among the two generations, only the proband had abnormal thyroid function and his parents had no relevant medical history.

Comprehensive examinations of proband and his parents were performed. Information about the age, height and weight, and heart rate of the proband and his parents is shown in Table III. The proband was shorter than his peers, however, other measurements for the proband were within the normal range. The thyroid function of the parents of the proband are shown in Table IV. The free triiodothyronine (FT3), free thyroxine (FT4) and TSH levels in the proband were higher than normal, while the thyroid function of his parents were normal. Peripheral blood examinations showed the following: Platelets, 348×109/l in routine blood tests (reference value, 100–300); trace element results, Zn 4.95 mg/l (reference value, 57.5–78), Ca 56.6 mg/l (reference value, 57.5–78), Fe 327 mg/l (reference value, 301–530), Mg 30.1 mg/l (reference value, 27–46), Pb 45.6 µg/l (reference value, 0–100), Cu 0.85 mg/l (reference value, 0.64–1.4); growth hormone 0.23 ng/ml (reference value, <3.0); insulin-like growth factor-1,151.00 ng/ml (reference value, 51–303); vitamin D 13.21 ng/ml (reference value, 30–60); intact parathyroid hormone 28.00 pg/ml (reference value, 16–87); bone alkaline phosphatase 210 U/l (reference value, 0–200); N-MID BGP (N-MID bone Gla protein) 63.83 ng/ml (reference value, 9.74–39.47); ferritin 96.77 ng/ml (reference value, 4.63–204), total cholesterol 4.11 mmol/l (reference value, 3.6–5.7), triglycerides 0.94 mmol/l (reference value, 0.8–1.8), creatine kinase 85 U/l (reference value, 38–240). The wrist anterior segment (Fig. 2) suggested that the bone age was less than the actual age. Thyroid color ultrasound examination showed normal. The thyroid iodine uptake was 10.2% at 2 h (reference value, 5–15%), 19.2% at 6 h (reference value, 15–25%), 31.8% at 24 h (reference value, 25–40%). The basal metabolic rate was 219 KJ/h. Pituitary nuclear magnetic imaging plain scan tips showed that the pituitary morphology was normal. The proband also had a normal electrocardiogram.

Table III.

Information about the proband and his parents.

Proband/patient	Sex	Age, years	Height, cm	Weight, kg	Heart rate (beats/min)
I:1	M	22	170	67	72
I:2	F	22	168	66	78
II:1	M	4	100	20	99

F, female; M, male.

Table IV.

Thyroid function of the parents of the proband.

Date	Parent	FT3 (3.1–6.8 pmol/l)	TT3 (1.14–3.02 nmol/l)	FT4 (9.8–22 pmol/l)	TT4 (75.45–148.18 nmol/l)	TSH (0.31–5.3 mIU/l)
03/22/2016	I:1	5.91	2.91	16.93	142.01	1.58
	I:2	4.83	2.87	15.72	112.40	0.83
08/11/2018	I:1	–	2.89	–	145.06	1.46
	I:2	–	2.81	–	105.97	0.72

The values in brackets indicate reference values. FT3, free triiodothyronine; FT4, free thyroxine; TSH, thyroid stimulating hormone; TT3, total triiodothyronine; TT4, total thyroxine.

Figure 2.

Wrist anterior segment of the proband. The wrist anterior segment suggested that the bone age was less than the actual age.

Gene sequencing suggested that the proband had a point mutation in the 10th exon of the THRβ gene (c. 824 T>C). T at position 824 of the THRβ gene coding sequence was replaced by C (Fig. 3), resulting in the replacement of isoleucine at position 275 with threonine (p. I275T). This is a novel mutation and, to the best of our knowledge, there have been no previous reports of this mutation. The parents of the proband had T at this position, which is consistent with the standard sequence. Computational mutation prediction was conducted (Fig. 4). The crystal structure of the THRβ-T3 complex was used as a reference wild-type structure. The THRβ conserved domain was analyzed using the NCBI/Structure/Cdd tool (https://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi). The amino acid at position 275 is located just inside the ligand binding site and the co-activation recognition site. The sequence-based mutation prediction for I275T was found to be damaging according to NCBI/Structure/Cdd.

Figure 3.

Partial sequencing results of exon 10 for the THRβ gene. (A) Reference gene sequence. The (B) father and (C) mother of the proband. (D) The proband. The arrows indicate the mutation site in the 10th exon of the THRβ gene (c. 824 T>C). The mutation results in T at position 824 of the THRβ gene being replaced with C, resulting in the replacement of the amino acid isoleucine at position 275 with threonine (p. I275T). THRβ, thyroid hormone receptor β.

Figure 4.

Computational mutation prediction. (A) Protein sequence of wild-type THRβ. The green triangle indicated by the red arrow is the isoleucine at position 275, which is located within the ligand binding site and the co-activation recognition site. (B) The protein sequence of the proband. The green triangle indicated by the red arrow is the threonine at position 275. (C) The crystal structure of wild-type THRβ-T3 complex and (D) the crystal structure of the THRβ-T3 complex from the proband. THRβ, thyroid hormone receptor β.

Discussion

Refetoff and Dumitrescu (1) first reported a case of RTH in 1967. It is thought that most cases of RTH are a result of autosomal dominant or recessive hereditary with familial onset, however, occasionally it is sporadic. RTH has no gender or ethnic differences and can occur in all age groups (7). According to the onset and clinical manifestations of RTH, the disease is divided into three categories, global resistance (GRTH), pituitary resistance to thyroid hormone (PRTH) and peripheral resistance to thyroid hormone (PrRTH). In total, ~80% of patients with RTH belong to the GRTH type, while PRTH is not common, and only a few cases of PrRTH have been reported (8). The pathogenesis of the disease is mainly through THR dysfunction caused by mutations of the gene encoding THRβ. However, ~10% of cases of RTH are due to other factors, including decreased number of THR, absence of THR and unknown causes (9). RTH is associated with the THRβ locus on chromosome 3 and THRβ gene mutations are considered to be the most important cause of RTH (10,11). In total, ~80% of RTH cases are caused by THRβ gene mutations, however, such mutations are not found in 10–15% of cases (7,8,10). The THRβ gene consists of 10 exons and 9 introns, encoding a protein of 461 amino acids. Gene sequencing is the golden standard for the diagnosis of RTH. To date, all THRβ mutations that have been identified are located in three hotspot regions between exons 7 and 10 (234–282, 310–353 and 429–461) (12–18).

RTH has various clinical manifestations and is difficult to diagnose, however, laboratory examinations identify specific characteristics; thyroid hormone and TSH levels are elevated during the course of the illness. The thyroid function of the proband showed that FT3, FT4 and TSH levels were elevated during the course of the illness. Even in the later period, FT3 and FT4 were higher than normal after the proband was treated with a high dose of levothyroxine tablets. Although TSH decreased, it did not reach normal levels. The THR gene of the proband was sequenced. The results of sequencing revealed a point mutation in the 10th exon of the THRβ gene (c. 824 T>C). This variant site is located within the three hotspot regions between exon 7 and 10. This point mutation caused the amino acid isoleucine at position 275 to be replaced with threonine (p. I275T). The physical and chemical properties of these two amino acids are quite different. Isoleucine is an aliphatic neutral amino acid, soluble in water and slightly soluble in ethanol. However, the pH of threonine is 5.0–6.5, it is soluble in water at high temperatures and insoluble in ethanol. In the present study, NCBI/Structure/Cdd was used to analyze the conserved domain of THRβ. The amino acid at position 275 is located just inside the ligand binding site and the co-activation recognition site. The sequence-based mutation prediction for I275T showed that this change was probably damaging to the protein. Therefore, it is hypothesized that the mutation site in the proband interferes with the binding of THR to ligands. When isoleucine is mutated to threonine at this site, the binding capacity of THR to the ligand may be greatly reduced, resulting in symptoms of RTH.

According to clinical features, laboratory examination, gene sequencing and treatment, the proband was diagnosed with RTH. The proband was treated with 100 µg of oral levothyroxine sodium daily for 1 year and clinical symptoms were gradually improved, however, TSH, FT3 and FT4 levels were still elevated. As the present treatment is effective, drug therapy continues. The current symptoms of the proband include slow growth and development, along with other manifestations, such as reduced activity and intelligence. It could be attributed to oral administration of a high dose levothyroxine sodium tablets, so the proband's growth retardation is improved compared to no treatment and IQ score is normal. As the proband is a child, follow-up treatment is important to promote growth, bone maturity and mental development. Therefore, it is recommended that the parents and physicians continue to observe the growth and mental development of the proband.

In conclusion, RTH has no specific clinical features, and may be associated with sinus tachycardia, goiter, hyperactivity, growth retardation and other symptoms, which makes diagnosing this disease difficult. Gene sequencing can improve diagnosis. Therefore, genetic sequencing is recommended for initial diagnosis and the appropriate treatment of patients with clinically suspected RTH.