Nonautoimmune Hyperthyroidism Caused by a Somatic Mosaic GNAS Mutation Involving Part of the Thyroid Gland

Introduction

Toxic multinodular goiter is a frequent cause of nonautoimmune hyperthyroidism. Although somatic mutations in the TSH receptor (TSHR) have been described as a common explanation for this disorder, GNAS defects can cause this phenotype as part of the complex McCune–Albright syndrome, and extremely rarely as isolated toxic thyroid adenomas due to activating GNAS mutations (1,2). In this study, we report a girl with nonautoimmune hyperthyroidism caused by a somatic mutation in the GNAS gene.

Case Report

A 4-year and 11-month-old girl of white European ancestry was found to be hyperthyroid when first evaluated for goiter and tall stature; height was +2.9 standard deviation score (SDS) and bone age 6 years and 10 months (+2.1 SDS). Her serum TSH was suppressed at <0.005 mU/L (0.4–3.6) and free thyroxine (fT4) was high at 4.52 ng/dL (0.85–1.75). Additional tests showed an elevation of her total T4 (TT4) of 17.4 μg/dL (5–11.6), TT3 of 513 ng/dL (90–180), and fT3 of 18.9 pg/mL (2–6). A thyroid autoimmunity screen (TSI, TRAb, TPO, and TG antibodies) was negative. Thyroid ultrasound showed a normal right upper lobe (RUL), while the right lower lobe and entire left lobe were heterogeneous with increased vascular flow (Supplementary Fig. S1). A ¹²³I uptake scan showed an increased uptake at 57% (reference range 8–40). There were no café-au-lait spots and no history of fractures or bone lesions. Treatment with methimazole (MMI) in doses ranging from 7.5 to 10 mg daily kept her thyroid function tests within the reference range, and by 9 years her height was in the 90th percentile and has remained between the 75th and 90th percentile until her current age of 10.5 years. At this time she is premenarchal (Tanner pubic hair 4 and breast development 3).

Further Investigations and Results from Genetic Analysis

Written informed consents were obtained from all family members before blood and tissue collection according to a protocol approved by the IRB. Her nonconsanguineous parents and sister had no manifestations of hyperthyroidism, and no abnormalities in their thyroid glands or tests (Fig. 1A). Sequencing of genomic DNA obtained from circulating leukocytes and cDNA generated from thyroid tissue obtained by fine needle aspiration revealed no TSHR variants.

OPEN IN VIEWER

FIG. 1.

Pedigree of the family with thyroid tests results and genetic analysis. (A) Family pedigree with results of thyroid function tests aligned below each symbol. Generations are indicated with Roman numerals and individuals with Arabic numbers above each symbol. The arrow indicates the proband (II-2) who was off MMI for two weeks before obtaining the blood sample and performing radioiodide scan (1B). High values are in bold and red. (B) Radioiodide scan of the proband's thyroid gland showing the locations from which samples were obtained by fine needle aspiration. The electropherograms show the region of GNAS cDNA in which heterozygous mutation (blue and red; C/T) in three of four quadrants or no mutation (WT; blue only; C) in the RUL. MMI, methimazole; RUL, right upper lobe.

MMI was discontinued for two weeks, which resulted in suppression of the serum TSH. A radioiodide scan was obtained, which showed no uptake in the RUL (Fig. 1B) where normal thyroid tissue was visualized by ultrasound (Supplementary Fig. S1). Total RNA was extracted from samples obtained by needle aspiration from each of the four quadrants of the thyroid gland and placed in Trizol^® reagent (Life Technologies). cDNA was generated using VILO^® reverse transcriptase (Life Technologies) following the manufacturer's instructions. Sanger sequencing revealedS a heterozygous missense mutation in the GNAS gene (ENST00000371100.4/NM_080425.3: c.2530C>T;p.Arg844Cys previously reported as NM_000516.5:c.601C>T;p.Arg201Cys) and referred herein as R201C, in three of four quadrants of the thyroid gland but not in the specimen obtained from the right upper quadrant showing suppressed radioiodide uptake (Fig. 1B). To determine the significance of the small peak in the corresponding electropherogram, synthesis of cDNA and sequencing was repeated along with those from a normal thyroid tissue and tissue from a papillary thyroid carcinoma. Small background peaks were observed in all three samples (Supplementary Fig. S2). Nevertheless, we cannot completely exclude the presence of a small number of cells expressing the mutant allele in the RUL. This mutation was absent in genomic DNA of the proband (Fig. 1C) and all family members, thus representing a somatic gain-of-function mutation. This mutation has been previously reported in dbSNP (rs11554273), COSMIC (COSM123397), and in ClinVar as pathogenic mutation associated with different phenotypes, such as McCune–Albright syndrome, Cushing syndrome, and pituitary adenoma. This mutation has not been reported in the gnomAD database (minor allele frequency = 0).

Given this finding, additional tests were obtained. Serum calcium, phosphorus, PTH, IGF1, IGF-BP3, cortisol, estrone, estradiol, FSH, and LH were all normal for sex and age.

Discussion

Gain-of-function mutations in genes of the cAMP-signaling pathway have been associated with autonomous hyperfunction of thyroid cells of mice and humans (1). The majority of activating mutations in patients with nonautoimmune hyperthyroidism involve TSHR defects, although GNAS mutations have been found in a small number of cases (2,3). In a study of 77 patients with hyperfunctioning toxic adenomas, only 5% were caused by somatic GNAS mutations (2). However, to our knowledge there is no report of a somatic GNAS mutation involving both lobes of the thyroid, yet sparing one portion. Activating GNAS mutations R201C and R201H have been reported in different endocrine tumors, involving the pituitary, testis, ovary, and thyroid adenomas and carcinomas (1). In vitro studies of R201C and R201H GNAS mutations have shown inhibition of GTPase activity leading to constitutive activation of adenylyl cyclase (4).

In conclusion, we report a case of nonautoimmune hyperthyroidism caused by a somatic mosaic GNAS mutation affecting part of the thyroid gland. Establishing that the mutation is present in only three of four quadrants of the thyroid gland allows for potentially selective surgical or radioiodide treatment, sparing the normal right upper thyroid lobe, and possibly maintaining euthyroidism without the need for lifelong thyroid hormone replacement.