The thyroid receptor beta gene and resistance to thyroid hormone: Implications for behavioral and brain research

The thyroid receptor beta gene and resistance to thyroid hormone: Implications for behavioral and brain research

Psychoneuroendocrinology, Vol. 19, No. 4, pp. iii-vii, 1994 1994 Elsevier Science Ltd Printed in the USA.All rights reserved Pergamon 0306-4530/94 ...

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Vol. 19, No. 4, pp. iii-vii, 1994 1994 Elsevier Science Ltd Printed in the USA.All rights reserved


0306-4530/94 $6.00+ .OO



21 February

1994; in final form 26 February


INTRODUCTION RESISTANCE TOTHYROIDhormone (RTH) is one of several hormone resistance syndromes that also includes androgen, glucocorticoid and vitamin D resistance syndromes. Since the description of pseudohypoparathyroidism and the introduction of the concept of hormone resistance over 50 years ago (Albright et al., 1942), numerous studies have defined the clinical and biochemical parameters of the various hormone resistance syndromes. It was not until recent advances in molecular genetic technology, however, that the underlying pathophysiological mechanism for the hormone resistance syndromes could be directly studied on a molecular genetic level. Molecular genetic studies have identified mutations in the genes that encode for the particular hormone receptor proteins. Futhermore, in vitro studies have shown that these mutations result in functional impairment of the receptor protein. Therefore, the mutations do not represent neutral polymorphisms. DEFINITION




The syndrome of resistance to thyroid hormone (RTH) is characterized by elevated thyroxine (T4) and triiodothyronine (T,) concentrations that are accompanied by inappropriately nonsupressed levels of thyroid-stimulating hormone (TSH) and a reduced responsiveness of pituitary and peripheral tissues to the actions of thyroid hormone (Refetoff et al., 1993). Although some subjects with RTH can have clinical features commonly associated with hypothyroidism, including short stature, delayed bone maturation, or goiter, the majority of them are clinically euthyroid since the elevated thyroid hormone concentrations reflect a compensatory physiological response of the thyroid-pituitary

Address correspondence and reprint requests to: Dr. Peter Hauser, Chief of Psychiatry, Baltimore Veterans Affairs Medical Center, 10 North Greene Street, Baltimore, Maryland 21201. The research studies described in this editorial were completed while Dr. Hauser was afXliated with the Molecular and Cellular Endocrinology Branch, National Institute of Diabetes and Digestive and Kidney Diseases (NIDDKD), NIH, Bethesda, MD 20892. ...




axis to tissue resistance. However, there is great variability in the degree of refractoriness to thyroid hormone action among subjects and in different target tissues within each individual (Magner et al., 1986). Although different kindreds exhibit discemable patterns of tissue resistance, the degree of resistance in a given tissue varies among affected subjects within each kindred. Years before the development of molecular genetic technology, the underlying pathophysiologic mechanism for RTH was postulated as an abnormality of the thyroid hormone receptor. However, it was not until the thyroid receptor gene (formerly known as the c-erb A gene) was identified (Weinberger et al., 1986), that the association of RTH to a defective receptor could be directly studied on a molecular genetic level. Two different thyroid receptor genes, alpha and beta, have been characterized and mapped to human chromosomes 17q11.2-21 and 3~21-25, respectively (Dayton et al., 1984; Gareau et al., 1988). The thyroid receptor is in the same nuclear hormone receptor superfamily as the glucocorticoid, estrogen, vitamin D, and retinoic acid receptors. These nuclear receptors share a common structure in that they have a ligand binding domain in the carboxyl-end of the receptor protein as well as a DNA-binding domain (Evans, 1988). The mechanism of action of thyroid hormone is initiated by hormone binding to the thyroid receptor. Once activated, the thyroid receptor protein can modulate the expression of specific thyroid responsive genes by binding to sequences in the regulatory region of these genes known as T, response elements or TREs. Employing the technique of restriction fragment length polymorphism (RFLP), initial studies from our laboratory demonstrated linkage of the RTH phenotype to the human thyroid receptor beta (hTR/3) gene on chromosome 3 (Usala et al., 1988). Subsequent studies of familial and sporadic cases of RTH have found distinct mutations in exons 9 or 10 of the hTRp gene, which codes for the hormone binding domain of the thyroid receptor (Mixson et al., 1992; Parilla et al., 1991; Usala et al., 1990). The vast majority of the more than 60 different mutations reported have involved only one of the two thyroid receptor p alleles of the affected members, confirming the previous familial studies suggesting that RTH is predominantly inherited in an autosomal dominant fashion (Magner et al., 1986; Refetoff et al., 1993). Most of the known mutations that have been identified are single base substitutions that cause a single nonconservative amino acid substitution in the thyroid receptor protein. In vitro functional studies of several different naturally occurring mutant thyroid receptors have shown a decreased or absent T, binding affinity compared to the normal receptor (Meier et al., 1992; Parilla et al., 1991). A recent study of four mutant thyroid receptors in transfected cell cultures directly correlated T, binding affinity and transcriptional activity (Meier et al., 1992). Moreover, the ability of the wild type hTR/3 to activate a positive TRE was not only reduced but inhibited by all four mutant thyroid receptors. This dominant-negative effect could be reversed by high concentrations of T3 in two mutant thyroid receptors with partial T, binding affinity, but not in the two mutant thyroid receptors with undetectable T, binding. However, studies to date have found no correlation between the degree of functional impairment of the mutant thyroid receptor and the severity or variability of the RTH phenotype. This suggests that additional mechanisms may contribute to the variability and severity of the clinical manifestations of RTH. Recent studies in two kindreds, using an allele-specific primer extension method, suggest that there is a differential expression of mutant and wild type alleles in subjects with RTH (Mixson et al., 1993). There was a modest correlation between the preferential expression of the mutant thyroid receptor mRNA and a more severe phenotypic form



of RTH. Also, in some RTH subjects, expression of the mutant receptor did decrease as age increased. Another explanation for the variability and severity of the clinical manifestations of RTH may be that the thyroid response elements in the regulatory region of various tissue-specific genes are of different types. Therefore, the interaction of a particular mutant thyroid receptor with a thyroid response element may differ according to the type of thyroid response element that is present. Furthermore, the mutant thyroid receptor could bind with certain auxiliary proteins necessary for gene transcription and thus decrease the availability of these accessory proteins for binding to the normal thyroid receptor. BEHAVIORAL



Several recent studies of subjects with RTH have underscored the importance of the human thyroid receptor beta gene in behavioral and brain development. Previous familial studies described poor school performance, learning disabilities and symptoms of hyperactivity as being among the more commonly reported somatic and neuropsychiatric presentations of RTH (Magner et al., 1986). It was not until our recent study of 104 affected and unaffected family members, however, that a systematic behavioral study, using structured psychiatric interviews, was undertaken in a large sample of subjects with RTH (Hauser et al., 1993a). In our study we showed a strong and specific association of attention deficit hyperactivity disorder (ADHD) and RTH. Fifty percent of RTH affected adults compared to 7% of unaffected adult family members, and 70% of RTH affected children compared to 20% of unaffected child family members met criteria for the diagnosis of ADHD. Other psychiatric diagnoses did not significantly differ between the two groups. Children with resistance had a IO-fold higher likelihood of the diagnosis of ADHD than their unaffected siblings. The identified mutations that cause RTH are clustered predominantly in either exon 9 or exon 10 of the hTRp gene (Parilla et al., 1991). Although subjects with mutations in either of these two clusters are similarly affected with ADHD, language disorders have been reported to be significantly more common in subjects with mutations in exon 9 than in exon 10 (Mixson et al., 1992). This difference is interesting to consider given that a significant percentage of children diagnosed with ADHD are codiagnosed with a specific developmental disorder or learning disability (Shaywitz & Shaywitz, 1991). Exon 9 and exon 10 mutations variably decrease thyroid receptor affinity for thyroid hormone, which could be the causative mechanism for the presence of ADHD in subjects with RTH. However, exon 9 mutations may additionally alter the thyroid receptor protein such that it no longer interacts with certain auxiliary proteins. This lack of interaction may subsequently change the activity of certain target genes during critical periods of language development in subjects with exon 9 mutations. Our recent IQ and magnetic resonance imaging (MRI) studies of subjects with resistance to thyroid hormone further suggest an important role for the thyroid receptor beta gene in brain and intellectual development (Hauser et al. 1993b). We evaluated intelligence in 59 subjects with resistance to thyroid hormone and 50 unaffected family members using the age-appropriate Wechsler scale and Wide Range Achievement Test-revised. The mean IQ scores of subjects with resistance were significantly lower than those of unaffected family members for verbal, performance and full scale IQ, as well as for reading and arithmetic achievement tests. Furthermore, a comparison of 16 of these resistance subjects to their unaffected sibling closest in age showed that the mean IQs



of the resistance group was over 10 points lower in all categories. However, only 3 of 59 subjects with resistance had a full scale IQ score under 70, the upper limit for a diagnosis of mild mental retardation. MRI scans were obtained on 43 of these resistance subjects and 32 unaffected family members. The scans were assessed by a rater blind to the subject diagnosis and sex for the presence of cerebral anomalies, particularly Sylvian fissure anomalies and multiple Heschl’s gyri. These types of anomalies have been demonstrated in subjects with learning disorders not associated with resistance (Leonard et al., 1993). Male resistance subjects had a significantly increased frequency of cerebral anomalies as compared with their unaffected relatives, but female subjects did not. Seventy percent of male resistance subjects had a left Sylvian fissure anomaly. Taken together these data emphasize the importance of the hTR/3 gene in behavior, intelligence and brain development. CONCLUSION The effects of thyroid hormone on brain growth and development are well-characterized. However, it is important to note that the mechanism of thyroid hormone action is initiated by its binding to the thyroid receptor. The activated receptor protein can in turn modulate the expression of specific thyroid responsive genes by binding to thyroid response elements in the regulatory region of these genes. Although the expression of myelin basic protein mRNA has been shown to be mediated by the interaction of the activated thyroid receptor with a thyroid response element in the promotor region of this gene in rat brain (Farsetti et al., 1991), thyroid response elements have not yet been identified in the regulatory region of human genes critical for normal brain development. In congenital hypothyroidism, the lack of thyroid hormone prevents activation of the thyroid receptor and, subsequently, the thyroid response elements of various target genes necessary for normal brain development. If not treated within the first 6 months of life, congenital hypothyroidism results in severe mental retardation that is relatively consistent from individual to individual. Unlike congenital hypothyroidism, resistance to thyroid hormone has a much more variable consequence on behavioral and intellectual outcome. Possible molecular genetic explanations for this variability include the location of the mutation and the differential expression of mutant receptor mRNA relative to the wild type. The location of the mutation can determine the degree of decrease in thyroid hormone binding affinity for the mutant thyroid receptor protein. Also, the location of the mutation may disrupt the normal interaction of the thyroid receptor with certain as yet unidentified auxiliary proteins necessary for the development of particular brain functions. The association of exon 9 mutations with language disorders may serve as an example. The preferential expression of the mutant receptor may not only explain differences described in phenotypic severity of resistance to thyroid hormone, but may also provide a mechanism for age-related changes in the severity of illness. This is an interesting consideration as some studies have suggested that ADHD symptoms improve with age. Finally, other factors, such as the compensatory ability of the organism to overcome the tissue resistance by increasing thyroid hormone concentrations, could ameliorate the severity of illness. Although thyroid receptor abnormalities may be discovered without obvious accompanying elevations of thyroid hormone concentrations, resistance to thyroid will probably remain an uncommon disorder. The value of resistance to thyroid hormone for brain and behavioral research is, however, as one genetic model for certain behavioral disorders



of childhood. Furthermore, this model allows us to study the interaction of various genetic and nongenetic factors with a known genetic abnormality and thereby test hypotheses that may explain the variability and severity of the clinical phenotype. Acknowledgments: I am indebted to John Matochik, PhD, and A. James Mixson, MD, for their critical review of the manuscript. Particular thanks to Marla Prather, PhD, for her excellent editorial assistance and to Bruce D. Weintraub, MD, who continues to be a source of inspiration.

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