Stefano Stagi a Loredana Cavalli b Laura Congiu c Maria Flora Scusa c Alessandra Ferlini d Stefania Bigoni d Alberto Benincasa c Bruno Rossi b Giorgio Pini c
a Health Sciences Department, University of Florence, Anna Meyer Children’s University Hospital, Florence , b Department of Neuroscience, Section of Neurorehabilitation, University of Pisa, Pisa , c Tuscany Rett Center, Versilia Hospital, Viareggio , and d Medical Genetic Section, Department of Medical Sciences, Ferrara University, Ferrara , Italy
patients (26.7%) showed higher FT 3 levels than the upper reference limit, significantly differing in respect to controls (2.0%, p < 0.0001). Finally, 5 patients (11.1%) showed higher levels of TSH, statistically differing from the control subjects (2.0%, p < 0.0001). However, evaluating the patients on the basis of different RTT genotype subgroups, patients with CDKL5 deletions showed significantly higher FT 4 values than patients with MeCP2 deletions (p < 0.05). On the other hand, patients with other types of MeCP2 mutations also showed FT 4 levels significantly higher than patients with MeCP2 de-letions (p < 0.05). In fact, out of 8 patients with FT 4 levels higher than the upper references limit, 3 of them presented with CDKL5 deletions (3 patients, 37.5%), 4 (50%) had MeCP2 mutations, and 1 (12.5%) belonged to the subgroup of MeCP2 deletions. However, when analyzing FT 3 levels of the 12 patients showing higher FT 3 levels than the upper refer-ences limit, 6 (50%) belonged to the subgroup with MeCP2 mutations, 4 (33.3%) to the subgroup with MeCP2 deletions, and 2 (16.7%) to the subgroups with CDKL5 deletions. Fur-thermore, no patient with RTT was positive for antithyro-globulin autoantibodies, antithyroid peroxidase, or anti-TSHr, with no statistical differences in respect to the con-trols. L -thyroxine treatment was not necessary for any patient. Conclusions: Abnormalities of thyroid function are not rare in RTT. The possible relationship between these dis-
Key Words
Children · Rett syndrome · Thyroid function · Thyroid autoimmunity · Subclinical hypothyroidism · Hypothyroidism · Thyroid hormone
Abstract
Introduction: Thyroid function in Rett syndrome (RTT) has rarely been studied with unanimous results. However, this aspect is of great concern regarding the effect thyroid hor-mones (TH) have on proper mammalian brain development. Objective: To evaluate the prevalence of abnormalities of thyroid function in a cohort of children with RTT. Patients
and Methods: Forty-five consecutive Caucasian girls (mean age: 8.6 ± 5.3 years, range: 2.0–26.1) meeting the clinical cri-teria for RTT were recruited. In all of the subjects, we evalu-ated the serum concentrations of free-T 3 (FT 3 ), free-T 4 (FT 4 ), thyroid-stimulating hormone (TSH), thyroperoxidase auto-antibodies, thyroglobulin autoantibodies (TgA), and TSH re-ceptor (TSHr) autoantibodies. The results were compared with a group of 146 age-matched healthy Caucasian chil-dren and adolescent girls (median age: 9.5 years, range: 1.8–14.6) from the same geographical area. Results: Mean FT 3 and TSH levels were not significantly different between the RTT patients and controls. Nevertheless, FT 4 levels were sig-nificantly higher in RTT patients than in controls (p < 0.005). In particular, 17.7% showed FT 4 levels higher than the upper reference limit (vs. 0.7% of controls, p < 0.0001), whereas 12
Received: August 25, 2014 Accepted: November 24, 2014 Published online: January 21, 2015
HORMONERESEARCH IN PÆDIATRICS
Dr. Loredana Cavalli Via Valfonda 7 IT–50123 Firenze (Italy) E-Mail cavallil.doc @ gmail.com
Rett syndrome (RTT; OMIM 312750) is an X-linked neurodevelopmental disorder that affects almost exclu-sively girls [1] . It is characterized by loss of spoken lan-guage and hand use with the development of distinctive hand stereotypies, and was originally described by An-dreas Rett in the 1960s [2, 3] . The prevalence of RTT ranges from 1: 10,000 to 1: 22,000 [4] .
Typical RTT girls have an apparently normal develop-ment until the 6- to 1-month period, and then present a progressive deterioration or loss of acquired skills such as purposeful hand function and communication, decelera-tion of head growth, and later the appearance of stereo-typic hand movements. This syndrome is associated to comorbidities including reduced somatic growth, gastro-intestinal disease, osteopenia, gait apraxia, scoliosis, au-tonomic dysfunction, breathing disturbances, and com-monly seizures [1, 3, 5–11] .
In addition to typical or classical RTT, some individu-als present with many, but not all, of the clinical features of RTT, thus there are ‘variant’ or ‘atypical’ RTT, which have been found to cluster in some distinct clinical group-ings, such as the ‘preserved speech variant’, which is char-acterized by the recovery of some degree of speech [12] , the ‘congenital variant’, which is recognized at birth and was recently identified with the mutation in FOXG1 gene [13] , and the ‘early-onset seizure variant’, which is also known as the ‘Hanefeld variant’ [14] .
In 90–95% of the cases with typical RTT, mutations in Methyl-CpG binding protein 2 gene ( MeCP2 ; OMIM 300005) have been detected. On the other hand, patients with Rett phenotype and early-onset of epilepsy are known to be caused by mutations in the cyclin-dependent kinase-like 5 gene ( CDKL5 ; OMIM 300203) [15] . In con-trast, males, who are hemizygous for comparable muta-tions, generally do not survive [16] .
Thyroid function has been rarely studied in RTT pa-tients, but one study reported a higher prevalence of lower mean serum total thyroxine (TT 4 ) [17] , whereas another study did not disclose problems in free-T 4 (FT 4 ) and thy-roid-stimulating hormone (TSH) levels in these patients [18] . However, this aspect is of great concern regarding the effect of thyroid hormones (TH) on proper mammalian
brain development [19] . Therefore, the purpose of this study was to evaluate the prevalence of abnormalities of thyroid function in a cohort of children with RTT.
Subjects and Methods
Forty-five consecutive Caucasian girls (mean age: 8.6 ± 5.3 years, range: 2.0–26.1) fulfilling the clinical criteria for RTT ac-cording to the lastest updated revision [3] were recruited from October 2006 to August 2013 at the Rett Center, Versilia Hospital, Lido di Camaiore, Lucca, Italy. Of these patients, 32 showed clas-sical RTT, 7 showed a preserved speech variant, and 6 showed a Hanefeld variant.
In all patients, MeCP2 and/or CDKL5 gene molecular analysis (sequencing and MLPA) was performed. All of the mutations identi-fied in the MeCP2 molecular analysis were divided into two main groups: we named the first group ‘Mutations’ (including variations as point mutations like missense mutations, nonsense mutations, and frameshift mutations) and the second one ‘Deletions’ (including variations such as exonic deletions and large deletions). In particular, MECP2 mutations were grouped into the following categories: early or late truncation (defined as truncating before amino acid 271 or after amino acid 271, respectively), missense, nonsense, and frame-shift. Finally, a third group was characterized by the involvement of CDKL5 gene, which presented deletions in all of the cases.
RTT clinical severity was assessed using either the total clinical severity score, a validated clinical rating specifically designed for RTT [20] or the use of the Rett Syndrome Behaviour Question-naire (RSBQ) [21] . These data are reported in table 1 .
Table 1. Demographic data and prevalence of thyroid dysfunction and autoimmunity in patients with RTT and controls
RTT Controls p
Patients, n 45 146 –Age, years 8.60±5.3 9.0±5.8 –Height, SDS –0.67±1.24 0.03±1.19 <0.005BMI, SDS –0.80±1.59 0.34±1.47 <0.0001Tanner stagePrepubertal, n (%) 32 (71.1%) 99 (67.8%) –Pubertal, n (%) 13 (28.9%) 47 (32.2%) –Subjects, n –
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Ethical approval was obtained from the ethics committee at Versilia Hospital. Written informed consent was obtained from the parents after an explanation of the nature of the study.
Study Protocol In all subjects the serum concentration of free-T 3 (FT 3 ), FT 4 ,
TSH, thyroperoxidase autoantibodies, thyroglobulin autoantibod-ies, and TSH receptor (TSHr) autoantibodies was evaluated. Insu-lin-like growth factor 1, a hormone which is routinely measured in RTT patients due to its important role in childhood growth, was also evaluated and reported.
Additionally, continuous data such as height, pubertal staging, weight and BMI were collected. As previously described [22] , height and BMI were considered as standard deviation scores (SDS), which were calculated according to the following formula: patient value − mean of age-related reference value/SD of the age-related reference value.
The age of pubertal onset was defined as the age at durable Tan-ner B2 stage for females or a testicular volume more than 4 ml for males (G2). Pubertal growth was defined as growth occurring be-tween the onset of puberty and the final height attained by the in-dividual [22] .
Control Group To compare thyroid function and autoimmunity, a group of
146 age-matched healthy Caucasian girls and adolescent females (median age: 9.5 years, range: 1.8–14.6, p = n.s.) from the same geographical area was selected. These control subjects, who were admitted to the hospital for minor surgery (e.g. adenotonsillecto-my, phimosis, dermoid cyst, herniotomy), and in whom a blood sample was collected before surgical treatment, had no neurologic disorders. Studies regarding part of this group have been previ-ously reported [23] .
Laboratory Methods All samplings from RTT patients and healthy controls were
carried out around 8 a.m. after overnight fasting. Blood was col-lected in heparinized tubes and all manipulations were carried out within 2 h after the sample collection was taken. The blood samples were centrifuged at 2,400 g for 15 min at 4 ° C; platelet-poor plasma was saved and the buffy coat was removed by aspiration. Plasma samples were stored at − 70 ° C until assayed.
TSH, FT 4 , and FT 3 serum levels were measured by immunomet-ric assays (Immulite TM 2000, Third Generation; DPC Diagnostic Products Corporation, Los Angeles, Calif., USA) and coefficients of variation were <8.5% for TSH, <7.5% for FT 4 , and <9.1% for FT 3 . The normal range of TH and TSH are, respectively: FT 4 : 9.27–20.46 pmol/l; FT 3 : 3.53–6.91 pmol/l; u-TSH: 0.4–4.0 μIU/ml [24] .
Subclinical hypothyroidism was defined as a TSH level above the upper reference limit for age in conjunction with normal se-rum TH levels. Overt hypothyroidism was defined as persistently increased TSH levels with decreased serum TH levels.
Thyroid autoimmunity was evaluated by fluorescence enzy-matic immunoassays of thyroglobulin autoantibodies and thyro-peroxidase autoantibodies; thyroglobulin autoantibody values were considered positive if ≥ 50 IU/ml and thyroperoxidase auto-antibody values were considered positive if ≥ 100 IU/ml [23] . TSHr was measured with THBIA (DiaSorin Spa, Vercelli, Italy) using a two-step radioreceptor assay; TSHr was considered positive when values were >9 U/l.
Statistical Analysis Statistical analyses were performed with SPSS (SPSS Inc.,
Chicago, Ill., USA). A χ 2 test or Fisher’s exact test, when appropri-ate, were used to compare differences between the patients and control subjects. Bonferroni’s correction for multiple comparisons was also applied under selected instances. Summaries of the con-tinuous variables are given as means ± SD and range. Spearman’s correlation test was used to determine correlation coefficients. A multiple stepwise regression analysis was used to identify variables that may correlate independently with FT 4 and TSH levels after adjusting for multiple potential confounding factors, such as age and degree of under- or overnutrition. Statistical tests were two-tailed and considered to be significant when p < 0.05.
Results
The primary characteristics of our study cohort and controls and the results of thyroidal function study are summarized in tables 1–3 .
In our cohort of 45 patients, we demonstrated MeCP2 gene deletions in 10 patients, other types of MeCP2 mu-tations in 29 patients, and CDKL5 deletions in 6 pa-tients.
Regarding biochemical analyses, the average FT 3 levels were not significantly different between the RTT patients (6.16 ± 1.10 pmol/l) and control subjects (6.43 ± 0.93 pmol/l, p = n.s.). TSH levels, which were sufficiently com-parable between the groups, were also not significantly different (2.41 ± 1.45 vs. 2.77 ± 1.31 μIU/ml, p = n.s.). Nevertheless, FT 4 levels were significantly higher in RTT
Table 2. Characteristics of thyroid function in three different RTT subtypes
hypothyroidism 14.7%^^^ 0%*** 0% * The MeCP2 mutations group shows higher values than the MeCP2 deletions group. ^ The MeCP2 mutations group shows higher values than the CDKL5 mutations group. † The CDKL5 mu-tations group shows higher values than the MeCP2 deletions group. The number of symbol *, ^, or † repeated after the values is proportional to the amplitude of the difference between the rela-tive groups.
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patients than in control subjects (17.15 ± 4.43 vs. 14.52 ± 4.12 pmol/l, p < 0.005).
In particular, 8 patients (17.7% of the RTT group) showed FT 4 levels higher than the upper references limit (0.7% of controls, p < 0.0001); on the contrary, none of them presented reduced FT 4 levels, thus were not signifi-cantly different from the control subjects. However, ana-lyzing each single girl’s biochemical parameters, 12 pa-tients (26.7%) showed FT 3 levels higher than the upper references limit, significantly differing in respect to con-trols (2.0%, p < 0.0001). On the other hand, no patients showed low FT 3 levels and were thus the same as the con-trols. Finally, evaluating TSH levels, 5 patients (10.2%) showed high levels of TSH, which differed from the con-trols in a statistically significant manner (2.0%, p < 0.0001).
Furthermore, evaluating the patients on the basis of the different RTT subgroups, patients with CDKL5 dele-tions showed significantly higher FT 4 values (20.72 ± 3.40 pmol/l) than patients with MeCP2 gene deletions (16.29 ± 3.00 pmol/l, p < 0.05) and MeCP2 mutations (16.71 ± 4.76 pmol/l, p < 0.05). On the other hand, none of the patients with MeCP2 mutations showed FT 4 levels significantly higher than the patients with MeCP2 deletions.
No statistically significant differences were found among the subgroups regarding the TSH levels, which were 1.74 ± 0.80 UIμ/ml in patients with CDKL5 dele-tions, 2.60 ± 1.49 μIU/ml in those with MeCP2 deletions, and 2.48 ± 1.54 μIU/ml in the ones with MeCP2 muta-tions.
Finally, when evaluating FT 3 levels, we noticed no sig-nificant differences among the subgroups: 6.31 ± 1.58 pmol/l in patients with CDKL5 deletions, 6.41 ± 1.04 pmol/l in MeCP2 deletions, and 5.75 ± 0.47 UIμ/ml for those with MeCP2 mutations.
Moreover, dividing the RTT patients on the basis of mutation types, girls with MeCP2 missense mutations did not present significant differences regarding FT 4 (16.73 ± 5.33 pmol/l), TSH (2.48 ± 1.59 μIU/ml), or FT 3 (6.09 ± 1.17 pmol/l) in respect to patients with MeCP2 nonsense, frameshift and deletion mutations (FT 4 : 16.44 ± 2.94 pmol/l; TSH: 2.54 ± 1.52 μIU/ml; FT 3 : 6.09 ± 0.89 pmol/l). Finally, there were also no differences when comparing FT 4 , FT 3 , and TSH levels in RTT patients with MeCP2 early and late truncating mutations.
Interestingly, we detected that the patients with FT 4 levels higher than the upper reference limit had CDKL5 deletions (3 patients, 37.5%) or MeCP2 mutations (4 pa-tients, 13.3%, p < 0.0001), whereas one of the patients with MeCP2 deletions showed abnormal FT 4 levels (10%, Pa
p < 0.0001). Additionally, among the 12 patients showing FT 3 levels higher than the upper reference limit, 6 (50%) patients belonged to the subgroup with MeCP2 muta-tions, 4 (33.3%) patients belonged to the subgroup with MeCP2 deletions (p < 0.0005), and 2 (16.4%) patients be-longed to the subgroup with CDKL5 deletions (p < 0.0001 vs. MeCP2 mutations, p < 0.0005 vs. MeCP2 deletions). However, the patients with higher TSH levels belonged to the subgroup with MeCP2 deletions (4/5: 80.0%) in re-spect to MeCP2 deletions (1/5, 20%, p < 0.0001) and CDKL5 mutations (0/5, p < 0.0001).
Concerning autoimmunity, no patient with RTT was positive for antithyroglobulin, antithyroid peroxidase, or antiTSHr, with no statistical differences in respect to the controls (1.14%, p = n.s.). Therefore L -thyroxine treat-ment was not necessary for any patients.
Analyzing the correlations between FT 4 levels, age, in-sulin growth factor 1, height SDS, BMI SDS, type of gene mutations, FT 3 levels, and TSH levels, we observed that FT 4 levels correlated with BMI SDS (r = –0.38, p < 0.005), height SDS (r = –0.62, p < 0.0001), and IGF-1 levels (r = –0.31, p < 0.05). However, FT 3 levels correlated with BMI SDS (r = –0.35, p < 0.05). As expected, height SDS showed a positive correlation with IGF-1 levels (r = 0.33, p < 0.05).
Discussion
To the best of our knowledge, our data is the first to show that patients with RTT may present a higher preva-lence of thyroid function abnormalities. In this regard, subtle changes of the TH levels in RTT patients have been reported in a previous study, although no evidence of clinical hypothyroidism was present in the affected pa-tients [17] . Normal age-appropriate plasma values for T 4 , TSH, and TSH-night rhythm were found in another study [18] .
Our results are of great concern because TH is essential for proper mammalian development [19] . In fact, the lack of a proper TH status results in cretinism, a condition characterized by mental retardation, ataxia, spasticity, and deafness [19] .
In contrast to congenital hypothyroidism, an RTT pa-tient’s brain does not show obvious signs of neurodegen-eration, atrophy, gliosis, demyelination, or neuronal mi-gration defects [25, 26] , suggesting that neurological symptoms may primarily stem from subtle defects of sub-cellular compartments such as dendrites, axons, or syn-aptic structures [27] .
MECP2 is thought to be necessary to stabilize and maintain the mature neuronal state, and its absence causes an abnormal expression of a large number of genes, with implications in the balance between synaptic excitation and inhibition [28] . However, MECP2 has a higher level of complexity than originally believed [29] . MECP2 has been coimmunoprecipitated with the silenc-ing mediator for retinoid and TH receptors [30] .
In fact, the disease-risk MeCP2 haplotype was associ-ated with increased expression of the MeCP2 transcrip-tion coactivator CREB1 and decreased expression of the corepressor histone deacetylase 1. CREB-binding protein has been shown to be a critical coactivator for TH recep-tors, even if in Rubinstein-Taybi syndrome, which is caused by a mutation of this protein and characterized by mental and growth retardation, TH resistance does not appear to be a typical feature [31] .
Our results may be compatible to this hypothesis, showing higher FT 3 and FT 4 levels in many patients with RTT associated with normal TSH levels. This may reflect the attempt, through increased levels of FT 3 and FT 4 , to increase the action of TH at the central nervous system level. Interestingly, in another condition known as Allan-Herndon-Dudley syndrome, a form characterized by con-genital hypotonia that progresses to spasticity with severe psychomotor delays, caused by the deficiency of mono-carboxylate transporter 8, the typical thyroid profile is characterized by elevated serum FT 3 and total T 3 , low or normal FT 4 and total T 4 , and normal TSH levels [32] .
TH action is mediated through the binding of T 3 to nuclear TH receptors within the target cell, and its degree of biologic activity is related to T 3 levels within the cell. In most target cells, T 4 is converted to T 3 by the iodothyro-nine deiodinases [33] . Once in the cells, it binds to a TH receptor, which then forms a complex with the retinoid X receptor. This complex binds to a T 3 -responsive element and, in turn, causes a change in the transcription of spe-cific genes and thus in the subsequent translation into proteins [33] .
In hypothyroid patients, dendritic spines are sites of synaptic connections; dendritic changes may thus direct-ly affect synapse number and formation, and thereby un-derlie cognitive deficits [34] . Interestingly, morphologi-cal changes observed in RTT also include alterations in dendritic shaft and spine number [35] . Although the dis-orders are quite different in some of their manifestations, the shared features of severe mental retardation and stunted axodendritic growth suggests some similarity with respect to disrupted gene expression in the develop-ing brain [36] .
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It has been observed that the MeCP2 gene is widely expressed in embryonic and adult tissues, although its ex-pression is at low levels early in development [37] . By means of the Northern blot technique in adult tissues, MeCP2 was expressed in brain and spinal, lung, kidney, tissues from the gastrointestinal tract, and thyroid and adrenal glands, whereas the smaller transcript is more abundant in cardiac and skeletal muscle, lymphoid tis-sues, liver, and the placenta [38] .
MeCP2 also interacts with two other corepressors, c-Ski and the nuclear receptor corepressor [39] . These are components of histone deacetylase complexes that can but do not always act together [39] . Neonatally induced hypo-thyroidism inhibits high levels of c-Ski expression [40] .
Interestingly, as regards transthyretin, a transporter for thyroxin from the bloodstream to the brain [41] , a re-cent study measured its levels in RTT patients with a pro-teomic approach [20] , and found that 15-kDa spot trans-thyretin was uniformly underexpressed in all of the exam-ined RTT patients, while the 30-kDa spot appeared to be normally expressed in the R306C and T158M patients but overexpressed in the R168X and MeCP2 large deletion types. In light of these findings, we can hypothesize that in RTT patients, TH levels may be in relation to the un-derexpression or overexpression of transthyretin in an at-
tempt to compensate for its levels in the cerebrospinal fluid.
No data are available to make a hypothesis about a mo-lecular connection between CDKL5 deletions and thyroid dysfunction, but our findings suggest that regardless of the specific mutation type, patients with CDKL5- related RTT present significantly different values of TH than control subjects.
In conclusion, abnormalities of thyroid function were shown in a cohort of 45 RTT patients. The possible rela-tionship between thyroid disorders and RTT phenotype should be confirmed and studied. Children with RTT should be screened for potential thyroid dysfunction.
Acknowledgements
The TIAMO Foundation (‘Tutti Insieme Associazione Malattie Orfane’) and the grants from RARER (Emilia Romagna Region, Area 1-Research and Innovation in Rare Diseases, 2012) and SIGN (Slovenian Italian Genetic Network) are acknowledged.
Disclosure Statement
The authors declare that they do not have neither financial nor non-financial conflicting interests.
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