NKX2.1-Related Disorders: a novel mutation with mild clinical presentation

Background A highly variable phenotype characterized by thyroid, respiratory and neurological defects has been reported in an already established group of disorders namely NKX2.1-related disorders. We describe here the case of an infant with a novel mutation of the NKX2.1 gene characterized by mild clinical presentation. Aim of the study was to elucidate the genotype-phenotype correlation in our patient. Methods We performed genetic analysis of the NKX2.1 gene in an infant with no neonatal respiratory distress and near-normal results at neonatal screening test for congenital hypothyroidism, choreoathetosis, ataxia and delayed independent walking. Results A novel mutation of the NKX2.1 gene has been identified, that is responsible for a mild framework of congenital hypothyroidism and neurological symptoms. Conclusions The frequency of congenital hypothyroidism cases associated with NKX2.1 mutations is expected to be higher in a subgroup of patients, selected according to the neurological presentation. In these patients the analysis of NKX2.1 mutational status is recommended.


Background
The management of the classical form of congenital hypothyroidism (CH) has changed thanks to the introduction of neonatal screening programs, which allow early diagnosis and replacement therapy. Over the last few years the progressive reduction of TSH-cutoff at neonatal screening and the advancements in the molecular analysis led to the definition of novel syndromic forms of CH, that are associated with mild thyroid deficiency [1].
Mutations of the NKX2.1 gene, previously known as TTF1 (thyroid transcription factor 1, OMIM*600635), mapping on chromosome 14q13, have been identified and associated with a syndromic form of CH (OMIM*610978) [2], initially named brain-lung-thyroid syndrome [3] due to the broad phenotypic spectrum including a variable combination of lung, thyroid and neurological defects.
So far, ninety-six mutations of the NKX2.1 gene have been reported , 70 of which are point mutations and 26 large deletions, often spanning the entire gene (HGMD Professional 2014.2).
The NKX2.1 gene has three exons and encodes for a member of the NK-2 family of transcription factors [14] that mediates thyroid-specific genes transcription. During embryogenesis the gene is expressed in the thyroid gland, in the lung and in the forebrain, especially in the hypothalamic area, infundibulum and basal ganglia region [15][16][17].
We identified a novel mutation of NKX2.1 in an infant showing neurological symptoms associated with nearnormal results at CH screening test, negative history of neonatal respiratory distress and recurrent pulmonary diseases.

Case presentation
Child of non consanguineous parents, the patient was born at 41 weeks of normal pregnancy, with normal birth weight (3965 g). The family history reported two cases of walking delay in the paternal line and several cases of autoimmune thyroid diseases in the maternal line. Both parents showed normal thyroid function. Neonatal adaptation was normal (APGAR 1°9, 5°10), with no respiratory distress at birth. At neonatal screening, TSH spot value was equal to the cut-off level (10 mUI/L). The test was repeated on blood spot and TSH levels became normal (4.4 mUI/L), thus no further analysis was performed. In the first months of life, the infant showed a progressive impairment in neurological development, characterized by choreoathetosic movements, disordered motility in the lower limbs, reduced motility of the upper limbs, axial hypotonia and development delay. At 8 months of age the analysis revealed a mild hypertireotropinemia with normal fT4 values. Thyroid ultrasound showed a normal in situ gland. Electroencephalography and brain MRI were normal at 12 months of age. Due to the progressive increase of TSH levels, the infant started the replacement therapy with L-Thyroxin at 13 months. Growth parameters and pulse rate were recorded, electrocardiography was performed and blood samples were taken in order to determine TSH and free T4 serum levels. Biochemical and clinical features measured at follow-up are reported in Table 1.
Serum free T4 and TSH levels were measured using a commercial chemiluminescent assay (Bayer, Fenwald, Germany). The clinical manifestation of the patient resembled defects that have been previously associated with mutations of the NKX2.1 gene. Therefore, DNA sequencing analysis was performed after obtaining informed consent. Genomic DNA was extracted from peripheral blood leukocytes of the child and his parents and the NKX2.1 gene coding region, comprising exon 1-3 of isoform 1, as well as the exon-intron boundaries were amplified by PCR and directly sequenced by Sanger sequencing.
The mutation was not detected in the parents, as shown in Fig. 1. Upon administration of the replacement therapy, the patient showed a progressive improvement in neuromotor and growth development. Of note, the height increased from −0,63 SDS to −0,12 SDS after one year of treatment.

Discussion
We report the case of an infant with a novel mutation of the NKX2.1 gene, showing neurological symptoms, with mild impairment of thyroid function, in the absence of neonatal respiratory distress, as a typical example of an already established group of disorders namely NKX2.1related disorders.
The phenotypic spectrum of NKX2.1-related disorders is highly variable and ranges from abnormalities in a single organ system to any combination of brain, thyroid and lung involvement. The neurological presentation may be the most frequent one. In particular BHC, the hallmark of NKX2.1-related disorders, is a classic early finding, as suggested by previous evidence [5], and is characterized by involuntary, irregular, jerk-like and continuous movements, frequently associated with hypotonia, ataxia and choreoathetosis. In some cases BHC is associated with respiratory distress syndrome or congenital hypothyroidism.
The case that we report here is characterized by mild clinical presentation. The movement disorders of the child were monitored starting from the first months of life and choreathetosis was evident at 8 months. Neonatal CH screening showed near-normal results. A mild form of hypothyroidism was detected in his first months of life and was responsive to a low dose of L-Thyroxine. No respiratory distress at birth and no respiratory symptoms during childhood were observed.
A novel NKX2.1 nonsense mutation (Y130X) and a single base exchange in intron 2(c. 463 + 41C > T), not reported in the SNP database (NCBI), were identified. The mutations were not detected in parents, despite the known history of walking delay in the paternal line. This evidence indicates that we identified de novo mutations.
The phenotype observed in our patient was mild despite the presence of NKX2.1 gene null mutation (Y130X). This discrepancy is not surprising in light of previous data showing the lack of genotype-phenotype correlation in NKX2.1-related disorders [7,11,18,19], likely due to the pathogenetic role of genetic and environmental factors that need to be further investigated [20]. The pathogenetic mechanism of the mutation, as other truncating mutations, might involve haploinsufficiency [2,8,10,12,[21][22][23].
However, we cannot exclude the potential activation of an alternative initiation site leading to a shorter product of 268 aminoacids with intact homeobox domain (HD) and TD and lacking the protein N-terminal. This product may partially preserve the protein function and therefore explain the mild phenotype of the patient.
The G142X nonsense mutation upstream to the NKX2.1 DNA binding domain was described by Salerno et al. [27] in a patient with CH and respiratory distress but not chorea, whose father was a immune carrier.
Nevertheless, the high phenotypic variability, even in cases carrying the same mutation, is not restricted to NKX2.1 nonsense mutations, but is a feature of all types of NKX2.1 mutations.
Therefore, the hypothesis of the activation of an alternative initiation site with intact HD in the novel Y130X nonsense mutation, upstream to the NKX2.1 DNA binding domain, seems to be remote. Functional studies are needed to definitively rule out this hypothesis.

Conclusions
Previously published data do not allow a clear interpretation of the pathogenetic role of NKX2.1 mutations in NKX2.1-related disorders, since a number of cases were not fully characterized for the phenotypic presentation of choreoathetosis. Moreover, the same mutation of the NKX2.1 gene can present with diverse phenotypes and highly variable clinical manifestations. Our analysis suggests that in the absence of severe thyroid deficit or respiratory distress, neurological evaluation should guide the patient's management. The mild hypothyroidism, frequently observed in these cases, may not be detected by neonatal screening. The low frequency of CH cases due to NKX2.1 mutations that are reported in literature is expected to be higher in a subgroup of patients selected according to the neurological presentation. In these patients, the analysis of NKX2.1 mutational status is recommended.

Consent
Written informed consent was obtained from the patient's parents for the publication of this case report and any accompanying images. A copy of the written consent is available for the review by the Editor-in-Chief of this journal.