OBJECTIVES: To verify the involvement of NKX2-1 gene in infants with 
brain-lung-thyroid (BLT) syndrome and hypothyroid phenotypes variable among 
congenital hypothyroidism (CH) or idiopathic mild hypothyroidism (IMH) of 
postnatal onset.
METHODS: The candidates were selected by a case-finding approach in 130 CH and 
53 IMH infants. The NKX2-1 gene was analyzed by direct sequencing and multiplex 
ligation-dependent probe amplification. The variants were studied in vitro, by 
expression analyses and luciferase bioassay.
RESULTS: Four cases (3 CH and 1 IMH) consistent with BLT syndrome were 
identified. Two children were affected with respiratory distress and CH, but 
wild-type NKX2-1 gene. The remaining two presented choreic movements and no 
pulmonary involvement, but discrepant thyroid phenotypes: one had severe CH with 
lingual ectopy and the other one IMH with gland in situ. They were carriers of 
new de novo heterozygous frameshift mutations of NKX2-1 (c.177delG and 
c.153_166del14). The c.177delG leads to a prematurely truncated protein 
(p.H60TfsX11) with undetectable activity in vitro. The c.153_166del14 leads to 
the generation of an elongated aberrant protein (p.A52RfsX351) able to 
translocate into the nucleus, but completely inactive on a responsive promoter.
CONCLUSIONS: Two novel heterozygous frameshift mutations of NKX2-1 were 
identified in 2 cases selected on the basis of a BLT-like phenotype among 183 
hypothyroid infants. The atypical hypothyroid phenotypes of these 2 children (CH 
with lingual ectopy or IMH of postnatal onset) further expand the clinical 
spectrum that can be associated with NKX2-1 mutations.