論文

Congenital hypothyroidism (CH) is the most prevalent congenital endocrine disorder and causes mental retardation. A male Japanese patient with first cousin marriage parents was diagnosed as CH at 10 months. He was born before introduction of mass screening for CH. With continuous thyroid hormone replacement therapy, normal thyroid hormone status was maintained until adulthood. Genetic screening of next-generation sequencing was performed at the age of 52 years, and we identified a new homozygous thyroid peroxidase (TPO) gene mutation (GRCh38.p13, chromosome 2 at position 1493997, c.1964 G>T, p.Cys655Phe). TPO is an important enzyme to produce thyroid hormone. As demonstrated by a homology analysis of TPO proteins among different species, cysteine 655 residue is highly conserved, suggesting an important role in maintaining TPO function and structure. An in silico study with three-dimensional structure of the novel mutation was performed and suggested that the mutation abolished disulfide bond between cysteines at positions 598 and 655. An in vitro functional analysis using HEK293 cells revealed that TPO activity of the mutant was significantly impaired compared with that of the wild type. Furthermore, study of immunohistochemistry showed that localization of TPO in cells did not differ between the wild type and the mutant. In conclusion, this single disulfide bond loss mutation of a new TPO homozygous mutation, p.Cys655Phe, reduced TPO activity and caused congenital hypothyroidism without affecting subcellular localization of TPO proteins.

Acromesomelic dysplasia, type Maroteaux (AMDM) is a congenital bone dysplasia characterized by disproportionate, acromesomelic shortening of the limbs and mild spondylar dysplasia. AMDM is caused by biallelic loss-of-function mutations in NPR2 encoding natriuretic peptide receptor-B. We report on a 25-yr-old Japanese woman with AMDM. Her height was 119.0 cm (-7.4 SD) and weight 35 kg (-2.3 SD). She had acromesomelic shortening of limbs and severe brachydactyly. Radiological examination showed that her metacarpals and phalanges were short and wide, and her vertebral bodies were mildly flattened. Molecular analysis revealed a novel homozygous NPR2 mutation (c.1163G>A, p.Arg388Gln). We performed in vitro functional studies using HA-tagged wild-type (WT) and Arg388Gln vectors (HA-WT-NPRB and HA-R388Q-NPRB). Cells expressing HA-R388Q-NPRB showed negligible cGMP responses to C-type natriuretic peptide (CNP) stimulation, indicating that the mutation led to severe loss-of-function. By immunofluorescence experiments under permeabilized conditions, HA-WT-NPRB was expressed on plasma membrane, while HA-R388Q-NPRB co-localized with an Endoplasmic Reticulum marker. Cells co-expressing R388Q and the WT exhibited lower responses under CNP treatment than cells co-expressing the WT and empty vectors. Thus, it was thought that R388Q caused a dominant-negative effect with a defect in cellular trafficking to the plasma membrane.