Carvalho Ana, Hermanns Pia, Rodrigues Ana-Luísa, Sousa Isabel, Anselmo João, Bikker Hennie, Cabral Rita, Pereira-Duarte Carlos, Mota-Vieira Luísa, Pohlenz Joachim
1 Department of Pediatrics, Hospital of Divino Espirito Santo , Ponta Delgada, Portugal .
Thyroid. 2013 Sep;23(9):1074-8. doi: 10.1089/thy.2012.0649.
Although thyroid dysgenesis is the most common cause of congenital hypothyroidism (CH), its molecular basis remains largely elusive. Indeed, in only a minority of cases with thyroid dysgenesis (2%-3%) was it possible to identify an underlying genetic defect. The objective of this study was to screen the PAX8 gene and the PAX2 gene in a family with six cases of CH spanning three generations and presenting urogenital malformations. Herein, we report a case series and in vitro characterization of the PAX8 gene mutation.
Investigations were conducted at a tertiary care referral center. The index case was diagnosed to have congenital hypothyroidism at 7 months of age when he presented with severe impairment of suckling, constipation, and poor development. Treatment with levothyroxine corrected the symptoms and was associated with catch-up growth. His progeny, including two sons, one daughter, and two granddaughters, were affected by CH, and three of them received the diagnosis at neonatal screening. Ultrasound demonstrated normally located thyroid glands with reduced volumes. Five of the six affected family members, including the index case, had urogenital malformations, including incomplete horseshoe kidney, undescended testicles, hydrocele, and ureterocele. Strabismus was found in three out of six affected patients. No other somatic malformations were found.
Direct sequencing of the PAX8 gene revealed a new heterozygous mutation (c.74C > G) in all affected individuals. This mutation leads to substitution of proline with arginine at codon 25 (P25R). Fluorescence microscopy showed that P25R is normally located in the nucleus. In transient transfection studies, this mutation causes reduced transcriptional activation ability when using a luciferase reporter construct under the control of a thyroglobulin promoter. This diminished transactivation ability is due to loss of DNA binding capability as shown in electrophoresis mobility shift assay. The sequencing analysis of the PAX2 gene was normal.
We conclude that this novel PAX8 mutation is responsible for a severe form of dominantly inherited CH. The mutation seems to be associated with abnormalities of the urogenital tract.
尽管甲状腺发育不全是先天性甲状腺功能减退症（CH）最常见的病因，但其分子基础仍不清楚。实际上，仅在少数甲状腺发育不全病例（2%-3%）中能够确定潜在的基因缺陷。本研究的目的是对一个三代中有6例CH且伴有泌尿生殖系统畸形的家系进行PAX8基因和PAX2基因筛查。在此，我们报告一系列病例以及PAX8基因突变的体外特征分析。
研究在一家三级医疗转诊中心进行。索引病例在7个月大时被诊断为先天性甲状腺功能减退症，当时他出现严重的吸吮障碍、便秘和发育不良。左甲状腺素治疗纠正了症状并伴有追赶生长。他的后代，包括两个儿子、一个女儿和两个孙女，均患有CH，其中三人在新生儿筛查时被诊断出来。超声显示甲状腺位置正常但体积减小。六名受影响的家庭成员中有五名，包括索引病例，患有泌尿生殖系统畸形，包括不完全马蹄肾、隐睾、鞘膜积液和输尿管囊肿。六名受影响患者中有三名患有斜视。未发现其他躯体畸形。
PAX8基因直接测序显示所有受影响个体中存在一个新的杂合突变（c.74C > G）。该突变导致第25密码子处脯氨酸被精氨酸替代（P25R）。荧光显微镜显示P25R正常定位于细胞核。在瞬时转染研究中，当使用甲状腺球蛋白启动子控制下的荧光素酶报告构建体时，该突变导致转录激活能力降低。如电泳迁移率变动分析所示，这种转录激活能力的降低是由于DNA结合能力丧失所致。PAX2基因测序分析正常。
我们得出结论，这种新的PAX8突变导致了一种严重的显性遗传CH形式。该突变似乎与泌尿生殖道异常有关。
