鋅指转录因子ZIC3在内脏异位中的研究现状及进展

内脏异位是由于左右非对称性发育异常所致,常与胸腹腔器官的异常偏侧化有关。心脏经常受累,且心脏受累的严重程度通常决定其预后效果。内脏异位患者有特征性的心血管畸形、内脏器官的异常排列以及中线结构发育畸形。在内脏异位患者中第一个被发现有突变的基因是编码锌指转录因子的ZIC3。很多研究证实,ZIC3突变可导致X连锁内脏异位,而且在孤立性先心病中也发现了ZIC3的突变。至今,在内脏异位患者中发现有13个ZIC3突变,其中包括无义突变、错义突变、沉默突变、移码突变以及易位突变等。然而,ZIC3基因在内脏异位,特别是伴复杂先心病中的致病机理仍不是很清楚。本文就ZIC3结构、作用、突变以及其在内脏异位伴先心病中的研究现状及存在的问题做一综述。     

The NuRD complex and macrocephaly associated neurodevelopmental disorders

The nucleosome remodeling and deacetylase (NuRD) complex is a major regulator of gene expression involved in pluripotency, lineage commitment, and corticogenesis. This important complex is composed of seven different proteins, with mutations in CHD3, CHD4, and GATAD2B being associated with neurodevelopmental disorders presenting with macrocephaly and intellectual disability similar to other overgrowth and intellectual disability (OGID) syndromes. Pathogenic variants in CHD3 and CHD4 primarily involve disruption of enzymatic function. GATAD2B variants include loss‐of‐function mutations that alter protein dosage and missense variants that involve either of two conserved domains (CR1 and CR2) known to interact with other NuRD proteins. In addition to macrocephaly and intellectual disability, CHD3 variants are associated with inguinal hernias and apraxia of speech; whereas CHD4 variants are associated with skeletal anomalies, deafness, and cardiac defects. GATAD2B‐associated neurodevelopmental disorder (GAND) has phenotypic overlap with both of these disorders. Of note, structural models of NuRD indicate that CHD3 and CHD4 require direct contact with the GATAD2B‐CR2 domain to interact with the rest of the complex. Therefore, the phenotypic overlaps of CHD3‐ and CHD4‐related disorders with GAND are consistent with a loss in the ability of GATAD2B to recruit CHD3 or CHD4 to the complex. The shared features of these neurodevelopmental disorders may represent a new class of OGID syndrome: the NuRDopathies.     

Functional and structural basis of the nuclear localization signal in the ZIC3 zinc finger domain

Disruptions in ZIC3 cause heterotaxy, a congenital anomaly of the left-right axis. ZIC3 encodes a nuclear protein with a zinc finger (ZF) domain that contains five tandem C2H2 ZF motifs. Missense mutations in the first ZF motif (ZF1) result in defective nuclear localization, which may underlie the pathogenesis of heterotaxy. Here we revealed the structural and functional basis of the nuclear localization signal (NLS) of ZIC3 and investigated its relationship to the defect caused by ZF1 mutation. The ZIC3 NLS was located in the ZF2 and ZF3 regions, rather than ZF1. Several basic residues interspersed throughout these regions were responsible for the nuclear localization, but R320, K337 and R350 were particularly important. NMR structure analysis revealed that ZF1-4 had a similar structure to GLI ZF, and the basic side chains of the NLS clustered together in two regions on the protein surface, similar to classical bipartite NLSs. Among the residues for the ZF1 mutations, C253 and H286 were positioned for the metal chelation, whereas W255 was positioned in the hydrophobic core formed by ZF1 and ZF2. Tryptophan 255 was a highly conserved inter-finger connector and formed part of a structural motif (tandem CXW-C-H-H) that is shared with GLI, Glis and some fungal ZF proteins. Furthermore, we found that knockdown of Karyopherin alpha1/alpha6 impaired ZIC3 nuclear localization, and physical interactions between the NLS and the nuclear import adapter proteins were disturbed by mutations in the NLS but not by W255G. These results indicate that ZIC3 is imported into the cell nucleus by the Karyopherin (Importin) system and that the impaired nuclear localization by the ZF1 mutation is not due to a direct influence on the NLS.     

A recognizable systemic connective tissue disorder with polyvalvular heart dystrophy and dysmorphism associated with TAB2 mutations

Deletions encompassing TAK1‐binding protein 2 (TAB2) associated with isolated and syndromic congenital heart defects. Rare missense variants are found in patients with a similar phenotype as well as in a single individual with frontometaphyseal dysplasia. We describe a family and an additional sporadic patient with polyvalvular heart disease, generalized joint hypermobility and related musculoskeletal complications, soft, velvety and hyperextensible skin, short limbs, hearing impairment, and facial dysmorphism. In the first family, whole‐exome sequencing (WES) disclosed the novel TAB2 c.1398dup (p.Thr467Tyrfs*6) variant that eliminates the C‐terminal zinc finger domain essential for activation of TAK1 (TGFβ‐activated kinase 1)‐dependent signaling pathways. The sporadic case carryed a ~2 Mb de novo deletion including 28 genes also comprising TAB2. This study reveal an association between TAB2 mutations and a phenotype resembling Ehlers‐Danlos syndrome with severe polyvalvular heart disease and subtle facial dysmorphism. Our findings support the existence of a wider spectrum of clinical phenotypes associated with TAB2 perturbations and emphasize the role of TAK1 signaling network in human development.     

T-box transcription factor TBX20 mutations in Chinese patients with congenital heart disease

Despite animal studies having demonstrated that Tbx20 is essential for heart development, few studies have been conducted about TBX20 and congenital heart disease (CHD) in humans. Recently two TBX20 mutations have been associated with human heart defects in two Caucasian families, but TBX20 mutations underlying the more common isolated forms of CHD are still unknown. To explore this question and to analyze the association between TBX20 and susceptibility to CHD 203 Chinese patients with a variety of predominantly sporadic CHD and 300 control subjects were investigated for TBX20 mutations. The exon 2–6 contributing to the T-box DNA-binding domain and their flanking intron sequences were amplified by polymerase chain reaction (PCR) and then were sequenced after purification. Three non-synonymous mutations (A63T, I121F, and T262M) were identified in 3 patients, which were not seen in 300 controls. I121F and T262M mutations occurred within the highly conserved T-box DNA-binding domain. Two synonymous sequence variants (N222N, T262T) and one intervening variant (IVS2-5insCT) were observed in 3 patients but not in the controls. In addition, eight SNPs were observed both in patients and controls and four (S167S, P177P, A181A, and I219I) of them are novel. These data indicate that the frequency of TBX20 missense mutations occurred in Chinese CHD children is low, but they probably contribute to the risk of atrial septal defect (ASD), total anomalous pulmonary venous connection (TAPVC) and tetralogy of Fallot (TOF) in a small subset of Chinese. The findings provide the first insight into TBX20 mutations for TOF and TAPVC. Functional study involved in the new sequence variants should be subject of further investigation.     

Comorbidity of congenital heart defects and holoprosencephaly is likely genetically driven and gene‐specific

Comorbidity of holoprosencephaly (HPE) and congenital heart disease (CHD) in individuals with genetic variants in known HPE‐related genes has been recurrently observed. Morphogenesis of the brain and heart from very early stages are regulated by several biological pathways, some of them involved in both heart and brain development as evidenced by genetic studies on model organisms. For instance, downregulation of Hedgehog or Nodal signaling pathways, both known as major triggers of HPE, has been shown to play a role in the pathogenesis of CHD, including structural defects and left–right asymmetry defects. In this study, individuals with various types of HPE were investigated clinically and by genomic sequencing. Cardiac phenotypes were assessed in 434 individuals with HPE who underwent targeted sequencing. CHDs were identified in 8% (n = 33) of individuals, including 10 (30%) cases of complex heart disease. Only four individuals (4/33) had damaging variants in the known HPE genes STAG2, SIX3, and SHH. Interestingly, no CHD was identified in the 37 individuals of our cohort with pathogenic variants in ZIC2. These findings suggest that CHD occurs more frequently in HPE‐affected individuals with or without identifiable genetic variants, and this co‐occurrence may be genetically driven and gene‐specific.