ZTTK syndrome is a severe multisystem developmental disorder characterized by delayed psychomotor development and ID. In addition to case indexing, the reported patients with ZTTK syndrome were also reviewed. The clinical symptoms and neuroimaging results of the patients with ZTTK syndrome were described in Table 1 and the further detailed information was shown in Supplementary Table S3.
At present, a total of 31 patients with ZTTK syndrome have been reported worldwide. The clinical manifestations of the disease are often different in the patients with distinct genotype, even in the patients with the same genotype and different race. However, ZTTK syndrome is most likely to occur in the early childhood of the patients. All the patients showed developmental delay (DD) (29/29), ID (31/31), facial dysmorphism (29/29), eye/vision abnormality (24/24), pregnancy abnormal of mater (13/13) and hearing defect (15/15). The majority of cases presented with failure-to-thrive (26/28; 92%), mouth abnormalities (10/11; 90%), musculoskeletal abnormalities (23/29; 79%), feeding difficulties (22/28; 78%), C-section (7/10; 70%) respiratory failure of born (6/10; 60%) and short stature (17/30; 56%). The remaining patients with ZTTK syndrome exhibited urogenital malformations (12/29; 41%), congenital heart defects (8/27; 29%), gastrointestinal malformations (7/29; 24%), autism spectrum disorder (7/27; 25%), craniosynostosis (2/22; 9%) and hands and feet abnormalities. Premature delivery (12/29; 41%) is inclined to be a warning. Moreover, dysplastic kidney including single kidney and horseshoe kidney is the major cause of urogenital malformations.
Neuroimaging findings were performed by VEEG and/or MRI for the patients with ZTTK syndrome. Brain abnormalities were observed in most patients (23/28; 82%). 17 of 27 cases (62%) developed enlarged ventricular. Corpus callosum abnormalities occurred in nearly half of the patients (12/28; 42%). 6 of 28 cases (21%) developed cortex malformation. In addition, 4 of 28 cases showed white-matter abnormalities and cerebellar abnormalities (14%), respectively. And more notably, 14 of 28 patients (50%) presented with seizures and most of the cases (21/28; 75%) showed hypotonia.
ZTTK syndrome is regarded as a severe multisystem developmental disorder and caused by a heterozygous mutation in SON gene. The mutations in SON gene that have been reported in previously published papers are summarized and depicted in Table 2 and Fig. 3, respectively. There are 28 different variants in SON gene have been reported, which mainly result in 4 molecular consequence including frameshift, nonsense, non-frameshift and missense. Due to that the size of the exon 3 in SON gene accounts for 82% of the entire coding region, the majority of these mutations localize on this exon, thus forming the repeat domain and RS-rich domain [8–10]. Interestingly, almost all the variants occur only once except that c.5753_5756delTTAG (p.Val1918Glufs*87) mutant occur 8 times in different patients, which suggested it is extremely hot-spot mutation of SON gene. To explore the effect of this mutation on the phenotype of the patients with ZTTK syndrome, the difference of the clinical features and neuroimaging findings among the patients with this mutation and other mutations were analyzed and compared (Supplementary Table S4). Most of the clinical and neuroimaging findings of the patients maintain consistent, such as DD, ID, facial dysmorphism and so on. However, the clinical symptoms including hypermetropia, cleft palate, horseshoe kidney, joint contractures, scoliosis/kyphosis, hemivertebrae, small hands and arachnodactyly belonged in hands aberrant, cerebellar abnormalities and craniosynostosis did not occur in the patients with the variant c.5753_5756delTTAG. This mutation was continuously compared with other variants in RS-domain and it was found that age of birth, short stature, strabismus, congenital heart defects, urogenital malformations, hand and feet abnormalities were distinction but the other features, especially neuroimaging findings and musculoskeletal abnormalities were similar (Supplementary Table S4). Currently, no suitable conclusion can be obtained due to the limited number of cases.
SON haploinsufficiency leads to defective RNA splicing of multiple genes that are critical for brain development, neuronal migration, metabolism and heterogeneous renal phenotypes [7, 12]. All the patients with ZTTK syndrome present with ID and DD (Table 1 and Supplementary Table S3) caused by the de nove LoF mutations in SON gene, which were further ascertained by the exome sequencing of 2104 trios [15] and 66 neonatal patients [16], respectively. At present, the mutations in more than 1,500 genes have been found to participate in ID and/or DD [17–21], a few of which were indeed significantly down-regulated in the individuals with SON haploinsufficiency through qPCR analysis [7]. These phenomena confirm that the de novo LoF mutations in SON gene are the main cause of a complex neurodevelopmental disorder associated with ID and/or DD and severe brain malformations [7].
SON, a nuclear speckle-localized protein, consists of 2,426 amino acids (Fig. 3). The N-terminal region of SON contains a Lysine-rich region and an extensive amino-acid repeat region that might serve as a scaffold for loading accessory proteins [6, 8, 9, 22, 23]. The RS domain and the G-patch in the C-terminal domain of SON are the core motifs necessary for the activity of SON during splicing process. However, the DSRM domain, which has been found in diverse proteins involved in RNA metabolisms and RNA interference (RNAi), such as dsRNA-dependent protein kinase (PKR) and Dicer [24, 25], is dispensable [8–10]. During the splicing process of constitutive RNA, SON ensures the efficient removal of intron from the transcripts containing suboptimal splice sites. Importantly, SON-mediated splicing is required for the proper processing of the selective transcripts related to cell cycle, microtubules, centrosome maintenance, genome stability and heterogeneous renal phenotypes [9, 10, 12, 26].
Due to the regulation effect of SON on RNA splicing and transcription, the heterozygous loss-of-function mutations in SON gene can reduce the gene expression of PKD1 and PKD2 through impairing the intron removal process at multiple splice sites during their pre-mRNA splicing [9, 27]. It has been demonstrated that the expression PKD1 and PKD2 mRNAs can be moderated and diffused in the neural tube, anterior horn of the spinal cord, neural ganglia and heart et al during the 5- to 6-week human embryo [28]. Subsequently, PKD1 mRNAs mainly express in brain and kidney as well as PKD1 mRNAs in lung and kidney during fetal issues [28]. In addition to the CAKUT, PKD1 and PKD2 gene, there are many genes of which the splicing process is regulated by SON, such as OSR1, PAX8, FRAS1, GDNF and BMP4, and WNT4 [12]. Osr1, a known transcription factor, homologizes with human OSR1 and plays a role in the second heart field (SHF) together with Tbx5 and Pcks6 gene, in which the mutation can cause common atrium in mouse embryos [29–31]. Furthermore, the function of both Osr1 and Osr2 gene is required for the maintenance of the expression of the signaling molecules critical for joint formation besides Gdf5, Wnt4 and Wnt9b [32]. By multiple tissue expression array analysis, it has been also demonstrated that PAX8 expresses in developing central nervous system and kidney, including ureteric bud and main collecting ducts [33, 34], and the expression of FRAS1 is relatively high in fetal kidney and heart [35]. The heterozygous missense mutations in FRAS1 gene cause non-syndromic CAKUT in humans [36]. BMP4 is a vital regulatory molecule that can play a role during the developmental process including mesoderm induction, tooth development, limb formation, bone induction, fracture repair, orofacial cleft and microphthalmia, even can regulate heart development [37–39]. In general, the hypothesis that the clinically aberrant features of ZTTK syndrome patients are regulated by the genes mentioned above due to the loss-of-function mutations in SON gene will be further certified in the future.
ZTTK syndrome is regarded as an extremely rare genetic disorder, and until recently, there is no known effective method to prevent the progression of this disease, which imposes great importance on the accurate DNA diagnosis and prenatal screening to this rare disease.
In summary, this study provided a female patient with ID, DD, facial dysmorphism, brain malformation and seizures caused by the mutation c.5753_5756delTTAG (p. Val1918Glufs*87) in SON gene, which can produce a truncate variant and disrupt RNA splicing of the genes associated with brain development and metabolism. Then, all the reported patients with ZTTK syndrome and the mutations in SON gene were systematically reviewed and analyzed. We found that the clinical manifestations of all the patients with ZTTK syndrome are somewhat variable, but the typical clinical symptoms presented in every patient. Furthermore, the variant c.5753_5756delTTAG (p.Val1918Glufs*87) is extremely hot-spot mutation of SON gene. These results obtained from the present study are hoped to provide guidance for the early diagnosis of ZTTK syndrome.