Galloway–Mowat syndrome (GAMOS) is a very rare recessively inherited disorder characterized by steroid-resistant nephrotic syndrome (SRNS) or early-onset NS, microcephaly, impaired brain development, and other features (Galloway and Mowat, 1968). GAMOS has high genetic heterogeneity. GAMOS (OMIM, PS251300) is associated with 10 different genes, including WDR73, LAGE3, OSGEP, TP53RK, TPRKB, WDR4, NUP107, NUP133, GON7, and YRDC. These genes are located in autosomes, except for the LAGE3 gene with X-linked inheritance (Rosti et al., 2017; Braun et al., 2018; Fujita et al., 2018; Arrondel et al., 2019; Domingo-Gallego et al., 2019). A recent study showed that PRDM15 gene variants can cause GAMOS (Mann et al., 2021). Galloway–Mowat syndrome-4 (OMIM, #617730) is a type of GAMOS caused by the TP53-regulating kinase (TP53RK) homozygous or compound heterozygous variant. However, very few GAMOS4 cases have been reported.

The TP53RK gene (OMIM, *608679) is located on chromosome 20q13.12 and has two exons. TP53RK encodes a 253-amino acid p53-related protein kinase or tp53 reactive kinase (PRPK or TPRK), belonging to the highly conserved kinase, endopeptidase, and other proteins of small size (KEOPS) complex (Treimer et al., 2022). PRPK promotes ATPase activity of the KEOPS complex, thus regulating N-6-threonyl-carbamoyl adenosine (t6A) synthesis and improving translational efficiency and accuracy (Treimer et al., 2022). PRPK is a human protein kinase involved in protein phosphorylation via protein serine/threonine kinase activity, p53 binding activity, transferase activity, and transferring phosphorus-containing groups. PRPK may play an essential role in cell apoptosis and cell cycle through phosphorylation of p53 serine-15 residue, thus enhancing transcriptional activity (Abe et al., 2001, 2006). Only nine GAMOS4 cases caused by eight deleterious variants in the TP53RK gene have been reported in four articles. Braun et al. (2017) discovered pathogenic TP53RK mutations (homozygous c.125G > A/p.G42D; homozygous c.728G > T/p.R243L; compound heterozygous c.179del/p.K60Sfs*61 and c.242C > G/p.T81R) in four patients from three unrelated GAMOS4 families with nephrotic syndrome onset before 1 year of age. A Korean case study also identified a homozygous mutation (c.194A > T/p. K65M) in the TP53RK of a GAMOS4 family, where all affected members showed very early onset nephrotic syndrome, microcephaly, facial abnormalities, and premature death (Hyun et al., 2018). A Chinese article reported a case of a 1-year-old girl with microcephaly, growth and intelligence retardation, and nephrotic syndrome. The whole-exome sequencing (WES) found that the child had compound heterozygous mutations c.107 T > C/p.L36P and c.728G > T/p.R243 (Zhuo et al., 2020). These studies show the infantile-onset nephrotic syndrome and serious manifestations of GAMOS4 with TP53RK deficiency. However, Treimer et al. (2022) described a 3-year-old boy with a homozygous TP53RK missense variant (c.163C > G/p.R55G), who presented with neurodevelopmental deficits and progressive microcephaly but no clinical history of renal failure.

In this study, four novel TP53RK pathogenic variants were identified in three Chinese children with GAMOS4 via WES. The children had heterozygous c.15_16dup/p. p.A6Efs*29, which was in compound heterozygous state with c.745A > G/p.R249G, c.185G > A/p.R62H, and c.335A > G/p.Y112C. One patient was alive up to 3 years of age, and another 6-year and the 3-month-old patient did not have nephrotic syndrome at the time of the most recent follow-up. This study provides different clinical characteristics associated with GAMOS4, thus improving the understanding of the TP53RK-associated phenotype.

Galloway–Mowat syndrome is a genetic disease characterized by various developmental and physical abnormalities. GAMOS is caused by mutations in several genes reported in 86 families, including 60 cases (49.2%) of WDR73 mutation, 26 cases (21.3%) of OSGEP mutation, 13 cases (10.6%) of GON7 mutation, 8 cases (6.6%) of TP53RK modification, 5 cases (4%) of NUP107 conversion, 3 cases (2.5%) of YRDC mutation, 3 cases (2.5%) of LAGE3 mutation, 2 cases (1.6%) of TPRKB transformation, 1 case (0.8%) of WDR4 mutation, and 1 case (0.8%) of NUP133 mutation (Rosti et al., 2017; Fujita et al., 2018; Wang et al., 2018; Arrondel et al., 2019; Domingo-Gallego et al., 2019; El Younsi et al., 2019; Racine and Golden, 2021). GAMOS has very heterogeneous signs and symptoms, including microcephaly (small head size), impaired motor development, intellectual disability, seizures, nephrotic syndrome, hiatal hernia, optic atrophy, and other physical abnormalities.

Galloway–Mowat syndrome-4 caused by the TP53RK gene is mainly characterized by microcephaly, nephrotic syndrome, and severe central nervous system abnormalities (severe global developmental delay, cerebral atrophy, visual impairment, and dystonia). The clinical features and TP53RK variants in the three patients and previously published cases with documented GAMOS4 are summarized in Table 1. The first GAMOS4 study found four patients from three consanguine families of different origins (European, South African, Indian, Thailand, and Morocco; Abe et al., 2001). In that study, all symptoms appeared earlier, especially renal phenotype (diffuse mesangial sclerosis or focal-segmental glomerulosclerosis). Two patients, B77_21 and B77_22, were siblings who developed early proteinuria at 2 months and died at 11 and 2.5 months of age. The other two patients, N2984 and N2194, had proteinuria at the age of 1 year and 10 months and later died of end-stage renal disease (ESRD) at the age of 2.5 years and 3 years, respectively. The second study reported a Korean GAMOS non-consanguineous family with three affected siblings (II-1, II-2, and II-4) related to a homozygous TP53RK mutation genetic testing was not conducted in one case (II-1; Abe et al., 2006). Three of the four children in the family showed similar phenotypes, including early-onset nephrotic syndrome, microcephaly, facial deformity, and early death (age of death: 10 months, 21 days, and 25 days, respectively). Another Chinese article (2020) found a 1-year-old girl with hypoalbuminemia, hyperlipidemia, massive proteinuria, renal tubule damage, low level of immunoglobulin, special facies (broad and flat nose, too wide eye space), speech delay, continuous tremor of hands and feet, and could not sit alone (Braun et al., 2017). The patient was diagnosed with cerebral palsy and nephrotic syndrome before genetic testing. The above-reported individuals displayed early-onset nephrotic syndrome within the first year of life and died before the age of 3 from renal illness. In this study, the three patients had some typical features of this syndrome, such as small head circumferences (primary microcephaly), abnormalities in their cranial imaging (cerebral hypoplasia), and a DD (severe intellectual motor dysfunction and speech impairment). However, the three patients are now over 3 years of age, especially P2, who are over 6 years of age, demonstrating the high degree of heterogeneity in TP53RK-related GAMOS4. The three children also had strabismus and dystonia, but none had nephrotic syndrome. However, P1 had asymptomatic proteinuria. The latest documented Turkey case also had little renal involvement and survived until the age of 3 years and 7 months (Treimer et al., 2022). Moreover, cranial imaging of individuals was variable. Abe et al. (2001) found patients with cerebellar hypoplasia (B77_22), polymicrogyria (B77_21 and B77_22), cerebral atrophy (N2984), and bilateral myelination defects (N2194). Axial T2-weighted MRI of case II-2 showed a pattern of simplified sulcation (Abe et al., 2006). Brain MRIs of the Turkey case detected cortical atrophy with increased extra-axial spaces, thin corpus callosum, simplified gyral pattern, mild cerebellar atrophy, and diffuse T2-hypointense signal of the thalamus (Treimer et al., 2022). In this study, P2 and P3 exhibited cerebellar atrophy, while P1 did not show cerebellar hypoplasia but had numerous aberrant signal shadows in the brain and enlargement of the cerebral gyrus, a typical cranial lesion that has not been documented. Abe et al. (2001) showed that two cases had mixed seizures while only P2 had concomitant seizures in this study. P1 did not have epilepsy at the time of reporting but had an abnormal EEG, which does not rule out the occurrence of unmonitored seizures. EEG results of P3 were not detected. Furthermore, the three patients had abnormal facial features, primarily wide eye spacing, ear deformities, and small jaws. P1 also had hand and foot deformities, which appeared as conical fingers and foot entropion (the first phenotypic feature detected in GAMOS4 patients).

The TP53-regulating kinase (PRPK and TP53RK) is a component of the KEOPS complex essential for tRNA (t6A) modification, transcription control, and telomere regulation (Treimer et al., 2022). KEOPS complex comprises five subunits: LAGE3, OSGEP, PRPK, TPRKB, and the recently discovered C14orf142/GON7 (Wan et al., 2017). LAGE3, OSGEP, and PRPK are found in human podocytes and may be the minimum set required for the transfer of the threonyl-carbamoyl group to the adenosine at position 37 (t₆A37) in tRNAs that recognize ANN codons. TPRKB (TP53RK-binding protein) significantly increases the activity between its interactions with PRPK N-lobe. PRPK-TPRKB has autophosphorylation activity, which is switched to ATPase activity after the addition of OSGEP. The PRPK C-terminus of helix αH can play a supporting role in the OSGEP catalytic center (Daugeron et al., 2011). Most typical GAMOS patients have a defect in the KEOPS complex. Knockdown of OSGEP, TP53RK, and TPRKB can inhibit cell proliferation and reduce cell migration of human podocytes, impair translation, upregulate endoplasmic reticulum (ER) stress, activate DNA damage response (DDR) signaling, increase apoptosis, and decrease actin cytoskeleton network (Braun et al., 2017). Therefore, mutations in the KEOPS complex significantly affect the etiology of GAMOS. Although Ensembl describes two transcript variants (ENST00000372114.3 and ENST00000372102.3) encoding two potential isoforms (ENST00000372114.3-253aa and ENSP00000361174-121aa) of TP53RK, the clinical pathological significance of each transcript isoform of TP53RK in GAMOS is unknown. In this study, the variants were described using the NM_033550 transcript reference sequence based on ACMG guidelines and reported literature.

To date, only 12 TP53RK pathogenic variants have been identified in eight GAMOS families with detailed clinical data. Nine missense and two frameshift mutations in TP53RK have been found in 12 individuals (Table 1). All mutations occur in the catalytic domain of protein kinases (residues33-253), except for the novel frameshift c.15_16dup, which occurs at the N-terminus (Figure 3B). In this study, the three children had the TP53RK frameshift c.15_16dup. The allele distribution frequency (MAF) of this variant is 0.002% (EXAC, GnomAD), 0.009% (EXAC Asia), 0.011% (GnomAD Asia), and 0.0405% (China’s in-house local database), indicating that this variant is common in East Asian or Chinese populations. Moreover, the c.15_16dup allele in the three children was absent in other populations, strongly suggesting that it may be a founder mutation at a low carrier frequency. Although this variant is very rare in the general population, it has been detected previously for dbSNP identifier rs774069989, most likely of South Korean ancestry. It is also associated with microcephaly. In addition, this variant was not present in the available data from the 1,000 Genomes Project but was present in the heterozygous state in 54 individuals from China’s in-house local database (see text footnote 1). Therefore, screening of homogeneous individuals and variant-specific core haplotypes should be conducted to further investigate the founder effect hypothesis. Another reported frameshift c.179del was found in a compound heterozygous state with T81R in 2 sibs, which inhibits TP53RK interaction with TPRKB according to immunoprecipitation tests (Braun et al., 2017). c.179del can produce a 113-amino acid protein that can partially destroy the kinase domain, while c.15_16dup can form a 33-truncated protein without the kinase domain. Therefore, c.179del may cause the similarity in the three patients, including facial dysmorphism, microcephaly, DDs, brain anomalies, a propensity for seizures, and no early-onset nephrotic syndrome. These missense variants are located in a highly conserved protein kinase domain and can attack the disease. Although high-resolution structures of the human KEOPS complex are unavailable, human PRPK, X-ray diffraction structure of human PRPK-TPRKB complex, and human OSGEP/LAGE3/GON7 complex (predicted by modeling based on AlphaFold) revealed that R249G, R62H, and Y112C can weaken interactions and destabilize the KEOPS complex by impeding electrostatic interactions (Figure 4E). Nevertheless, further research is needed to fully understand the functional structural domain of TP53RK protein and identify additional genotype–phenotype associations since past medical history, and the progression of clinical symptoms is unclear in some patients. The discovery of the four variants broadens the range of pathogenic mutations in GAMOS4.

The severity of the renal symptoms of GAMOS ranges from mild non-nephrotic proteinuria to SRNS, quickly followed by end-stage renal disease and death. Renal disease is the only known cause of death among GAMOS patients. The impact of the WDR73 variant on GAMOS1 mortality is about 36.7% (22/60; Colin et al., 2014; Ben-Omran et al., 2015; Jinks et al., 2015; Vodopiutz et al., 2015; Rosti et al., 2016). Furthermore, there is no cure for this kidney disease, and all treatments are only supportive. The age at death range from 1.4 to 28 years (mean, 8.4 years). Herein, P1 only had minor chronic proteinuria with no nephrotic syndrome signs, while P2 and P3 did not show any kidney disease. In contrast, seven of the previously reported GAMOS4 patients had early onset nephrotic syndrome and passed away from renal insufficiency before the age of 3 years (mean, 1.1 years). The prognosis of GAMOS patients may be correlated with the early onset of the nephrotic syndrome. The patient’s intellectual and motor development is relatively mild if the onset of the syndrome is late, and their renal function also gradually declines. However, the patient can live longer if hormonal or immunosuppressive therapy is effective (Udler et al., 1997). To date, the oldest GAMOS survivor without nephropathy was about 35 years. The survivors had GAMOS1 caused by the WDR73 mutation (Jiang et al., 2017). To the best of our knowledge, P2 is the oldest child that has ever survived GAMOS4 caused by the TP53RK gene variant. The three patients did not have nephrotic syndrome, and only P1 had slight proteinuria, indicating that their overall survival was relatively long. However, urine cycle, liver and kidney function, and a renal ultrasound should be dynamically monitored. In addition, the detection of a combination of recurrent urinary tract infections and renal tissue biopsy is necessary.

In conclusion, four novel TP53RK variants were identified in three unrelated neurodevelopmental disorder patients using WES. This study provides additional clinical findings of GAMOS4 and enriches the mutation spectrum of the TP53RK gene. Nevertheless, this study provides a basis for the future clinical diagnosis and identification of GAMOS4 disease.

The original contributions presented in the study are publicly available. Information about TP53RK variants in this study can be accessed online through the lovd database using id #0000918160 (c.15_16dup) (https://databases.lovd.nl/shared/variants/0000918160#00021665), #0000918161(c.745A>G) (https://databases.lovd.nl/shared/variants/0000918161#00021665), #0000918162(c.185G>A) (https://databases.lovd.nl/shared/variants/0000918162#00021665), #0000918165(c.335A>G) (https://databases.lovd.nl/shared/variants/0000918165#00021665).

The studies involving human participants were reviewed and approved by Wuhan Children’s Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China. Written informed consent to participate in this study was provided by the participants’ legal guardian/next of kin. Written informed consent was obtained from the minor(s)' legal guardian/next of kin for the publication of any potentially identifiable images or data included in this article.

JC and H-mZ designed the study. PX, G-BY, and J-RH gathered clinical information from the family members and drafted the manuscript. MP and W-YG performed the sequencing, as well as analyzed and interpreted the data. All authors coordinated the study coordination, revised the manuscript, and read and approved the final version of the manuscript.

This research was supported by the Hubei Provincial Science and Technology Plan Project for the Clinical Research Center of Neurodevelopmental Disorders in Children (No. 2022DCC020).

MP and W-YG were employed by the Beijing Chigene Translational Medical Research Centre Co. Ltd.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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