Novel compound mutations of SMARCAL1 associated with severe Schimke immuno-osseous dysplasia in a Chinese patient

Loss of helicase C-terminal domain of SMARCAL1 protein associated with severe Schimke immuno-osseous dysplasia

Schimke immuno-osseous dysplasia (SIOD) is a rare multi-system condition caused by biallelic loss-of-function mutations in the SMARCAL1 gene. This disorder is characterized by disproportionate growth failure, T-cell deficiency, and renal dysfunction. Pathogenic variants in the SMARCAL1 gene have been reported in only approximately half of SIOD-affected individuals. Among these alterations, nonsense and frameshift mutations generally lead to a severe phenotype with early onset. In this study, we identified novel mutations in an Iranian patient with SIOD. A 4-year-old girl with developmental delay and facial dysmorphism was referred to our center for molecular diagnosis. We applied whole-exome and Sanger sequencing for co-segregation analysis. Subsequently, bioinformatic analysis was performed to assess the pathogenic effects of the variants and their post-transcriptional effects. We discovered two novel mutations (c.2281delT and c.2283delA) in exon 15 of the SMARCAL1 gene, resulting in a truncated protein with a loss of 193 amino acids (p.S761Rfs*1). Variant effect predictors indicated that these variants are pathogenic, and multi-sequence alignments revealed high conservation of this region among different species. Given that our patient exhibited severe a phenotype and passed away soon after receiving a definitive molecular diagnosis, we propose that the loss of the helicase C-terminal domain in the deleted part of SMARCAL1 may lead to the severe form of SIOD. Besides, the combination of growth retardation and bone abnormalities also plays a crucial role in the early diagnosis of the disease.

T-cell receptor signaling in Schimke immuno-osseous dysplasia is SMARCAL1-independent

Schimke immuno-osseous dysplasia (SIOD) caused by mutations in SMARCAL1 is an ultra-rare disease characterized by specific facial features, skeletal dysplasia, and steroid-resistant nephrotic syndrome, which often leads to kidney failure and requires transplantation. Cellular (T-cell) deficiency, lymphopenia, and infections have been frequently reported, but whether they are due to T-cell-intrinsic defects in T-cell receptor (TCR) signaling associated with SMARCAL1 deficiency or to T-cell-extrinsic effects such as the impaired proliferation of hematopoietic precursors or T-cell-specific immunosuppression after renal transplantation remains unknown. We have explored the effects of SMARCAL1 deficiency on T-cell receptor signaling in primary and immortalized T cells from a 9-year-old SIOD patient under immunosuppression treatment when compared to healthy donors. Immortalized T cells recapitulated the SMARCAL1 deficiency of the patient, as judged by their impaired response to gamma irradiation. The results indicated that TCR-mediated signaling was normal in SIOD-derived immortalized T cells but strongly impaired in the primary T cells of the patient, although rescued with TCR-independent stimuli such as PMA + ionomycin, suggesting that SIOD-associated T-cell signaling is not intrinsically defective but rather the result of the impaired proliferation of hematopoietic precursors or of T-cell-specific immunosuppression. The lack of early thymic emigrants in our patients may support the former hypothesis.

Expanding Phenotype of Schimke Immuno-Osseous Dysplasia: Congenital Anomalies of the Kidneys and of the Urinary Tract and Alteration of NK Cells

Schimke immuno-osseous dysplasia (SIOD) is a rare multisystemic disorder with a variable clinical expressivity caused by biallelic variants in SMARCAL1. A phenotype-genotype correlation has been attempted and variable expressivity of biallelic SMARCAL1 variants may be associated with environmental and genetic disturbances of gene expression. We describe two siblings born from consanguineous parents with a diagnosis of SIOD revealed by whole exome sequencing (WES). Results: A homozygous missense variant in the SMARCAL1 gene (c.1682G>A; p.Arg561His) was identified in both patients. Despite carrying the same variant, the two patients showed substantial renal and immunological phenotypic differences. We describe features not previously associated with SIOD-both patients had congenital anomalies of the kidneys and of the urinary tract and one of them succumbed to a classical type congenital mesoblastic nephroma. We performed an extensive characterization of the immunophenotype showing combined immunodeficiency characterized by a profound lymphopenia, lack of thymic output, defective IL-7Rα expression, and disturbed B plasma cells differentiation and immunoglobulin production in addition to an altered NK-cell phenotype and function. Conclusions: Overall, our results contribute to extending the phenotypic spectrum of features associated with SMARCAL1 mutations and to better characterizing the underlying immunologic disorder with critical implications for therapeutic and management strategies.

Whole Exome Sequencing Identified a Novel Biallelic SMARCAL1 Mutation in the Extremely Rare Disease SIOD

Schimke immuno-osseous dysplasia (SIOD) is an extremely rare autosomal recessive pleiotropic disease. Although biallelic mutations in SMARCAL1 gene have been reported to be the genetic etiology of SIOD, its molecular diagnosis has been challenging in a relatively proportion of cases due to the extreme rarity. Here, we made a definitive SIOD diagnosis of a 5-year-old girl with an extremely mild phenotype by applying whole exome sequencing (WES). As a result, a novel maternal mutation (c.2141+5G > A) confirmed to create a novel splice donor site combined with a known paternal mutation (c.1933C > T; p.Arg645Cys) were detected. In addition, previous reported SIOD cases showed excessive enrichment for mutations in the helicase ATP-binding and C-terminal domains of SMARCAL1. Similarly, the novel mutation we identified caused a mutant protein truncated in the SMARCAL1 C-terminus. Interestingly, based on the phenotypic profile, compared to reported cases, the patient in our study exhibited milder symptoms with renal dysfunctions limited to asymptomatic proteinuria, but no neurological signs or recurrent infections. Moreover, we identified 73 SMARCAL1-interacting genes, which formed a significant interconnected interaction network with roles in disease-related pathways such as double-strand break repair via homologous recombination, DNA repair, and replication fork processing. Notably, the top 15 SMARCAL1-interacting genes all showed a similar renal temporal expression pattern. Altogether, to our knowledge, the case in this study is the first case diagnosed originally based on a genetic test via WES rather than a characteristic phenotype. The identification of the novel allelic mutation (c.2141+5G > A) extends the phenotypic spectrum of SMARCAL1 mutations and the following bioinformatics analysis presents additional genetic evidence to illustrate the role of SMARCAL1 in SIOD.

A novel compound heterozygous mutation of the SMARCAL1 gene leading to mild Schimke immune-osseous dysplasia: a case report

BACKGROUND

Schimke immune-osseous dysplasia (SIOD, OMIM 242900) is characterized by spondyloepiphyseal dysplasia, T-cell deficiency, renal dysfunction and special facial features. SMARCAL1 gene mutations are determined in approximately 50% of patients diagnosed with SIOD.

CASE PRESENTATION

The case presented here is that of a 6-year-old boy who was born at 33 weeks to healthy, non-consanguineous Chinese parents. He presented with short stature (95 cm; <3rd percentile) and proteinuria. Initially suspected of having IgM nephropathy, the patient was finally diagnosed with mild Schimke immune-osseous dysplasia. One novel mutation (p.R817H) and one well-known mutation (p.R645C) was identified in the SMARCAL1 gene.

CONCLUSION

This report describes a clinical and genetic diagnostic model of mild SIOD. It also highlights the importance of molecular testing or clinical diagnosis and the guidance it provides in disease prognosis.

Clinical features and mutation of NPHP5 in two Chinese siblings with Senior-Løken syndrome

Senior-Løken syndrome is a rare syndromic form of nephronophthisis that is associated with retinal dystrophy. Presently, seven genes (NPHP1-6 and NPHP10) have been associated with Senior-Løken syndrome. NPHP5 mutations are known to cause classical Senior-Løken syndrome. Here, we report two sisters (II-4, II-5) from a Chinese Han ethnic family who presented with classical Senior-Løken syndrome. Both affected sisters exhibited Leber's congenital amaurosis and juvenile nephronophthisis that progressed to end-stage renal disease by the age of 16 years and 9 months in patient II-4 and 12 years and 9 months in patient II-5. Sequence analysis showed a homozygous truncated mutation in NPHP5, c.1090C>T (p.R364X), in the patient II-4. This mutation is predicted to introduce a new open reading frame that results in the truncation of the C-terminal 235 amino acids of nephrocystin-5 and its consequent loss of function. Both parents carried a single heterozygous mutation in the same position, and no homozygous deletion of NPHP1 was found in this pedigree.

Structure of RPA32 bound to the N-terminus of SMARCAL1 redefines the binding interface between RPA32 and its interacting proteins

Replication protein A subunit RPA32 contains a C-terminal domain that interacts with a variety of DNA damage response proteins including SMARCAL1, Tipin, UNG2 and XPA. We have solved the high-resolution crystal structure of RPA32 C-terminal domain (RPA32C) in complex with a 26-amino-acid peptide derived from the N-terminus of SMARCAL1 (SMARCAL1N). The RPA32C-SMARCAL1N structure reveals a 1 : 1 binding stoichiometry and displays a well-ordered binding interface. SMARCAL1N adopts a long α-helical conformation with the highly conserved 11 residues aligned on one face of the α-helix showing extensive interactions with the RPA32C domain. Extensive mutagenesis experiments were performed to corroborate the interactions observed in crystal structure. Moreover, the α1/α2 loop of the RPA32C domain undergoes a conformational rearrangement upon SMARCAL1N binding. NMR study has further confirmed that the RPA32C-SMARCAL1N interaction induces conformational changes in RPA32C. Isothermal titration calorimetry studies have also demonstrated that the conserved α-helical motif defined in the current study is required for sufficient binding of RPA32C. Taken together, our study has provided convincing structural information that redefines the common recognition pattern shared by RPA32C interacting proteins.

DATABASE

The atomic coordinates of RPA32C in complex with 26-aa SMARCAL1 (SMARCAL1N) peptide have been deposited at the Protein Data Bank with accession code 4MQV.

STRUCTURED DIGITAL ABSTRACT

RPA32 and SMARCAL1 bind by isothermal titration calorimetry(1, 2, 3, 4, 5, 6, 7, 8, 9) RPA32 and SMARCAL1 bind by molecular sieving (View interaction) RPA32 and SMARCAL1 bind by x-ray crystallography (View interaction) Tipin and RPA32 bind by isothermal titration calorimetry (1, 2) RPA32 and UNG2 bind by isothermal titration calorimetry (1, 2, 3) SMARCAL1 and RPA32 bind by nuclear magnetic resonance (View interaction) UNG2 and RPA32 bind by nuclear magnetic resonance (View interaction) Tipin and RPA32 bind by nuclear magnetic resonance (View interaction).