A novel splice site mutation in SMARCAL1 results in aberrant exon definition in a child with Schimke immunoosseous dysplasia

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.

Different phenotypes of Schimke immuno-osseous dysplasia (SIOD) in two sisters with the same mutation in the SMARCAL1 gene: case reports

BACKGROUND

Schimke immuno-osseous dysplasia (SIOD) is a very rare autosomal recessive genetic disease caused by mutations in the SMARCAL1 gene. It is characterized by spondyloepiphyseal dysplasia, T-cell immunodeficiency, hypercromic nevi, hypercholesterolemia and steroid-resistant nephrotic syndrome with progressive renal failure to end-stage kidney disease.

CASE PRESENTATION

We report two cases of SIOD in sisters, diagnosed after the debut of nephrotic syndrome. Both had a personal history of short stature, acetabular hip dysplasia and hypercholesterolemia. The first case, a 6 year-old girl, presented peripheral refractory edema, severe arterial hypertension and progressive decrease of the glomerular filtration rate. Steroid-resistance of nephrotic syndrome was confirmed, treated with tacrolimus without response. Renal function worsened over the following 4 months, so haemodialysis was started. Her sister, a 5 years-old girl, had steroid-resistant nephrotic syndrome, who is currently with normal blood pressure and normal renal function under enalapril treatment. In view of the suspicion of SIOD, the genetic studies were carried out, revealing the same mutation in homozygosis.

CONCLUSIONS

SIOD has a variable expression with multi-systemic involvement with a short life expectancy. The early diagnosis is important, which can encourage the early start of treatment and anticipation of complications that may be life-threatening.

Temporally distinct post-replicative repair mechanisms fill PRIMPOL-dependent ssDNA gaps in human cells

PRIMPOL repriming allows DNA replication to skip DNA lesions, leading to ssDNA gaps. These gaps must be filled to preserve genome stability. Using a DNA fiber approach to directly monitor gap filling, we studied the post-replicative mechanisms that fill the ssDNA gaps generated in cisplatin-treated cells upon increased PRIMPOL expression or when replication fork reversal is defective because of SMARCAL1 inactivation or PARP inhibition. We found that a mechanism dependent on the E3 ubiquitin ligase RAD18, PCNA monoubiquitination, and the REV1 and POLζ translesion synthesis polymerases promotes gap filling in G2. The E2-conjugating enzyme UBC13, the RAD51 recombinase, and REV1-POLζ are instead responsible for gap filling in S, suggesting that temporally distinct pathways of gap filling operate throughout the cell cycle. Furthermore, we found that BRCA1 and BRCA2 promote gap filling by limiting MRE11 activity and that simultaneously targeting fork reversal and gap filling enhances chemosensitivity in BRCA-deficient cells.

Time for remodeling: SNF2-family DNA translocases in replication fork metabolism and human disease

Over the course of DNA replication, DNA lesions, transcriptional intermediates and protein-DNA complexes can impair the progression of replication forks, thus resulting in replication stress. Failure to maintain replication fork integrity in response to replication stress leads to genomic instability and predisposes to the development of cancer and other genetic disorders. Multiple DNA damage and repair pathways have evolved to allow completion of DNA replication following replication stress, thus preserving genomic integrity. One of the processes commonly induced in response to replication stress is fork reversal, which consists in the remodeling of stalled replication forks into four-way DNA junctions. In normal conditions, fork reversal slows down replication fork progression to ensure accurate repair of DNA lesions and facilitates replication fork restart once the DNA lesions have been removed. However, in certain pathological situations, such as the deficiency of DNA repair factors that protect regressed forks from nuclease-mediated degradation, fork reversal can cause genomic instability. In this review, we describe the complex molecular mechanisms regulating fork reversal, with a focus on the role of the SNF2-family fork remodelers SMARCAL1, ZRANB3 and HLTF, and highlight the implications of fork reversal for tumorigenesis and cancer therapy.

Schimke immuno-osseous dysplasia, two new cases with peculiar EEG pattern

Schimke Immuno-Osseous Dysplasia (SIOD) is an autosomal recessive multisystem disorder caused by pathogenic variants in the gene SMARCAL1. The clinical picture is characterized by spondyloepiphyseal dysplasia resulting in growth failure, nephropathy and T-cell deficiency. Neurologic manifestations include microcephaly, cognitive impairment, migraine-like headaches and cerebrovascular manifestations such as cerebral atherosclerotic vascular disease and reversible cerebral vasoconstriction. The role of SMARCAL1 deficiency in non-vascular neurological complications is still under debate. Epilepsy has been reported in a few patients, even in the absence of brain abnormalities. Data regarding electroencephalographic (EEG) patterns in SIOD are scarce METHODS: We describe the clinical, neuroradiological and EEG findings in two unrelated patients with SIOD showing a peculiar pseudo-periodic EEG pattern apparently not related to the cerebrovascular complications, since it was recognized both before and after cerebrovascular events CONCLUSION: Our observations support the hypothesis that SMARCAL1plays an important role in neurodevelopment and brain function and expand the spectrum of neurological abnormalities related to SIOD.

Genomic medicine for kidney disease

Technologies such as next-generation sequencing and chromosomal microarray have advanced the understanding of the molecular pathogenesis of a variety of renal disorders. Genetic findings are increasingly used to inform the clinical management of many nephropathies, enabling targeted disease surveillance, choice of therapy, and family counselling. Genetic analysis has excellent diagnostic utility in paediatric nephrology, as illustrated by sequencing studies of patients with congenital anomalies of the kidney and urinary tract and steroid-resistant nephrotic syndrome. Although additional investigation is needed, pilot studies suggest that genetic testing can also provide similar diagnostic insight among adult patients. Reaching a genetic diagnosis first involves choosing the appropriate testing modality, as guided by the clinical presentation of the patient and the number of potential genes associated with the suspected nephropathy. Genome-wide sequencing increases diagnostic sensitivity relative to targeted panels, but holds the challenges of identifying causal variants in the vast amount of data generated and interpreting secondary findings. In order to realize the promise of genomic medicine for kidney disease, many technical, logistical, and ethical questions that accompany the implementation of genetic testing in nephrology must be addressed. The creation of evidence-based guidelines for the utilization and implementation of genetic testing in nephrology will help to translate genetic knowledge into improved clinical outcomes for patients with kidney disease.

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.

Functions of SMARCAL1, ZRANB3, and HLTF in maintaining genome stability

A large number of SNF2 family, DNA and ATP-dependent motor proteins are needed during transcription, DNA replication, and DNA repair to manipulate protein-DNA interactions and change DNA structure. SMARCAL1, ZRANB3, and HLTF are three related members of this family with specialized functions that maintain genome stability during DNA replication. These proteins are recruited to replication forks through protein-protein interactions and bind DNA using both their motor and substrate recognition domains (SRDs). The SRD provides specificity to DNA structures like forks and junctions and confers DNA remodeling activity to the motor domains. Remodeling reactions include fork reversal and branch migration to promote fork stabilization, template switching, and repair. Regulation ensures these powerful activities remain controlled and restricted to damaged replication forks. Inherited mutations in SMARCAL1 cause a severe developmental disorder and mutations in ZRANB3 and HLTF are linked to cancer illustrating the importance of these enzymes in ensuring complete and accurate DNA replication. In this review, we examine how these proteins function, concentrating on their common and unique attributes and regulatory mechanisms.

The role of SMARCAL1 in replication fork stability and telomere maintenance

SMARCAL1 (SWI/SNF Related, Matrix Associated, Actin Dependent Regulator Of Chromatin, Subfamily A-Like 1), also known as HARP, is an ATP-dependent annealing helicase that stabilizes replication forks during DNA damage. Mutations in this gene are the cause of Schimke immune-osseous dysplasia (SIOD), an autosomal recessive disorder characterized by T-cell immunodeficiency and growth dysfunctions. In this review, we summarize the main roles of SMARCAL1 in DNA repair, telomere maintenance and replication fork stability in response to DNA replication stress.

Identification of a Substrate Recognition Domain in the Replication Stress Response Protein Zinc Finger Ran-binding Domain-containing Protein 3 (ZRANB3)

DNA damage and other forms of replication stress can cause replication forks to stall. Replication stress response proteins stabilize and resolve stalled forks by mechanisms that include fork remodeling to facilitate repair or bypass of damaged templates. Several enzymes including SMARCAL1, HLTF, and ZRANB3 catalyze these reactions. SMARCAL1 and HLTF utilize structurally distinct accessory domains attached to an ATPase motor domain to facilitate DNA binding and catalysis of fork remodeling reactions. Here we describe a substrate recognition domain within ZRANB3 that is needed for it to recognize forked DNA structures, hydrolyze ATP, catalyze fork remodeling, and act as a structure-specific endonuclease. Thus, substrate recognition domains are a common feature of fork remodeling, SNF2-family, DNA-dependent ATPases, and our study provides further mechanistic understanding of how these enzymes maintain genome integrity during DNA replication.