Human RAD50 gene mutations cause Nijmegen Breakage Syndrome-like disease, characterized by severe prenatal and postpartum growth retardation and microcephaly. It is very rare (less than 5 cases) with limited clinical data and treatment experience. Clinical information was collected on a boy with microcephaly and severe growth restriction, including birth history, clinical features, unplanned response to recombinant human growth hormone treatment, and five-year follow-up after growth hormone discontinuation. The child underwent trio-based whole-exome sequencing and Sanger sequencing to validate the mutation. Constructed variant plasmids were used for in vitro functional experiments and Western blots to evaluate the potential impact of the variants. The boy was born at full term, with substantial growth retardation from infancy to early childhood. At the age of 4.5 years, the child with syndromic short stature was prescribed recombinant human growth hormone for height correction by junior resident physicians, with no genetic evaluation performed prior to treatment. After 5 years and 9 months of recombinant human growth hormone treatment, genetic analysis was done due to his evident microcephaly and distinctive facial features. Two novel variants (p.His1269Argfs2 and p.Ser844Asn) were identified in the RAD50 gene. Western blotting revealed the presence of the Flag-tag and EGFP in RAD50-wt, but not in RAD50-mut (p.His1269Argfs2), indicating the frameshift mutation may markedly impair RAD50 protein expression or stability. Although recombinant human growth hormone significantly improved the patient's growth rate (from -3.35 SD to -1.28 SD), these variants may serve as a potential molecular basis for Nijmegen Breakage Syndrome-like disease and could also increase the risk of tumor formation. Treatment with recombinant human growth hormone was discontinued when the patient was 10 years and 3 months old. A five-year follow-up showed no evidence of a tumor was observed; The 15-year-old patient's height ceased to increase and remained at 151.5 cm. In the current study, we identified and characterized a patient with two RAD50 mutations. This report expands the clinical and genetic scope of RAD50 mutations. For the first time, it describes the response to unplanned recombinant human growth hormone therapy and the risk of long-term tumors. This report is intended to raise clinical awareness of the risk-benefit balance of recombinant human growth hormone therapy for syndromic short stature.

Nijmegen breakage syndrome-like disorder (NBSLD) is a rare chromosomal instability syndrome caused by biallelic pathogenic variants in RAD50, which is a key component of the MRE11-RAD50-NBS1 (MRN) complex involved in DNA double-strand break repair. Merely few cases have been reported worldwide, and its phenotypic spectrum remains incompletely defined. We report a 6-year-old Chinese boy, who presented with bilateral cryptorchidism, severe microcephaly, growth retardation, multiple café-au-lait macules, brachydactyly, and distinctive craniofacial features, including a sloping forehead, midface prominence, and receding mandible. Mild intellectual impairment was confirmed on formal neurocognitive testing. The immunological assessment revealed borderline lymphopenia without overt immunodeficiency. Whole-exome sequencing (WES) identified compound heterozygosity for two variants in RAD50 (NM_005732.3): a paternally inherited frameshift variant c.2165_2166insT (p.Lys722Asnfs*6) and a maternally inherited splice-site variant c.3752 + 4_3752 + 7dup. The minigene splicing assay revealed that the latter disrupts normal splicing, leading to partial intron retention and a premature stop codon (p.Ile1252*). Based on the clinical features and molecular confirmation, the patient was diagnosed with NBSLD. Allogeneic hematopoietic stem cell transplantation (HSCT) has been proposed as a potential therapeutic option for DNA damage repair disorders. However, it is not presently required for this patient, who is managed with regular surveillance. The present case expands the clinical and mutational spectrum of RAD50-associated NBSLD. It emphasizes the importance of combining clinical assessment, genomic analysis, and functional assays for the accurate diagnosis of rare chromosomal breakage disorders.

DNA is susceptible to various factors in vitro and in vivo and experience different forms of damage,among which double-strand break(DSB)is a deleterious form.To maintain the stability of genetic information,organisms have developed multiple mechanisms to repair DNA damage.Among these mechanisms,homologous recombination(HR)is praised for the high accuracy.The MRE11-RAD50-NBS1(MRN)complex plays an important role in HR and is conserved across different species.The knowledge on the MRN complex mainly came from the previous studies in Saccharomyces cerevisiae and Caenorhabditis elegans,while studies in the last decades have revealed the role of mammalian MRN complex in DNA repair of higher animals.In this review,we first introduces the MRN complex regarding the composition,structure,and roles in HR.In addition,we discuss the human diseases such as ataxia-telangiectasia-like disorder,Nijmegen breakage syndrome,and Nijmegen breakage syndrome-like disorder that are caused by dysfunctions in the MRN complex.Furthermore,we summarize the mouse models established to study the clinical phenotypes of the above diseases. 生物体的DNA常遭受着来自体外和体内各种因素的攻击,其中DNA双链断裂(DSB)是严重的一种DNA损伤方式。为了保证遗传信息的稳定性,生物体自身存在应对DNA损伤的修复机制。同源重组修复是精确的修复DSB的方式,MRE11-RAD50-NBS1(MRN)复合物是参与同源重组修复的关键蛋白,在不同物种之间存在保守性。从前关于MRN复合物的功能研究主要来源于酿酒酵母和线虫等低等生物,近些年来对哺乳动物MRN复合物的研究提示MRN复合物在高等动物DNA损伤修复中存在功能。本文综述了MRN复合物的组成和结构及其在DNA损伤同源重组修复中的功能,同时也介绍了MRN复合物异常所带来的人类疾病共济失调性毛细血管扩张综合征类似病症、奈梅亨断裂综合征和奈梅亨断裂综合征类似病症,并对这 3类DNA损伤修复缺陷疾病的临床表型和相关小鼠模型研究进行了总结。.

The MRE11-RAD50-NBN (MRN) complex plays a key role in recognizing and signaling DNA double-strand breaks. Pathogenic variants in NBN and MRE11 give rise to the autosomal-recessive diseases, Nijmegen breakage syndrome (NBS) and ataxia telangiectasia-like disorder, respectively. The clinical consequences of pathogenic variants in RAD50 are incompletely understood. We aimed to characterize a newly identified RAD50 deficiency/NBS-like disorder (NBSLD) patient with bone marrow failure and immunodeficiency. We report on a girl with microcephaly, mental retardation, bird-like face, short stature, bone marrow failure and B-cell immunodeficiency. We searched for candidate gene by whole-exome sequencing and analyzed the cellular phenotype of patient-derived fibroblasts using immunoblotting, radiation sensitivity assays and lentiviral complementation experiments. Compound heterozygosity for two variants in the RAD50 gene (p.Arg83His and p.Glu485Ter) was identified in this patient. The expression of RAD50 protein and MRN complex formation was maintained in the cells derived from this patient. DNA damage-induced activation of the ATM kinase was markedly decreased, which was restored by the expression of wild-type (WT) RAD50. Radiosensitivity appeared inconspicuous in the patient-derived cell line as assessed by colony formation assay. The RAD50R83H missense substitution did not rescue the mitotic defect in complementation experiments using RAD50-deficient fibroblasts, whereas RAD50WT did. The RAD50E485X nonsense variant was associated with in-frame skipping of exon 10 (p.Glu485_545del). These findings indicate important roles of RAD50 in human bone marrow and immune cells. RAD50 deficiency/NBSLD can manifest as a distinct inborn error of immunity characterized by bone marrow failure and B-cell immunodeficiency.

DNA double-strand breaks (DSBs) are highly toxic DNA lesions that can lead to chromosomal instability, loss of genes and cancer. The MRE11/RAD50/NBN (MRN) complex is keystone involved in signaling processes inducing the repair of DSB by, for example, in activating pathways leading to homologous recombination repair and nonhomologous end joining. Additionally, the MRN complex also plays an important role in the maintenance of telomeres and can act as a stabilizer at replication forks. Mutations in NBN and MRE11 are associated with Nijmegen breakage syndrome (NBS) and ataxia telangiectasia (AT)-like disorder, respectively. So far, only one single patient with biallelic loss of function variants in RAD50 has been reported presenting with features classified as NBS-like disorder. Here, we report a long-term follow-up of an unrelated patient with facial dysmorphisms, microcephaly, skeletal features, and short stature who is homozygous for a novel variant in RAD50. We could show that this variant, c.2524G > A in exon 15 of the RAD50 gene, induces aberrant splicing of RAD50 mRNA mainly leading to premature protein truncation and thereby, most likely, to loss of RAD50 function. Using patient-derived primary fibroblasts, we could show abnormal radioresistant DNA synthesis confirming pathogenicity of the identified variant. Immunoblotting experiments showed strongly reduced protein levels of RAD50 in the patient-derived fibroblasts and provided evidence for a markedly reduced radiation-induced AT-mutated signaling. Comparison with the previously reported case and with patients presenting with NBS confirms that RAD50 mutations lead to a similar, but distinctive phenotype.