Stable maintenance of the Mre11-Rad50-Nbs1 complex is sufficient to restore the DNA double-strand break response in cells lacking RecQL4 helicase activity

A Chinese patient with Rothmund-Thomson syndrome

INTRODUCTION

Rothmund-Thomson syndrome (RTS) is a rare autosomal recessive disorder that has been reported in all ethnicities, with several identifiable pathogenic variants. There have been reported cases indicating that RTS may lead to low birth weight in fetuses, but specific data on the fetal period are lacking. Genetic testing for RTS II is currently carried out by identifying pathogenic variants in RECQL4.

METHODS

In order to determine the cause, we performed whole-genome sequencing (WGS) analysis on the patient and his parents. Variants detected by WGS were confirmed by Sanger sequencing and examined in family members.

RESULTS

After analyzing the WGS data, we found a heterozygous nonsense mutation c.2752G>T (p.Glu918Ter) and a novel frameshift insertion mutation c.1547dupC (p.Leu517AlafsTer23) of RECQL4, which is a known pathogenic/disease-causing variant of RTS. Further validation indicated these were compound heterozygous mutations from parents.

CONCLUSION

Our study expands the mutational spectrum of the RECQL4 gene and enriches the phenotype spectrum of Chinese RTS patients. Our information can assist the patient's parents in making informed decisions regarding their future pregnancies. This case offers a new perspective for clinicians to consider whether to perform prenatal diagnosis.

Deubiquitinase USP2 stabilizes the MRE11-RAD50-NBS1 complex at DNA double-strand break sites by counteracting the ubiquitination of NBS1

The MRE11-RAD50-NBS1 (MRN) complex plays essential roles in the cellular response to DNA double-strand breaks (DSBs), which are the most cytotoxic DNA lesions, and is a target of various modifications and controls. Recently, lysine 48-linked ubiquitination of NBS1, resulting in premature disassembly of the MRN complex from DSB sites, was observed in cells lacking RECQL4 helicase activity. However, the role and control of this ubiquitination during the DSB response in cells with intact RECQL4 remain unknown. Here, we showed that USP2 counteracts this ubiquitination and stabilizes the MRN complex during the DSB response. By screening deubiquitinases that increase the stability of the MRN complex in RECQL4-deficient cells, USP2 was identified as a new deubiquitinase that acts at DSB sites to counteract NBS1 ubiquitination. We determined that USP2 is recruited to DSB sites in a manner dependent on ATM, a major checkpoint kinase against DSBs, and stably interacts with NBS1 and RECQL4 in immunoprecipitation experiments. Phosphorylation of two critical residues in the N terminus of USP2 by ATM is required for its recruitment to DSBs and its interaction with RECQL4. While inactivation of USP2 alone does not substantially influence the DSB response, we found that inactivation of USP2 and USP28, another deubiquitinase influencing NBS1 ubiquitination, results in premature disassembly of the MRN complex from DSB sites as well as defects in ATM activation and homologous recombination repair abilities. These results suggest that deubiquitinases counteracting NBS1 ubiquitination are essential for the stable maintenance of the MRN complex and proper cellular response to DSBs.

Non-Melanoma Skin Cancers and Other Cutaneous Manifestations in Bone Marrow Failure Syndromes and Rare DNA Repair Disorders

Although most non-melanoma skin cancers are felt to be sporadic in origin, these tumors do play a role in several cancer predisposition syndromes. The manifestations of skin cancers in these hereditary populations can include diagnosis at extremely early ages and/or multiple primary cancers, as well as tumors at less common sites. Awareness of baseline skin cancer risks for these individuals is important, particularly in the setting of treatments that may compromise the immune system and further increase risk of cutaneous malignancies. Additionally, diagnosis of these disorders and management of non-cutaneous manifestations of these diseases have profound implications for both the patient and their family. This review highlights the current literature on the diagnosis, features, and non-melanoma skin cancer risks associated with lesser-known cancer predisposition syndromes, including bone marrow failure disorders, genomic instability disorders, and base excision repair disorders.