Cerebroretinal microangiopathy with calcifications and cysts associated with CTC1 and NDP mutations

Case Report: CTC1 mutations in a patient with diffuse hepatic and splenic hemangiomatosis complicated by Kasabach-Merritt syndrome

Diffuse hemangiomatosis of the liver and spleen is rare. Currently, few studies are available on diffuse hepatic and splenic hemangiomatosis accompanied by Kasabach-Merritt syndrome (KMS). The conserved telomere maintenance component 1 (CTC1) gene contributes to telomere maintenance and replication by forming the telomeric capping complex. Herein, we report a case of diffuse hemangiomatosis in the liver and spleen accompanied by KMS in a 59-year-old woman who carried two novel heterozygous CTC1 variants: c.435+9A>C and c.3074C>T (p.Ala1025Val). Using next-generation sequencing, we detected mutations in the CTC1 gene in our patient, who had chief complaints of fatigue and abdominal distension complicated by severe thrombocytopenia and consumptive coagulopathy. Clinical symptoms, laboratory tests, and imaging findings led to the diagnosis of diffuse hepatic and splenic hemangiomatosis accompanied by KMS. The patient was treated with prednisone, thalidomide, and sirolimus, and her general condition was ameliorated at the 4-month follow-up with improved platelet count and coagulation function. A CTC1 gene mutation may be involved in the pathological process of vascular diseases. A combination treatment regimen of prednisone, thalidomide, and sirolimus may be effective for KMS.

Structural and functional analysis of an OB-fold in human Ctc1 implicated in telomere maintenance and bone marrow syndromes

The human CST (Ctc1, Stn1 and Ten1) complex binds the telomeric overhang and regulates telomere length by promoting C-strand replication and inhibiting telomerase-dependent G-strand synthesis. Structural and biochemical studies on the human Stn1 and Ten1 complex revealed its mechanism of assembly and nucleic acid binding. However, little is known about the structural organization of the multi-domain Ctc1 protein and how each of these domains contribute to telomere length regulation. Here, we report the structure of a central domain of human Ctc1. The structure reveals a canonical OB-fold with the two identified disease mutations (R840W and V871M) contributing to the fold of the protein. In vitro assays suggest that although this domain is not contributing directly to Ctc1’s substrate binding properties, it affects full-length Ctc1 localization to telomeres and Stn1-Ten1 binding. Moreover, functional assays show that deletion of the entire OB-fold domain leads to significant increase in telomere length, frequency of internal single G-strands and fragile telomeres. Our findings demonstrate that a previously unknown OB-fold domain contributes to efficient Ctc1 telomere localization and chromosome end maintenance.

Human CST suppresses origin licensing and promotes AND-1/Ctf4 chromatin association

Human CTC1-STN1-TEN1 (CST) is an RPA-like single-stranded DNA-binding protein that interacts with DNA polymerase α-primase (pol α) and functions in telomere replication. Previous studies suggest that CST also promotes replication restart after fork stalling. However, the precise role of CST in genome-wide replication remains unclear. In this study, we sought to understand whether CST alters origin licensing and activation. Replication origins are licensed by loading of the minichromosome maintenance 2-7 (MCM) complex in G1 followed by replisome assembly and origin firing in S-phase. We find that CST directly interacts with the MCM complex and disrupts binding of CDT1 to MCM, leading to decreased origin licensing. We also show that CST enhances replisome assembly by promoting AND-1/pol α chromatin association. Moreover, these interactions are not dependent on exogenous replication stress, suggesting that CST acts as a specialized replication factor during normal replication. Overall, our findings implicate CST as a novel regulator of origin licensing and replisome assembly/fork progression through interactions with MCM, AND-1, and pol α.

Emerging roles of CST in maintaining genome stability and human disease

The human CTC1-STN1-TEN1 (CST) complex is a single-stranded DNA binding protein that shares homology with RPA and interacts with DNA polymerase alpha/primase. CST complexes are conserved from yeasts to humans and function in telomere maintenance. A common role of CST across species is in the regulation of telomere extension by telomerase and C-strand fill-in synthesis. However, recent studies also indicate that CST promotes telomere duplex replication as well the rescue of stalled DNA replication at non-telomeric sites. Furthermore, CST dysfunction and mutation is associated with several genetic diseases and cancers. In this review, we will summarize what is known about CST with a particular focus on the emerging roles of CST in DNA replication and human disease.

Cerebroretinal microangiopathy with calcifications and cysts: A case report

RATIONAL

Cerebroretinal microangiopathy with calcifications and cysts (CRMCC) is believed to be an autosomal recessive genetic disease, with disorders in multisystem organs. Its characteristic neurological disorders manifested on neuroimaging are a triad of leukoencephalopathy, intracranial calcifications, and parenchymal cysts. In this paper, we report a CRMCC patient with multisystem involvement, focusing on the neuroimaging features, to get a better understanding of the rare disease and improve our diagnostic ability.

PATIENT CONCERNS

The 23-year-old female patient firstly presented with an adolescence onset of ophthalmological manifestations. Four years later, hematological and neurological disorders occurred, the latter of which demonstrated a relatively slow progression in the following 7 years preceding her presentation to our hospital.

INTERVENTIONS

During hospitalization, disorders involving digestive, cardiovascular and respiratory systems were also detected. In addition, a more comprehensive depiction of neurological disorders on neuroimaging was also obtained.

DIAGNOSES

On the basis of multiple system disorders and the detection of mutations in conserved telomere maintenance component 1(CTC1) gene, a diagnosis of CRMCC was made.

OUTCOMES

After supportive therapy during her 4-week hospitalization, the patient's general condition improved and was released from the hospital.

LESSONS

CRMCC could be primarily diagnosed with the aid of its multiple system disorders and remarkable neuroimaging features. Cerebral micro hemorrhages determined by the combination of CT and T2-weighted magnetic resonance images in our case could provide some additional information for diagnosis. Furthermore, several other associated disorders were depicted for the first time in our case, expanding the clinical spectrum of CRMCC.

Norrin/Frizzled4 signalling in the preneoplastic niche blocks medulloblastoma initiation

The tumor microenvironment is a critical modulator of carcinogenesis; however, in many tumor types, the influence of the stroma during preneoplastic stages is unknown. Here we explored the relationship between pre-tumor cells and their surrounding stroma in malignant progression of the cerebellar tumor medulloblastoma (MB). We show that activation of the vascular regulatory signalling axis mediated by Norrin (an atypical Wnt)/Frizzled4 (Fzd4) inhibits MB initiation in the Ptch^+/− mouse model. Loss of Norrin/Fzd4-mediated signalling in endothelial cells, either genetically or by short-term blockade, increases the frequency of pre-tumor lesions and creates a tumor-permissive microenvironment at the earliest, preneoplastic stages of MB. This pro-tumor stroma, characterized by angiogenic remodelling, is associated with an accelerated transition from preneoplasia to malignancy. These data expose a stromal component that regulates the earliest stages of tumorigenesis in the cerebellum, and a novel role for the Norrin/Fzd4 axis as an endogenous anti-tumor signal in the preneoplastic niche.

DOI:http://dx.doi.org/10.7554/eLife.16764.001

Tying up the Ends: Plasticity in the Recognition of Single-Stranded DNA at Telomeres

Telomeres terminate nearly exclusively in single-stranded DNA (ssDNA) overhangs comprised of the G-rich 3' end. This overhang varies widely in length from species to species, ranging from just a few bases to several hundred nucleotides. These overhangs are not merely a remnant of DNA replication but rather are the result of complex further processing. Proper management of the telomeric overhang is required both to deter the action of the DNA damage machinery and to present the ends properly to the replicative enzyme telomerase. This Current Topic addresses the biochemical and structural features used by the proteins that manage these variable telomeric overhangs. The Pot1 protein tightly binds the single-stranded overhang, preventing DNA damage sensors from binding. Pot1 also orchestrates the access of telomerase to that same substrate. The remarkable plasticity of the binding interface exhibited by the Schizosaccharomyces pombe Pot1 provides mechanistic insight into how these roles may be accomplished, and disease-associated mutations clustered around the DNA-binding interface in the hPOT1 highlight the importance of this function. The budding yeast Cdc13-Stn1-Ten1, a telomeric RPA complex closely associated with telomere function, also interacts with ssDNA in a fashion that allows degenerate sequences to be recognized. A related human complex composed of hCTC1, hSTN1, and hTEN1 has recently emerged with links to both telomere maintenance and general DNA replication and also exhibits mutations associated with telomere pathologies. Overall, these sequence-specific ssDNA binders exhibit a range of recognition properties that allow them to perform their unique biological functions.