POLθ prevents MRE11-NBS1-CtIP-dependent fork breakage in the absence of BRCA2/RAD51 by filling lagging-strand gaps.

POLθ promotes repair of DNA double-strand breaks (DSBs) resulting from collapsed forks in homologous recombination (HR) defective tumors. Inactivation of POLθ results in synthetic lethality with the loss of HR genes BRCA1/2, which induces under-replicated DNA accumulation. However, it is unclear whether POLθ-dependent DNA replication prevents HR-deficiency-associated lethality. Here, we isolated Xenopus laevis POLθ and showed that it processes stalled Okazaki fragments, directly visualized by electron microscopy, thereby suppressing ssDNA gaps accumulating on lagging strands in the absence of RAD51 and preventing fork reversal. Inhibition of POLθ DNA polymerase activity leaves fork gaps unprotected, enabling their cleavage by the MRE11-NBS1-CtIP endonuclease, which produces broken forks with asymmetric single-ended DSBs, hampering BRCA2-defective cell survival. These results reveal a POLθ-dependent genome protection function preventing stalled forks rupture and highlight possible resistance mechanisms to POLθ inhibitors.

POLθ processes ssDNA gaps and promotes replication fork progression in BRCA1-deficient cells.

Polymerase theta (POLθ) is an error-prone DNA polymerase whose loss is synthetically lethal in cancer cells bearing breast cancer susceptibility proteins 1 and 2 (BRCA1/2) mutations. To investigate the basis of this genetic interaction, we utilized a small-molecule inhibitor targeting the POLθ polymerase domain. We found that POLθ processes single-stranded DNA (ssDNA) gaps that emerge in the absence of BRCA1, thus promoting unperturbed replication fork progression and survival of BRCA1 mutant cells. A genome-scale CRISPR-Cas9 knockout screen uncovered suppressors of the functional interaction between POLθ and BRCA1, including NBN, a component of the MRN complex, and cell-cycle regulators such as CDK6. While the MRN complex nucleolytically processes ssDNA gaps, CDK6 promotes cell-cycle progression, thereby exacerbating replication stress, a feature of BRCA1-deficient cells that lack POLθ activity. Thus, ssDNA gap formation, modulated by cell-cycle regulators and MRN complex activity, underlies the synthetic lethality between POLθ and BRCA1, an important insight for clinical trials with POLθ inhibitors.

A Previously Unrecognized Molecular Landscape of Lynch Syndrome in the Mexican Population.

Lynch syndrome (LS) is the main hereditary colorectal cancer syndrome. There have been few reports regarding the clinical and molecular characteristics of LS patients in Latin America; this is particularly true in the Mexican population, where no information is available. The present study aims to describe the clinical and molecular spectrum of variants in a cohort of patients diagnosed with LS in Mexico. We present a retrospective analysis of 412 patients with suspected LS, whose main site of cancer diagnosis was the colon (58.25%), followed by the endometrium (18.93%). Next-generation sequencing analysis, with an extensive multigene panel, showed that 27.1% (112/414) had a variant in one of the genes of the mismatch repair pathway (MMR); 30.4% (126/414) had a variant in non-MMR genes such as CHEK2, APC, MUTYH, BRCA1, and BRCA2; and 42.5% (176/414) had no genetic variants. Most of the variants were found in MLH1. Pathogenic variants (PVs) in MMR genes were identified in 65.7% (96/146) of the total PVs, and 34.24% (45/146) were in non-MMR genes. Molecular and clinical characterization of patients with LS in specific populations allowed personalized follow-up, with the option for targeted treatment with immune checkpoint inhibitors and the development of public health policies. Moreover, such characterization allows for family cascade testing and consequent prevention strategies.

Familial erythrocytosis 2 and von Hippel-Lindau disease in the same pediatric patient / Eritrocitosis familiar 2 y enfermedad de von Hippel-Lindau en el mismo paciente

Abstract Background: Patients with familial erythrocytosis type 2 have no increased risk of von Hippel-Lindau-associated tumors, although mutations in the VHL gene cause both pathologies. Case report: We present a case of a compound heterozygote patient with von Hippel-Lindau disease and familial erythrocytosis type 2. One of the mutations found in our patient, c.416C>G (p.Ser139Cys) of the VHL gene, has not been previously reported. This case is the second one reported where von Hippel-Lindau disease and familial erythrocytosis type 2 coexist in the same individual. Conclusions: Despite the low frequency of familial erythrocytosis type 2 in patients with von Hippel-Lindau disease, the possibility of this diagnosis should be considered to avoid unnecessary invasive studies to explain the polyglobulia in these patients and guarantee an adequate follow-up and vigilance of both diseases.

Resumen Introducción: Los pacientes con eritrocitosis familiar tipo 2 no muestran un riesgo incrementado de desarrollar tumores asociados con la enfermedad de von Hippel-Lindau, a pesar de que ambas afecciones están causadas por variantes patogénicas en el gen VHL. Caso clínico: Se presenta el caso de un paciente heterocigoto compuesto con enfermedad de von Hippel-Lindau y eritrocitosis familiar tipo 2. Una de las variantes patogénicas en el paciente, VHL c.416C>G (p.Ser139Cys), no ha sido previamente reportada. Este es el segundo reporte de caso en que la enfermedad de von Hippel-Lindau y la eritrocitosis familiar tipo 2 coexisten en el mismo individuo. Conclusiones: A pesar de la baja frecuencia de la eritrocitosis familiar tipo 2 en pacientes con enfermedad de von Hippel-Lindau, la posibilidad del diagnóstico debe considerarse con el fin de evitar estudios invasivos innecesarios para explicar la presencia de poliglobulia en estos pacientes y para garantizar un adecuado seguimiento y una correcta vigilancia de ambas enfermedades.

The human nucleoporin Tpr protects cells from RNA-mediated replication stress.

Although human nucleoporin Tpr is frequently deregulated in cancer, its roles are poorly understood. Here we show that Tpr depletion generates transcription-dependent replication stress, DNA breaks, and genomic instability. DNA fiber assays and electron microscopy visualization of replication intermediates show that Tpr deficient cells exhibit slow and asymmetric replication forks under replication stress. Tpr deficiency evokes enhanced levels of DNA-RNA hybrids. Additionally, complementary proteomic strategies identify a network of Tpr-interacting proteins mediating RNA processing, such as MATR3 and SUGP2, and functional experiments confirm that their depletion trigger cellular phenotypes shared with Tpr deficiency. Mechanistic studies reveal the interplay of Tpr with GANP, a component of the TREX-2 complex. The Tpr-GANP interaction is supported by their shared protein level alterations in a cohort of ovarian carcinomas. Our results reveal links between nucleoporins, DNA transcription and replication, and the existence of a network physically connecting replication forks with transcription, splicing, and mRNA export machinery.

Familial erythrocytosis 2 and von Hippel-Lindau disease in the same pediatric patient.

BACKGROUND: Patients with familial erythrocytosis type 2 have no increased risk of von Hippel-Lindau-associated tumors, although mutations in the VHL gene cause both pathologies. CASE REPORT: We present a case of a compound heterozygote patient with von Hippel-Lindau disease and familial erythrocytosis type 2. One of the mutations found in our patient, c.416C>G (p.Ser139Cys) of the VHL gene, has not been previously reported. This case is the second one reported where von Hippel-Lindau disease and familial erythrocytosis type 2 coexist in the same individual. CONCLUSIONS: Despite the low frequency of familial erythrocytosis type 2 in patients with von Hippel-Lindau disease, the possibility of this diagnosis should be considered to avoid unnecessary invasive studies to explain the polyglobulia in these patients and guarantee an adequate follow-up and vigilance of both diseases.

Mexican BRCA1 founder mutation: Shortening the gap in genetic assessment for hereditary breast and ovarian cancer patients.

The deletion of exons 9 to 12 of BRCA1 (9-12 del BRCA1) is considered a founder mutation in the Mexican population. We evaluate the usefulness of the target detection of 9-12 del BRCA1 as the first molecular diagnostic strategy in patients with Hereditary Breast and Ovarian Cancer (HBOC). We performed the genetic assessment of 637 patients with suspected HBOC. The region corresponding to the breakpoints for the 9-12 del BRCA1 was amplified by polymerase chain reaction (PCR). An analysis of the clinical data of the carriers and non-carriers was done, searching for characteristics that correlated with the deletion. The 9-12 del BRCA1 was detected in 5% of patients with suspected HBOC (30/637). In patients diagnosed with ovarian cancer, 13 of 30 were 9-12 del BRCA1 carriers, which represents 43%. We found a significant association between the 9-12 del BRCA1 carriers with triple negative breast cancer and high-grade papillary serous ovarian cancer. We concluded that the detection of the 9-12 del BRCA1 is useful as a first molecular diagnostic strategy in the Mexican population. In particular, it shortens the gap in genetic assessment in patients with triple negative breast cancer and ovarian cancer.

MAD2γ, a novel MAD2 isoform, reduces mitotic arrest and is associated with resistance in testicular germ cell tumors.

BACKGROUND: Prolonged mitotic arrest in response to anti-cancer chemotherapeutics, such as DNA-damaging agents, induces apoptosis, mitotic catastrophe, and senescence. Disruptions in mitotic checkpoints contribute resistance to DNA-damaging agents in cancer. MAD2 has been associated with checkpoint failure and chemotherapy response. In this study, a novel splice variant of MAD2, designated MAD2γ, was identified, and its association with the DNA damage response was investigated. METHODS: Endogenous expression of MAD2γ and full-length MAD2 (MAD2α) was measured using RT-PCR in cancer cell lines, normal foreskin fibroblasts, and tumor samples collected from patients with testicular germ cell tumors (TGCTs). A plasmid expressing MAD2γ was transfected into HCT116 cells, and its intracellular localization and checkpoint function were evaluated according to immunofluorescence and mitotic index. RESULTS: MAD2γ was expressed in several cancer cell lines and non-cancerous fibroblasts. Ectopically expressed MAD2γ localized to the nucleus and reduced the mitotic index, suggesting checkpoint impairment. In patients with TGCTs, the overexpression of endogenous MAD2γ, but not MAD2α, was associated with resistance to cisplatin-based chemotherapy. Likewise, cisplatin induced the overexpression of endogenous MAD2γ, but not MAD2α, in HCT116 cells. CONCLUSIONS: Overexpression of MAD2γ may play a role in checkpoint disruption and is associated with resistance to cisplatin-based chemotherapy in TGCTs.