Severe Immune Thrombocytopenic Purpura in a Pediatric Patient With Fanconi Anemia: A Case Report.

Fanconi anemia (FA) is a rare inherited disorder characterized by progressive bone marrow failure, chromosomal instability, and an increased predisposition to malignancies. Autoimmune manifestations are uncommon in FA, with immune thrombocytopenic purpura (ITP) being a particularly rare presentation. ITP is an autoimmune disorder marked by immune-mediated platelet destruction, leading to severe thrombocytopenia and an increased risk of bleeding. This case report presents a pediatric patient with FA and severe ITP, illustrating the clinical challenges of managing autoimmune complications in the context of FA. This case report describes the case of a six-year-old boy with known FA who presented with a three-day history of spontaneous bruising, petechiae, and epistaxis. He had severe thrombocytopenia (platelet count: 8,000/µL) without other significant cytopenias. The initial workup excluded viral infections and other secondary causes, leading to the diagnosis of ITP. The patient was treated with intravenous immunoglobulin (IVIG) and corticosteroids, resulting in a transient improvement. However, his platelet counts declined, prompting treatment with rituximab, which achieved a sustained response. He was discharged after four weeks of rituximab therapy and remained stable at follow-up with a platelet count of 100,000/µL. This case highlights the rare occurrence of ITP in FA and the successful use of rituximab for refractory thrombocytopenia. The findings suggest a need for ongoing research into the mechanisms of immune dysregulation in FA and the development of optimized therapeutic strategies for managing autoimmune manifestations in this complex patient population.

FANCD2 genome binding is nonrandom and is enriched at large transcriptionally active neural genes prone to copy number variation.

Fanconi anemia (FA) is a rare genetic disease characterized by congenital abnormalities and increased risk for bone marrow failure and cancer. Central nervous system defects, including acute and irreversible loss of neurological function and white matter lesions with calcifications, have become increasingly recognized among FA patients, and are collectively referred to as Fanconi Anemia Neurological Syndrome or FANS. The molecular etiology of FANS is poorly understood. In this study, we have used a functional integrative genomics approach to further define the function of the FANCD2 protein and FA pathway. Combined analysis of new and existing FANCD2 ChIP-seq datasets demonstrates that FANCD2 binds nonrandomly throughout the genome with binding enriched at transcription start sites and in broad regions spanning protein-coding gene bodies. FANCD2 demonstrates a strong preference for large neural genes involved in neuronal differentiation, synapse function, and cell adhesion, with many of these genes implicated in neurodevelopmental and neuropsychiatric disorders. Furthermore, FANCD2 binds to regions of the genome that replicate late, undergo mitotic DNA synthesis (MiDAS) under conditions of replication stress, and are hotspots for copy number variation. Our analysis describes an important targeted role for FANCD2 and the FA pathway in the maintenance of large neural gene stability.

Small pituitary volume and central nervous system anomalies in Fanconi Anemia.

Introduction: Fanconi anemia (FA) is a genomic instability disorder associated with congenital abnormalities, including short stature and the presence of central nervous system anomalies, especially in the hypothalamic-pituitary area. Thus, differences in pituitary size could associate with the short stature observed in these patients. Our aim was to evaluate whether central nervous system abnormalities and pituitary gland volume correlate with height and hormone deficiencies in these patients. Methods: In this cross-sectional exploratory study 21 patients diagnosed with FA between 2017 and 2022 in a Spanish Reference Center were investigated. Magnetic resonance imaging (MRI) was performed and pituitary volume calculated and corelated with height and other endocrine parameters. Results: The percentage of abnormalities in our series was 81%, with a small pituitary (pituitary volume less than 1 SD) being the most frequent, followed by Chiari malformation type 1. The median value of pituitary volume was -1.03 SD (IQR: -1.56, -0.36). Short stature was found in 66.7% [CI95% 43-85.4]. Total volume (mm3) increases significantly with age and in pubertal stages. There were no differences between volume SD and pubertal stage, or the presence of endocrine deficiencies. No correlations were found between pituitary volume and the presence of short stature. The intraclass correlation index (ICC) average for volume was 0.85 [CI95% 0.61-0.94] indicating a good-to-excellent correlation of measurements. Discussion: Central nervous system anomalies are part of the FA phenotype, the most frequent after pituitary hypoplasia being posterior fossa abnormalities, which may have clinical repercussions in the patient. It is therefore necessary to identify those who could be candidates for neurosurgical intervention. The size of the pituitary gland is smaller in these patients, but this does not seem to be related to hormone deficiency and short stature or exposure to a low dose of total body irradiation.

Haploidentical hematopoietic stem cell transplantation with post-transplant cyclophosphamide for Fanconi anemia with/without anti-thymocyte globulin.

Background: We present comparative data of children with Fanconi anemia undergoing haploidentical hematopoietic stem cell transplantation (HSCT) with or without the addition of rabbit anti-thymocyte globulin (r-ATG) to the conditioning regimen. Patients and methods: This retrospective study included children with Fanconi anemia aged up to 18 years who underwent haploidentical HSCT between January 2015 and December 2022. The children were included in two cohorts in this study. Cohort 1 included children who received conditioning with fludarabine/cyclophosphamide/single fraction of 2 Gy TBI. The children in cohort 2 received the same conditioning along with r-ATG. Post-transplant cyclophosphamide was administered at a dose of 25 mg/kg on day3 and day4 in both cohorts. Results: A total of 35 children were included in the study, 25 in cohort 1 and 10 in cohort 2. Neutrophil engraftment was documented around day 14-16 post infusion in 21 children (84%) in cohort 1 and in 8 children (80%) in cohort 2. There was a significant difference in the incidence of the severity of graft versus host disease (GVHD) between the two cohorts (p = 0.003). In cohort 1, acute GVHD was documented in 17 children (68%), with grade 1/2 skin GVHD in 10 children, and grade 3/4 skin and gut GVHD in 7 children. Grade 4 gut GVHD was the cause of death in three children in cohort 1. In cohort 2, acute GVHD was documented in one child (10%) who had grade 4 skin and gut GVHD and succumbed to the above. Chronic GVHD was noted in nine (36%) children in cohort 1, and in one child (10%) in cohort 2. Cytomegalovirus reactivation was documented in 11 children (44%) in cohort 1 and three children (30%) in cohort 2. Overall survival was found to be 16/25 (64%) in cohort 1, with a median follow-up of 49 months, and 7/10 (70%) in cohort 2, with a median follow-up of 12 months. Conclusion: Serotherapy with r-ATG significantly reduced the incidence of GVHD from 68% to 10% in children with Fanconi anemia, with an increase in overall survival from 64% to 70%, although it did not affect graft failure. Further studies should focus on decreasing graft failure rates with early HSCT before multiple transfusions.

The Fanconi anemia pathway induces chromothripsis and ecDNA-driven cancer drug resistance.

Chromothripsis describes the catastrophic shattering of mis-segregated chromosomes trapped within micronuclei. Although micronuclei accumulate DNA double-strand breaks and replication defects throughout interphase, how chromosomes undergo shattering remains unresolved. Using CRISPR-Cas9 screens, we identify a non-canonical role of the Fanconi anemia (FA) pathway as a driver of chromothripsis. Inactivation of the FA pathway suppresses chromosome shattering during mitosis without impacting interphase-associated defects within micronuclei. Mono-ubiquitination of FANCI-FANCD2 by the FA core complex promotes its mitotic engagement with under-replicated micronuclear chromosomes. The structure-selective SLX4-XPF-ERCC1 endonuclease subsequently induces large-scale nucleolytic cleavage of persistent DNA replication intermediates, which stimulates POLD3-dependent mitotic DNA synthesis to prime shattered fragments for reassembly in the ensuing cell cycle. Notably, FA-pathway-induced chromothripsis generates complex genomic rearrangements and extrachromosomal DNA that confer acquired resistance to anti-cancer therapies. Our findings demonstrate how pathological activation of a central DNA repair mechanism paradoxically triggers cancer genome evolution through chromothripsis.

The role of SLFN11 in DNA replication stress response and its implications for the Fanconi anemia pathway.

Fanconi anemia (FA) is a hereditary disorder characterized by a deficiency in the repair of DNA interstrand crosslinks and the response to replication stress. Endogenous DNA damage, most likely caused by aldehydes, severely affects hematopoietic stem cells in FA, resulting in progressive bone marrow failure and the development of leukemia. Recent studies revealed that expression levels of SLFN11 affect the replication stress response and are a strong determinant in cell killing by DNA-damaging cancer chemotherapy. Because SLFN11 is highly expressed in the hematopoietic system, we speculated that SLFN11 may have a significant role in FA pathophysiology. Indeed, we found that DNA damage sensitivity in FA cells is significantly mitigated by the loss of SLFN11 expression. Mechanistically, we demonstrated that SLFN11 destabilizes the nascent DNA strands upon replication fork stalling. In this review, we summarize our work regarding an interplay between SLFN11 and the FA pathway, and the role of SLFN11 in the response to replication stress.

Oral health-related quality of life in Fanconi anemia: a cross-sectional study.

PURPOSE: To evaluate the oral health-related quality of life (OHRQoL) of individuals diagnosed with Fanconi anemia (FA). METHODS: A cross-sectional study was conducted with FA patients from two Brazilian referral centers. Participants underwent a complete dental, periodontal, and oral mucosa examination, as well as assessment of resting salivary flow. The short version of the Oral Health Impact Profile (OHIP-14) questionnaire was administered. Descriptive and bivariate analyses were performed, followed by multivariate analysis to examine the impact of independent variables on OHRQoL. RESULTS: The study included 20 (57.1%) males and 15 (42.9%) females, with a mean age of 18.9 years. Oral leukoplakia (OL) was found in 18 individuals. The overall OHIP-14 score was 9.9 ± 10.5. Individuals aged ≥ 16 years had higher OHIP-14 scores, indicating worse OHRQoL for physical pain (p = 0.007), psychological discomfort (p = 0.001), physical disability (p = 0.03), psychological disability (p = 0.001), handicap (p = 0.004), and overall score (p = 0.007). Females reported more negative OHRQoL than males for physical pain (p = 0.02), psychological discomfort (p = 0.03), psychological disability (p = 0.009), and overall score (p = 0.02). Individuals with OL had an overall OHIP-14 score 1.83 times higher than those without OL (95% CI: 1.02-3.28; p = 0.04). Lower salivary flow correlated with higher overall OHIP-14 scores (95% CI: 0.14-0.84; p = 0.01). CONCLUSION: This study represents the first attempt to evaluate OHRQoL in individuals with FA. The presence of OL and reduced salivary flow were identified as predictors of a negative impact on OHRQoL. It is imperative to integrate patients' quality of life in the clinical treatment protocols for the FA population.

A Rare Case Report of Fanconi Anemia-Associated Nuclease 1 Mutation Causing Nephrotic Syndrome.

A 25-year-old African male patient presented with a history of frothy urination for one month. He had a significant family history of early onset chronic kidney disease (CKD) in his older brother. On evaluation, he was found to have deranged renal function and nephrotic-range proteinuria of 6152 mg/day. Urine examination revealed proteinuria and glycosuria. Viral serology and autoimmune screening results were negative. Ultrasonography revealed contracted kidneys that were not amenable to biopsy. Genetic analysis revealed a Fanconi anemia-associated nuclease 1 (FAN 1) mutation in exon 4 (c.1399G>A) and exon 12 (c.2786A>C). The patient was managed conservatively with a maximum dose of angiotensin receptor blockers with a reduction in proteinuria on follow-up. This case report highlights the rare manifestation of FAN 1 mutation and its variable effects on the kidney.

Cisplatin-induced bone marrow failure in an adult patient with Fanconi anemia.

INTRODUCTION: Fanconi anemia (FA) is a genetic disorder characterized by bone marrow failure typically developing in the first decade of life, congenital abnormalities, and an increased predisposition to malignancy. However, patients with FA can remain undiagnosed until adulthood and present with solid organ malignancies. Due to impaired DNA repair mechanisms, patients with FA are highly susceptible to severe bone marrow toxicity when treated with cisplatin. CASE REPORT: A 38-year-old woman, diagnosed with locally advanced squamous cell carcinoma (SCC) of the uterine cervix, underwent treatment with weekly cisplatin concurrent with radiotherapy. After the second week of cisplatin treatment, she presented with severe pancytopenia. The prolonged and severe pancytopenia following cisplatin and radiation, along with cervical SCC in the absence of risk factors and the presence of parental consanguinity, raised the possibility of FA as the underlying cause. Whole exome sequencing revealed a homozygous FANCI c.668A > C (p.Lys223Thr) missense variant confirming the diagnosis of FA. MANAGEMENT AND OUTCOME: The pancytopenia exhibited a protracted course, necessitating admission and supportive treatment with antibiotics, red blood cell and platelet transfusions, as well as filgrastim and eltrombopag. Eventually, the pancytopenia improved after approximately 40 days of hospitalization. DISCUSSION: SCC of the head and neck or gynecologic organs in a young adult without known risk factors should prompt consideration of FA. Cisplatin should be avoided in patients with FA.

Longitudinal clinical manifestations of Fanconi anemia: A systematized review.

Fanconi anemia (FA) is a rare and complex inherited genetic disorder characterized by impaired DNA repair mechanisms leading to genomic instability. Individuals with FA have increased susceptibility to congenital anomalies, progressive bone marrow failure, leukemia and malignant tumors, endocrinopathies and other medical issues. In recent decades, steadily improved approaches to hematopoietic cell transplantation (HCT), the only proven curative therapy for the hematologic manifestations of FA, have significantly increased the life expectancy of affected individuals, illuminating the need to understand the long-term consequences and multi-organ ramifications. Utilizing a systematized review approach with narrative synthesis of each primary issue and organ system, we shed light on the challenges and opportunities for optimizing the care and quality of life for individuals with FA and identify knowledge gaps informing future research directions.

A comprehensive study evaluating germline FANCG variants in predisposition to breast and ovarian cancer.

BACKGROUND: Monoallelic germline pathogenic variants (GPVs) in five Fanconi anemia (FA) genes (BRCA1/FANCS, BRCA2/FANCD1, PALB2/FANCN, BRIP1/FANCJ, and RAD51C/FANCO) confer an increased risk of breast (BC) and/or ovarian (OC) cancer, but the role of GPVs in 17 other FA genes remains unclear. METHODS: Here, we investigated the association of germline variants in FANCG/XRCC9 with BC and OC risk. RESULTS: The frequency of truncating GPVs in FANCG did not differ between BC (20/10,204; 0.20%) and OC (8/2966; 0.27%) patients compared to controls (6/3250; 0.18%). In addition, only one out of five tumor samples showed loss-of-heterozygosity of the wild-type FANCG allele. Finally, none of the nine functionally tested rare recurrent missense FANCG variants impaired DNA repair activities (FANCD2 monoubiquitination and FANCD2 foci formation) upon DNA damage, in contrast to all tested FANCG truncations. CONCLUSION: Our study suggests that heterozygous germline FANCG variants are unlikely to contribute to the development of BC or OC.

Role of the mesenchymal stromal cells in bone marrow failure of Fanconi Anemia patients.

Introduction: Fanconi anemia (FA) is an inherited disorder characterized by bone marrow failure, congenital malformations, and predisposition to malignancies. Alterations in hematopoietic stem cells (HSC) have been reported, but little is known regarding the bone marrow (BM) stroma. Thus, the characterization of Mesenchymal Stromal Cells (MSC) would help to elucidate their involvement in the BM failure. Methods: We characterized MSCs of 28 FA patients (FA-MSC) before and after treatment (hematopoietic stem cell transplantation, HSCT; or gene therapy, GT). Phenotypic and functional properties were analyzed and compared with MSCs expanded from 26 healthy donors (HD-MSCs). FA-MSCs were genetically characterized through, mitomycin C-test and chimerism analysis. Furthermore, RNA-seq profiling was used to identify dysregulated metabolic pathways. Results: Overall, FA-MSC had the same phenotypic and functional characteristics as HD-MSC. Of note, MSC-GT had a lower clonogenic efficiency. These findings were not confirmed in the whole FA patients' cohort. Transcriptomic profiling identified dysregulation in HSC self-maintenance pathways in FA-MSC (HOX), and was confirmed by real-time quantitative polymerase chain reaction (RT-qPCR). Discussion: Our study provides a comprehensive characterization of FA-MSCs, including for the first time MSC-GT and constitutes the largest series published to date. Interestingly, transcript profiling revealed dysregulation of metabolic pathways related to HSC self-maintenance. Taken together, our results or findings provide new insights into the pathophysiology of the disease, although whether these niche defects are involved in the hematopoietic defects seen of FA deserves further investigation.

Reduced anti-Müllerian hormone levels in males with inherited bone marrow failure syndromes.

Fanconi anemia (FA), dyskeratosis congenita-related telomere biology disorders (DC/TBD), and Diamond-Blackfan anemia (DBA) are inherited bone marrow failure syndromes (IBMFS) with high risks of bone marrow failure, leukemia, and solid tumors. Individuals with FA have reduced fertility. Previously, we showed low levels of anti-Müllerian hormone (AMH), a circulating marker of ovarian reserve, in females with IBMFS. In males, AMH may be a direct marker of Sertoli cell function and an indirect marker of spermatogenesis. In this study, we assessed serum AMH levels in pubertal and postpubertal males with FA, DC/TBD, or DBA and compared this with their unaffected male relatives and unrelated healthy male volunteers. Males with FA had significantly lower levels of AMH (median: 5 ng/mL, range: 1.18-6.75) compared with unaffected male relatives (median: 7.31 ng/mL, range: 3.46-18.82, P = 0.03) or healthy male volunteers (median: 7.66 ng/mL, range: 3.3-14.67, P = 0.008). Males with DC/TBD had lower levels of AMH (median: 3.76 ng/mL, range: 0-8.9) compared with unaffected relatives (median: 5.31 ng/mL, range: 1.2-17.77, P = 0.01) or healthy volunteers (median: 5.995 ng/mL, range: 1.57-14.67, P < 0.001). Males with DBA had similar levels of AMH (median: 3.46 ng/mL, range: 2.32-11.85) as unaffected relatives (median: 4.66 ng/mL, range: 0.09-13.51, P = 0.56) and healthy volunteers (median: 5.81 ng/mL, range: 1.57-14.67, P = 0.10). Our findings suggest a defect in the production of AMH in postpubertal males with FA and DC/TBD, similar to that observed in females. These findings warrant confirmation in larger prospective studies.

A Nutrigenomic View on the Premature-Aging Disease Fanconi Anemia.

Fanconi anemia, a rare disorder with an incidence of 1 in 300,000, is caused by mutations in FANC genes, which affect the repair of DNA interstrand crosslinks. The disease is characterized by congenital malformations, bone marrow failure within the first decade of life, and recurrent squamous cell carcinomas of the oral cavity, esophagus, and anogenital regions starting around age 20. In this review, we propose that Fanconi anemia should be considered a premature-aging syndrome. Interestingly, the onset and severity of the life-limiting clinical features of Fanconi anemia can be influenced by lifestyle choices, such as a healthy diet and physical activity. These factors shape the epigenetic status of at-risk cell types and enhance the competence of the immune system through nutritional signaling. Fanconi anemia may serve as a model for understanding the aging process in the general population, addressing research gaps in its clinical presentation and suggesting prevention strategies. Additionally, we will discuss how the balance of genetic and environmental risk factors-affecting both cancer onset and the speed of aging-is interlinked with signal transduction by dietary molecules. The underlying nutrigenomic principles will offer guidance for healthy aging in individuals with Fanconi anemia as well as for the general population.

Advanced Analysis and Validation of a microRNA Signature for Fanconi Anemia.

Some years ago, we reported the generation of a Fanconi anemia (FA) microRNA signature. This study aims to develop an analytical strategy to select a smaller and more reliable set of molecules that could be tested for potential benefits for the FA phenotype, elucidate its biochemical and molecular mechanisms, address experimental activity, and evaluate its possible impact on FA therapy. In silico analyses of the data obtained in the original study were thoroughly processed and anenrichment analysis was employed to identify the classes of genes that are over-represented in the FA-miRNA population under study. Primary bone marrow mononuclear cells (MNCs) from sixFA patients and sixhealthy donors as control samples were employed in the study. RNAs containing the small RNA fractions were reverse-transcribed and real-time PCR was performed in triplicate using the specific primers. Experiments were performed in triplicate.The in-silico analysis reported six miRNAs as likely contributors to the complex pathological spectrum of FA. Among these, three miRNAs were validated by real-time PCR. Primary bone marrow mononuclear cells (MNCs) reported a significant reduction in the expression level of miRNA-1246 and miRNA-206 in the FA samples in comparison to controls.This study highlights several biochemical pathways as culprits in the phenotypic manifestations and the pathophysiological mechanisms acting in FA. A relatively low number of miRNAs appear involved in all these different phenotypes, demonstrating the extreme plasticity of the gene expression modulation. This study further highlights miR-206 as a pivotal player in regulatory functions and signaling in the bone marrow mesenchymal stem cell (BMSC) process in FA. Due to this evidence, the activity of miR-206 in FA deserves specific experimental scrutiny. The results, here presented, might be relevant in the management of FA.

Failure to Thrive With Severe Hypokalemia Yields Cystinosis Diagnosis in a 19-Month-Old Female Child: A Case Report.

Cystinosis is a rare, genetically inherited disease that affects lysosomal storage of cysteine. It is the most common cause of Fanconi syndrome. Mutations have led to early-onset end-stage renal disease as well as other systemic organ failures. In this case, we report a 19-month-old female child who presented acutely to the outpatient clinic with nausea, vomiting, and diarrhea. The patient was previously diagnosed with unspecified renal tubular acidosis and treated with oral electrolytes. Early labs during her acute presentation showed severe hypokalemia and electrolyte imbalance, which necessitated a transfer to a pediatric ICU. Through confirmatory testing, a diagnosis of cystinosis was made. This case is an example of the recognition and treatment of a rare inherited disease.

The splicing factor CCAR1 regulates the Fanconi anemia/BRCA pathway.

The twenty-three Fanconi anemia (FA) proteins cooperate in the FA/BRCA pathway to repair DNA interstrand cross-links (ICLs). The cell division cycle and apoptosis regulator 1 (CCAR1) protein is also a regulator of ICL repair, though its possible function in the FA/BRCA pathway remains unknown. Here, we demonstrate that CCAR1 plays a unique upstream role in the FA/BRCA pathway and is required for FANCA protein expression in human cells. Interestingly, CCAR1 co-immunoprecipitates with FANCA pre-mRNA and is required for FANCA mRNA processing. Loss of CCAR1 results in retention of a poison exon in the FANCA transcript, thereby leading to reduced FANCA protein expression. A unique domain of CCAR1, the EF hand domain, is required for interaction with the U2AF heterodimer of the spliceosome and for excision of the poison exon. Taken together, CCAR1 is a splicing modulator required for normal splicing of the FANCA mRNA and other mRNAs involved in various cellular pathways.

Diagnosing Fanconi Anemia: A Rare Case Report From Rural India.

Fanconi anemia is a rare but most prevalent form of inherited aplastic anemia, predominantly transmitted in an autosomal recessive manner, except for one X-linked variant. It arises from mutations in the genes across 16 different complementation groups that are crucial for DNA stability. It is marked by a wide range of congenital malformations, progressive pancytopenia, and an increased risk of both hematological malignancies and solid tumors. The congenital abnormalities associated with it can affect various organ systems, including the skeletal system, with significant variability among patients. One similar case has been reported here, which had the typical clinical features of FA. Due to varied phenotypic presentation, diagnosing FA can be challenging. A Chromosomal Breakage Study using mitomycin C (MMC) or diepoxybutane (DEB) is a distinctive cellular marker that aids in the diagnosis.

CCAR1 promotes DNA repair via alternative splicing.

DNA repair is directly performed by hundreds of core factors and indirectly regulated by thousands of others. We massively expanded a CRISPR inhibition and Cas9-editing screening system to discover factors indirectly modulating homology-directed repair (HDR) in the context of ∼18,000 individual gene knockdowns. We focused on CCAR1, a poorly understood gene that we found the depletion of reduced both HDR and interstrand crosslink repair, phenocopying the loss of the Fanconi anemia pathway. CCAR1 loss abrogated FANCA protein without substantial reduction in the level of its mRNA or that of other FA genes. We instead found that CCAR1 prevents inclusion of a poison exon in FANCA. Transcriptomic analysis revealed that the CCAR1 splicing modulatory activity is not limited to FANCA, and it instead regulates widespread changes in alternative splicing that would damage coding sequences in mouse and human cells. CCAR1 therefore has an unanticipated function as a splicing fidelity factor.

Dual role of proliferating cell nuclear antigen monoubiquitination in facilitating Fanconi anemia-mediated interstrand crosslink repair.

The Fanconi anemia (FA) repair pathway governs repair of highly genotoxic DNA interstrand crosslinks (ICLs) and relies on translesion synthesis (TLS). TLS is facilitated by REV1 or site-specific monoubiquitination of proliferating cell nuclear antigen (PCNA) (PCNA-Ub) at lysine 164 (K164). A PcnaK164R/K164R but not Rev1-/- mutation renders mammals hypersensitive to ICLs. Besides the FA pathway, alternative pathways have been associated with ICL repair (1, 2), though the decision making between those remains elusive. To study the dependence and relevance of PCNA-Ub in FA repair, we intercrossed PcnaK164R/+; Fancg-/+ mice. A combined mutation (PcnaK164R/K164R; Fancg-/- ) was found embryonically lethal. RNA-seq of primary double-mutant (DM) mouse embryonic fibroblasts (MEFs) revealed elevated levels of replication stress-induced checkpoints. To exclude stress-induced confounders, we utilized a Trp53 knock-down to obtain a model to study ICL repair in depth. Regarding ICL-induced cell toxicity, cell cycle arrest, and replication fork progression, single-mutant and DM MEFs were found equally sensitive, establishing PCNA-Ub to be critical for FA-ICL repair. Immunoprecipitation and spectrometry-based analysis revealed an unknown role of PCNA-Ub in excluding mismatch recognition complex MSH2/MSH6 from being recruited to ICLs. In conclusion, our results uncovered a dual function of PCNA-Ub in ICL repair, i.e. exclude MSH2/MSH6 recruitment to channel the ICL toward canonical FA repair, in addition to its established role in coordinating TLS opposite the unhooked ICL.