Classification of Brain Tumors by Nanopore Sequencing of Cell-Free DNA from Cerebrospinal Fluid.

BACKGROUND: Molecular brain tumor diagnosis is usually dependent on tissue biopsies or resections. This can pose several risks associated with anesthesia or neurosurgery, especially for lesions in the brain stem or other difficult-to-reach anatomical sites. Apart from initial diagnosis, tumor progression, recurrence, or the acquisition of novel genetic alterations can only be proven by re-biopsies. METHODS: We employed Nanopore sequencing on cell-free DNA (cfDNA) from cerebrospinal fluid (CSF) and analyzed copy number variations (CNV) and global DNA methylation using a random forest classifier. We sequenced 129 samples with sufficient DNA. These samples came from 99 patients and encompassed 22 entities. Results were compared to clinical diagnosis and molecular analysis of tumor tissue, if available. RESULTS: 110/129 samples were technically successful, and 50 of these contained detectable circulating tumor DNA (ctDNA) by CNV or methylation profiling. ctDNA was detected in samples from patients with progressive disease but also from patients without known residual disease. CNV plots showed diagnostic and prognostic alterations, such as C19MC amplifications in embryonal tumors with multilayered rosettes or Chr.1q gains and Chr.6q losses in posterior fossa group A ependymoma, respectively. Most CNV profiles mirrored the profiles of the respective tumor tissue. DNA methylation allowed exact classification of the tumor in 22/110 cases and led to incorrect classification in 2/110 cases. Only 5/50 samples with detected ctDNA contained tumor cells detectable through microscopy. CONCLUSIONS: Our results suggest that Nanopore sequencing data of cfDNA from CSF samples may be a promising approach for initial brain tumor diagnostics and an important tool for disease monitoring.

Transcriptomic and epigenetic dissection of spinal ependymoma (SP-EPN) identifies clinically relevant subtypes enriched for tumors with and without NF2 mutation.

Ependymomas encompass multiple clinically relevant tumor types based on localization and molecular profiles. Tumors of the methylation class "spinal ependymoma" (SP-EPN) represent the most common intramedullary neoplasms in children and adults. However, their developmental origin is ill-defined, molecular data are scarce, and the potential heterogeneity within SP-EPN remains unexplored. The only known recurrent genetic events in SP-EPN are loss of chromosome 22q and NF2 mutations, but neither types and frequency of these alterations nor their clinical relevance have been described in a large, epigenetically defined series. Transcriptomic (n = 72), epigenetic (n = 225), genetic (n = 134), and clinical data (n = 112) were integrated for a detailed molecular overview on SP-EPN. Additionally, we mapped SP-EPN transcriptomes to developmental atlases of the developing and adult spinal cord to uncover potential developmental origins of these tumors. The integration of transcriptomic ependymoma data with single-cell atlases of the spinal cord revealed that SP-EPN display the highest similarities to mature adult ependymal cells. Unsupervised hierarchical clustering of transcriptomic data together with integrated analysis of methylation profiles identified two molecular SP-EPN subtypes. Subtype A tumors primarily carried previously known germline or sporadic NF2 mutations together with 22q loss (bi-allelic NF2 loss), resulting in decreased NF2 expression. Furthermore, they more often presented as multilocular disease and demonstrated a significantly reduced progression-free survival as compared to SP-EP subtype B. In contrast, subtype B predominantly contained samples without NF2 mutation detected in sequencing together with 22q loss (monoallelic NF2 loss). These tumors showed regular NF2 expression but more extensive global copy number alterations. Based on integrated molecular profiling of a large multi-center cohort, we identified two distinct SP-EPN subtypes with important implications for genetic counseling, patient surveillance, and drug development priorities.

Characteristics of H3K27M-mutant diffuse gliomas with a non-midline location.

PURPOSE: Diffuse midline gliomas (DMG) with H3K27 alterations (H3K27M-DMG) are a highly aggressive form of brain cancer. In rare cases, H3K27 mutations have been observed in diffuse non-midline gliomas (DNMG). It is currently unclear how these tumors should be classified. Herein, we analyze the characteristics of DNMG with H3K27M mutations. METHODS: We reviewed the clinical, radiological and histological characteristics of all patients with an H3K27M mutated diffuse glioma diagnosed in our institution, between 2016 and 2023, to identify cases with a non-midline location. We then performed a molecular characterization (DNA methylation profiling, whole genome and transcriptome sequencing or targeted sequencing) of patients with an H3K27M-mutant DNMG and reviewed previously reported cases. RESULTS: Among 51 patients (18 children and 33 adults) diagnosed with an H3K27M diffuse glioma, we identified two patients (4%) who had a non-midline location. Including our two patients, 39 patients were reported in the literature with an H3K27M-mutant DNMG. Tumors were most frequently located in the temporal lobe (48%), affected adolescents and adults, and were associated with a poor outcome (median overall survival was 10.3 months (0.1-84)). Median age at diagnosis was 19.1 years. Tumors frequently harbored TP53 mutations (74%), ATRX mutations (71%) and PDGFRA mutations or amplifications (44%). In DNA methylation analysis, H3K27M-mutant DNMG clustered within or close to the reference group of H3K27M-mutant DMG. Compared to their midline counterpart, non-midline gliomas with H3K27M mutations seemed more frequently associated with PDGFRA alterations. CONCLUSION: DNMG with H3K27M mutations share many similarities with their midline counterpart, suggesting that they correspond to a rare anatomical presentation of these tumors. This is of paramount importance, as they may benefit from new therapeutic approaches such as ONC201.

Pediatric sellar teratoma - Case report and review of the literature.

BACKGROUND: Intracranial teratoma represents a rare neoplasm, occurring predominantly during childhood. Characteristic symptoms depend on the location but are mainly hydrocephalus, visual disturbances, hypopituitarism, and diabetes insipidus. Initial diagnosis can be challenging due to similar radiological features in both teratomas and other lesions such as craniopharyngiomas. Gross total resection is recommended if feasible and associated with a good prognosis. CASE DESCRIPTION: A 10-year-old girl presented with newly diagnosed growth retardation, fatigue, cephalgia and bilateral hemianopia. Further laboratory analysis confirmed central hypothyroidism and hypercortisolism. Cranial magnetic resonance imaging showed a cystic space-occupying lesion in the sellar and suprasellar compartment with compression of the optic chiasm without hydrocephalus present, suspicious of craniopharyngioma. Subsequently, an endonasal endoscopic transsphenoidal near-total tumor resection with decompression of the optic chiasm was performed. During postoperative recovery the patient developed transient diabetes insipidus, the bilateral hemianopia remained unchanged. The patient could be discharged in a stable condition, while hormone replacement for multiple pituitary hormone deficiency was required. Surprisingly, histopathology revealed conspicuous areas of skin with formation of hairs and squamous epithelia, compatible with a mature teratoma. CONCLUSIONS: We present an extremely rare case of pediatric sellar teratoma originating from the pituitary gland and a review of literature focusing on the variation in presentation and treatment. Sellar teratomas are often mistaken for craniopharyngioma due to their similar radiographic appearances. However, the primary goal of treatment for both pathologies is to decompress eloquent surrounding structures such as the optic tract, and if applicable, resolution of hydrocephalus while avoiding damage to the pituitary stalk and especially the hypothalamic structures. If feasible, the aim of surgery should be gross total resection.

Molecular refinement of pilocytic astrocytoma in adult patients.

AIM: Pilocytic astrocytomas (PA) in adults are rare and may be challenging to identify based only on histomorphology. Compared to their paediatric counterparts, they are reportedly molecularly more diverse and associated with a worse prognosis. We aimed to describe the characteristics of adult PAs more precisely by comprehensively profiling a series of 79 histologically diagnosed adult cases (≥18 years). METHODS: We performed global DNA methylation profiling and DNA and RNA panel sequencing, and integrated the results with clinical data. We further compared the molecular characteristics of adult and paediatric PAs that had a significant match to one of the established PA methylation classes in the Heidelberg brain tumour classifier. RESULTS: The mean age in our cohort was 33 years, and 43% of the tumours were located supratentorially. Based on methylation profiling, only 39% of the cases received a significant match to a PA methylation class. Sixteen per cent matched a different tumour type and 45% had a Heidelberg classifier score <0.9 with an affiliation to diverse established methylation classes in t-SNE analyses. Although the KIAA1549::BRAF fusion was found in 98% of paediatric PAs, this was true for only 27% of histologically defined and 55% of adult PAs defined by methylation profiling. CONCLUSIONS: A particularly high fraction of adult tumours with histological features of PA do not match current PA methylation classes, indicating ambiguous histology and an urgent need for molecular profiling. Moreover, even in adult PAs with a match to a PA methylation class, the distribution of genetic drivers differs significantly from their paediatric counterparts (p<0.01).

Robust methylation-based classification of brain tumours using nanopore sequencing.

BACKGROUND: DNA methylation-based classification of cancer provides a comprehensive molecular approach to diagnose tumours. In fact, DNA methylation profiling of human brain tumours already profoundly impacts clinical neuro-oncology. However, current implementation using hybridisation microarrays is time consuming and costly. We recently reported on shallow nanopore whole-genome sequencing for rapid and cost-effective generation of genome-wide 5-methylcytosine profiles as input to supervised classification. Here, we demonstrate that this approach allows us to discriminate a wide spectrum of primary brain tumours. RESULTS: Using public reference data of 82 distinct tumour entities, we performed nanopore genome sequencing on 382 tissue samples covering 46 brain tumour (sub)types. Using bootstrap sampling in a cohort of 55 cases, we found that a minimum set of 1000 random CpG features is sufficient for high-confidence classification by ad hoc random forests. We implemented score recalibration as a confidence measure for interpretation in a clinical context and empirically determined a platform-specific threshold in a randomly sampled discovery cohort (N = 185). Applying this cut-off to an independent validation series (n = 184) yielded 148 classifiable cases (sensitivity 80.4%) and demonstrated 100% specificity. Cross-lab validation demonstrated robustness with concordant results across four laboratories in 10/11 (90.9%) cases. In a prospective benchmarking (N = 15), the median time to results was 21.1 h. CONCLUSIONS: In conclusion, nanopore sequencing allows robust and rapid methylation-based classification across the full spectrum of brain tumours. Platform-specific confidence scores facilitate clinical implementation for which prospective evaluation is warranted and ongoing.

Glioneuronal tumor with ATRX alteration, kinase fusion and anaplastic features (GTAKA): a molecularly distinct brain tumor type with recurrent NTRK gene fusions.

Glioneuronal tumors are a heterogenous group of CNS neoplasms that can be challenging to accurately diagnose. Molecular methods are highly useful in classifying these tumors-distinguishing precise classes from their histological mimics and identifying previously unrecognized types of tumors. Using an unsupervised visualization approach of DNA methylation data, we identified a novel group of tumors (n = 20) that formed a cluster separate from all established CNS tumor types. Molecular analyses revealed ATRX alterations (in 16/16 cases by DNA sequencing and/or immunohistochemistry) as well as potentially targetable gene fusions involving receptor tyrosine-kinases (RTK; mostly NTRK1-3) in all of these tumors (16/16; 100%). In addition, copy number profiling showed homozygous deletions of CDKN2A/B in 55% of cases. Histological and immunohistochemical investigations revealed glioneuronal tumors with isomorphic, round and often condensed nuclei, perinuclear clearing, high mitotic activity and microvascular proliferation. Tumors were mainly located supratentorially (84%) and occurred in patients with a median age of 19 years. Survival data were limited (n = 18) but point towards a more aggressive biology as compared to other glioneuronal tumors (median progression-free survival 12.5 months). Given their molecular characteristics in addition to anaplastic features, we suggest the term glioneuronal tumor with ATRX alteration, kinase fusion and anaplastic features (GTAKA) to describe these tumors. In summary, our findings highlight a novel type of glioneuronal tumor driven by different RTK fusions accompanied by recurrent alterations in ATRX and homozygous deletions of CDKN2A/B. Targeted approaches such as NTRK inhibition might represent a therapeutic option for patients suffering from these tumors.

Genetic and epigenetic analysis of hepatocellular adenomas with atypical morphological features.

BACKGROUND: Hepatocellular adenoma (HCA) is a rare liver tumour, which can have atypical morphological features such as cytological atypia, pseudoglandular architecture, and altered reticulin framework. Little is known about the genetic and epigenetic alterations of such HCAs and whether they show the alterations classically found in hepatocellular carcinoma (HCC) or in HCA without atypical morphology. METHODS: We analysed five HCAs with atypical morphological features and one HCA with transition to HCC. Every tumour was subtyped by immunohistochemistry, sequenced by a targeted NGS panel, and analysed on a DNA methylation microarray. RESULTS: Subtyping of the five HCAs with atypical features revealed two ß-catenin mutated HCA (b-HCA), two ß-catenin mutated inflammatory HCA (b-IHCA), and one sonic hedgehog activated HCA (shHCA). None of them showed mutations typically found in HCC, such as, e.g. TERT or TP53 mutations. The epigenomic pattern of HCAs with atypical morphological features clustered with reference data for HCAs without atypical morphological features but not with HCC. Similarly, phyloepigenetic trees using the DNA methylation data reproducibly showed that HCAs with morphological atypia are much more similar to nonmalignant samples than to malignant samples. Finally, atypical HCAs showed no relevant copy number variations (CNV). CONCLUSION: In our series, mutational, DNA methylation, as well as CNV analyses, supported a relationship of atypical HCAs with nonatypical HCAs rather than with HCC. Therefore, in cases with difficult differential diagnosis between HCC and HCA, it might be advisable to perform targeted sequencing and/or combined methylation/copy number profiling.

Immunosuppressive effects of circulating bile acids in human endotoxemia and septic shock: patients with liver failure are at risk.

BACKGROUND: Sepsis-induced immunosuppression is a frequent cause of opportunistic infections and death in critically ill patients. A better understanding of the underlying mechanisms is needed to develop targeted therapies. Circulating bile acids with immunosuppressive effects were recently identified in critically ill patients. These bile acids activate the monocyte G-protein coupled receptor TGR5, thereby inducing profound innate immune dysfunction. Whether these mechanisms contribute to immunosuppression and disease severity in sepsis is unknown. The aim of this study was to determine if immunosuppressive bile acids are present in endotoxemia and septic shock and, if so, which patients are particularly at risk. METHODS: To induce experimental endotoxemia in humans, ten healthy volunteers received 2 ng/kg E. coli lipopolysaccharide (LPS). Circulating bile acids were profiled before and after LPS administration. Furthermore, 48 patients with early (shock onset within < 24 h) and severe septic shock (norepinephrine dose > 0.4 µg/kg/min) and 48 healthy age- and sex-matched controls were analyzed for circulating bile acids. To screen for immunosuppressive effects of circulating bile acids, the capability to induce TGR5 activation was computed for each individual bile acid profile by a recently published formula. RESULTS: Although experimental endotoxemia as well as septic shock led to significant increases in total bile acids compared to controls, this increase was mild in most cases. By contrast, there was a marked and significant increase in circulating bile acids in septic shock patients with severe liver failure compared to healthy controls (61.8 µmol/L vs. 2.8 µmol/L, p = 0.0016). Circulating bile acids in these patients were capable to induce immunosuppression, as indicated by a significant increase in TGR5 activation by circulating bile acids (20.4% in severe liver failure vs. 2.8% in healthy controls, p = 0.0139). CONCLUSIONS: Circulating bile acids capable of inducing immunosuppression are present in septic shock patients with severe liver failure. Future studies should examine whether modulation of bile acid metabolism can improve the clinical course and outcome of sepsis in these patients.

Mitochondrial misreading in skeletal muscle accelerates metabolic aging and confers lipid accumulation and increased inflammation.

We have recently reported on an experimental model of mitochondrial mistranslation conferred by amino acid exchange V338Y in the mitochondrial ribosomal protein MrpS5. Here we used a combination of RNA-Seq and metabolic profiling of homozygous transgenic MrpS5V338Y/V338Y mice to analyze the changes associated with the V338Y mutation in post-mitotic skeletal muscle. Metabolic profiling demonstrated age-dependent metabolic changes in the mutant V338Y animals, which included enhanced levels of age-associated metabolites and which were accompanied by increased glycolysis, lipid desaturation and eicosanoid biosynthesis, and alterations of the pentose phosphate pathway. In addition, transcriptome signatures of aged V338Y mutant muscle pointed to elevated inflammation, likely reflecting the increased levels of bioactive lipids. Our findings indicate that mistranslation-mediated chronic impairment of mitochondrial function affects specific bioenergetic processes in muscle in an age-dependent manner.

Severe oligomeric tau toxicity can be reversed without long-term sequelae.

Tau is a microtubule stabilizing protein that forms abnormal aggregates in many neurodegenerative disorders, including Alzheimer's disease. We have previously shown that co-expression of fragmented and full-length tau in P301SxTAU62on tau transgenic mice results in the formation of oligomeric tau species and causes severe paralysis. This paralysis is fully reversible once expression of the tau fragment is halted, even though P301S tau expression is maintained. Whereas various strategies to target tau aggregation have been developed, little is known about the long-term consequences of reverted tau toxicity. Therefore, we studied the long-term motor fitness of recovered, formerly paralysed P301SxTAU62on-off mice. To assess the seeding competence of oligomeric toxic tau species, we also inoculated ALZ17 mice with brainstem homogenates from paralysed P301SxTAU62on mice. Counter-intuitively, after recovery from paralysis due to oligomeric tau species expression, ageing P301SxTAU62on-off mice did not develop more motor impairment or tau pathology when compared to heterozygous P301S tau transgenic littermates. Thus, toxic tau species causing extensive neuronal dysfunction can be cleared without inducing seeding effects. Moreover, these toxic tau species also lack long-term tau seeding effects upon intrahippocampal inoculation into ALZ17 mice. In conclusion, tau species can be neurotoxic in the absence of seeding-competent tau aggregates, and mice can clear these tau forms permanently without tau seeding or spreading effects. These observations suggest that early targeting of non-fibrillar tau species may represent a therapeutically effective intervention in tauopathies. On the other hand, the absent seeding competence of early toxic tau species also warrants caution when using seeding-based tests for preclinical tauopathy diagnostics.

Diagnostic DNA methylome profiling and the WHO 5th edition CNS tumor classification.

The long-awaited 5th edition of the WHO brain tumor classification has put considerable emphasis on the importance of diagnostic DNA methylation profiling. In this article, the authors comparatively discuss selected practical aspects as well as general advantages and limitations of array- versus nanopore sequencing-based approaches to methylome profiling.

Larotrectinib Response in NTRK3 Fusion-Driven Diffuse High-Grade Glioma.

High-grade glioma (HGG) and glioblastoma are the most common adult malignant brain tumors. The standard treatment consists of surgical resection followed by radiochemotherapy with temozolomide. The prognosis and the therapeutic options of these malignant brain tumors however are limited. Here, we describe a case of a patient with HGG with a previously unknown NTRK3 fusion that showed an extraordinary response to treatment with larotrectinib. This case supports regular testing for NTRK fusion proteins.

Trans-seeding of Alzheimer-related tau protein by a yeast prion.

Abnormal tau protein aggregates constitute a hallmark of Alzheimer's disease. The mechanisms underlying the initiation of tau aggregation in sporadic neurodegeneration remain unclear. Here we investigate whether a non-human prion can seed tau aggregation. Due to their structural similarity with tau aggregates, we chose Sup35NM yeast prion domain fibrils for explorative tau seedings. Upon in vitro incubation with tau monomers, Sup35NM fibrils promoted the formation of morphologically distinct tau fibril strains. In vivo, intrahippocampal inoculation of Sup35NM fibrils accentuated tau pathology in P301S tau transgenic mice. Thus, our results provide first in vivo evidence for heterotypic cross-species seeding of a neurodegenerative human prion-like protein by a yeast prion. This opens up the conceptual perspective that non-mammalian prions present in the human microbiome could be involved in the initiation of protein misfolding in neurodegenerative disorders, a mechanism for which we propose the term "trans-seeding."

Supratentorial ependymoma in childhood: more than just RELA or YAP.

Two distinct genetically defined entities of ependymoma arising in the supratentorial compartment are characterized by the presence of either a C11orf95-RELA or a YAP-MAMLD1 fusion, respectively. There is growing evidence that supratentorial ependymomas without these genetic features exist. In this study, we report on 18 pediatric non-RELA/non-YAP supratentorial ependymomas that were systematically characterized by means of their histology, immunophenotype, genetics, and epigenomics. Comprehensive molecular analyses included high-resolution copy number analysis, methylation profiling, analysis of fusion transcripts by Nanostring technology, and RNA sequencing. Based upon histological and immunohistochemical features two main patterns were identified-RELA-like (n = 9) and tanycytic ependymomas (n = 6). In the RELA-like group histologically assigned to WHO grade III and resembling RELA-fused ependymomas, tumors lacked nuclear expression of p65-RelA as a surrogate marker for a pathological activation of the NF-κB pathway. Three tumors showed alternative C11orf95 fusions to MAML2 or NCOA1. A methylation-based brain tumor classifier assigned two RELA-like tumors to the methylation class "EP, RELA-fusion"; the others demonstrated no significant similarity score. Of the tanycytic group, 5/6 tumors were assigned a WHO grade II. No gene fusions were detected. Methylation profiling did not show any association with an established methylation class. We additionally identified two astroblastoma-like tumors that both presented with chromothripsis of chromosome 22 but lacked MN1 breaks according to FISH analysis. They revealed novel fusion events involving genes in chromosome 22. One further tumor with polyploid cytogenetics was interpreted as PFB ependymoma by the brain tumor methylation classifier but had no relation to the posterior fossa. Clinical follow-up was available for 16/18 patients. Patients with tanycytic and astroblastoma-like tumors had no relapse, while 2 patients with RELA-like ependymomas died. Our data indicate that in addition to ependymomas discovered so far, at least two more supratentorial ependymoma types (RELA-like and tanycytic) exist.

Clear cell meningiomas are defined by a highly distinct DNA methylation profile and mutations in SMARCE1.

Clear cell meningioma represents an uncommon variant of meningioma that typically affects children and young adults. Although an enrichment of loss-of-function mutations in the SMARCE1 gene has been reported for this subtype, comprehensive molecular investigations are lacking. Here we describe a molecularly distinct subset of tumors (n = 31), initially identified through genome-wide DNA methylation screening among a cohort of 3093 meningiomas, of which most were diagnosed histologically as clear cell meningioma. This cohort was further supplemented by an additional 11 histologically diagnosed clear cell meningiomas for analysis (n = 42). Targeted DNA sequencing revealed SMARCE1 mutations in 33/34 analyzed samples, accompanied by a nuclear loss of expression determined via immunohistochemistry and a decreased SMARCE1 transcript expression in the tumor cells. Analysis of time to progression or recurrence of patients within the clear cell meningioma group (n = 14) in comparison to those with meningioma WHO grade 2 (n = 220) revealed a similar outcome and support the assignment of WHO grade 2 to these tumors. Our findings indicate the existence of a highly distinct epigenetic signature of clear cell meningiomas, separate from all other variants of meningiomas, with recurrent mutations in the SMARCE1 gene. This suggests that these tumors may arise from a different precursor cell population than the broad spectrum of the other meningioma subtypes.

Glioblastomas with primitive neuronal component harbor a distinct methylation and copy-number profile with inactivation of TP53, PTEN, and RB1.

Glioblastoma IDH-wildtype presents with a wide histological spectrum. Some features are so distinctive that they are considered as separate histological variants or patterns for the purpose of classification. However, these usually lack defined (epi-)genetic alterations or profiles correlating with this histology. Here, we describe a molecular subtype with overlap to the unique histological pattern of glioblastoma with primitive neuronal component. Our cohort consists of 63 IDH-wildtype glioblastomas that harbor a characteristic DNA methylation profile. Median age at diagnosis was 59.5 years. Copy-number variations and genetic sequencing revealed frequent alterations in TP53, RB1 and PTEN, with fewer gains of chromosome 7 and homozygous CDKN2A/B deletions than usually described for IDH-wildtype glioblastoma. Gains of chromosome 1 were detected in more than half of the cases. A poorly differentiated phenotype with frequent absence of GFAP expression, high proliferation index and strong staining for p53 and TTF1 often caused misleading histological classification as carcinoma metastasis or primitive neuroectodermal tumor. Clinically, many patients presented with leptomeningeal dissemination and spinal metastasis. Outcome was poor with a median overall survival of only 12 months. Overall, we describe a new molecular subtype of IDH-wildtype glioblastoma with a distinct histological appearance and genetic signature.

Differentiation of rare brain tumors through unsupervised machine learning: Clinical significance of in-depth methylation and copy number profiling illustrated through an unusual case of IDH wildtype glioblastoma.

Methylation profiling has become a mainstay in brain tumor diagnostics since the introduction of the first publicly available classification tool by the German Cancer Research Center in 2017. We demonstrate the capability of this system through an example of a rare case of IDH wildtype glioblastoma diagnosed in a patient previously treated for T-cell acute lymphoblastic leukemia. Our novel in-house diagnostic tool EpiDiP provided hints arguing against a radiation-induced tumor, identified a novel recurrent genetic aberration, and thus informed about a potential therapeutic target.

CHCHD2 accumulates in distressed mitochondria and facilitates oligomerization of CHCHD10.

Mutations in paralogous mitochondrial proteins CHCHD2 and CHCHD10 cause autosomal dominant Parkinson Disease (PD) and Amyotrophic Lateral Sclerosis/Frontotemporal Dementia (ALS/FTD), respectively. Using newly generated CHCHD2, CHCHD10 and CHCHD2/10 double knockout cell lines, we find that the proteins are partially functionally redundant, similarly distributed throughout the mitochondrial cristae, and form heterodimers. Unexpectedly, we also find that CHCHD2/CHCHD10 heterodimerization increases in response to mitochondrial stress. This increase is driven by differences in the proteins' stability and mutual affinity: CHCHD2 is preferentially stabilized by loss of mitochondrial membrane potential, and CHCHD10 oligomerization depends on CHCHD2 expression. Exploiting the dependence of CHCHD10 oligomerization on CHCHD2, we developed a heterodimer incorporation assay and demonstrate that CHCHD2 and CHCHD10 with disease-causing mutations readily form heterodimers. As we also find that both proteins are highly expressed in human Substantia nigra and cortical pyramidal neurons, mutant CHCHD2 and CHCHD10 may directly interact with their wild-type paralogs in the context of PD and ALS/FTD pathogenesis. Together, these findings demonstrate that differences in the stability and mutual affinity of CHCHD2 and CHCHD10 regulate their heterodimerization in response to mitochondrial distress, revealing an unanticipated link between PD and ALS/FTD pathogenesis.