Training an automated circulating tumor cell classifier when the true classification is uncertain

Circulating tumor cell (CTC) and tumor-derived extracellular vesicle (tdEV) loads are prognostic factors of survival in patients with carcinoma. The current method of CTC enumeration relies on operator review and, unfortunately, has moderate interoperator agreement (Fleiss' kappa 0.60) due to difficulties in classifying CTC-like events. We compared operator review, ACCEPT automated image processing, and refined the output of a deep-learning algorithm to identify CTC and tdEV for the prediction of survival in patients with metastatic and nonmetastatic cancers. Operator review is only defined for CTC. Refinement was performed using automatic contrast maximization CM-CTC of events detected in cancer and in benign samples (CM-CTC). We used 418 samples from benign diseases, 6,293 from nonmetastatic breast, 2,408 from metastatic breast, and 698 from metastatic prostate cancer to train, test, optimize, and evaluate CTC and tdEV enumeration. For CTC identification, the CM-CTC performed best on metastatic/nonmetastatic breast cancer, respectively, with a hazard ratio (HR) for overall survival of 2.6/2.1 vs. 2.4/1.4 for operator CTC and 1.2/0.8 for ACCEPT-CTC. For tdEV identification, CM-tdEV performed best with an HR of 1.6/2.9 vs. 1.5/1.0 with ACCEPT-tdEV. In conclusion, contrast maximization is effective even though it does not utilize domain knowledge.

Ultra-sensitive CTC-based liquid biopsy for pancreatic cancer enabled by large blood volume analysis

The limited sensitivity of circulating tumor cell (CTC) detection in pancreatic adenocarcinoma (PDAC) stems from their extremely low concentration in the whole circulating blood, necessitating enhanced detection methodologies. This study sought to amplify assay-sensitivity by employing diagnostic leukapheresis (DLA) to screen large blood volumes. Sixty patients were subjected to DLA, with a median processed blood volume of ~ 2.8 L and approximately 5% of the resulting DLA-product analyzed using CellSearch (CS). Notably, DLA significantly increased CS-CTC detection to 44% in M0-patients and 74% in M1-patients, yielding a 60-fold increase in CS-CTC enumeration. DLA also provided sufficient CS-CTCs for genomic profiling, thereby delivering additional genomic information compared to tissue biopsy samples. DLA CS-CTCs exhibited a pronounced negative prognostic impact on overall survival (OS), evidenced by a reduction in OS from 28.6 to 8.5 months (univariate: p = 0.002; multivariable: p = 0.043). Additionally, a marked enhancement in sensitivity was achieved (by around 3-4-times) compared to peripheral blood (PB) samples, with positive predictive values for OS being preserved at around 90%. Prognostic relevance of CS-CTCs in PDAC was further validated in PB-samples from 228 PDAC patients, consolidating the established association between CTC-presence and reduced OS (8.5 vs. 19.0 months, p < 0.001). In conclusion, DLA-derived CS-CTCs may serve as a viable tool for identifying high-risk PDAC-patients and aiding the optimization of multimodal treatment strategies. Moreover, DLA enables comprehensive diagnostic profiling by providing ample CTC material, reinforcing its utility as a reliable liquid-biopsy approach. This high-volume liquid-biopsy strategy presents a potential pathway for enhancing clinical management in this malignancy.

StarDist Image Segmentation Improves Circulating Tumor Cell Detection

Simple Summary

Automated enumeration of circulating tumor cells (CTC) from immunofluorescence images starts with a selection of areas containing potential CTC. The CellSearch system has a built-in selection algorithm that has been observed to fail in samples with high cell density, thereby underestimating the true CTC load. We evaluated the deep learning method StarDist for the selection of possible CTC. In whole blood sample images, StarDist recovered 99.95% of CTC detected by CellSearch and segmented 10% additional CTC. In diagnostic leukapheresis (DLA) samples, StarDist segmented 20% additional CTC and performed well, whereas CellSearch had serious failures in 9% of samples.

Abstract

After a CellSearch-processed circulating tumor cell (CTC) sample is imaged, a segmentation algorithm selects nucleic acid positive (DAPI+), cytokeratin-phycoerythrin expressing (CK-PE+) events for further review by an operator. Failures in this segmentation can result in missed CTCs. The CellSearch segmentation algorithm was not designed to handle samples with high cell density, such as diagnostic leukapheresis (DLA) samples. Here, we evaluate deep-learning-based segmentation method StarDist as an alternative to the CellSearch segmentation. CellSearch image archives from 533 whole blood samples and 601 DLA samples were segmented using CellSearch and StarDist and inspected visually. In 442 blood samples from cancer patients, StarDist segmented 99.95% of CTC segmented by CellSearch, produced good outlines for 98.3% of these CTC, and segmented 10% more CTC than CellSearch. Visual inspection of the segmentations of DLA images showed that StarDist continues to perform well when the cell density is very high, whereas CellSearch failed and generated extremely large segmentations (up to 52% of the sample surface). Moreover, in a detailed examination of seven DLA samples, StarDist segmented 20% more CTC than CellSearch. Segmentation is a critical first step for CTC enumeration in dense samples and StarDist segmentation convincingly outperformed CellSearch segmentation.

Proficiency Testing to Assess Technical Performance for CTC-Processing and Detection Methods in CANCER-ID

BACKGROUND

Multiple technologies are available for detection of circulating tumor cells (CTCs), but standards to evaluate their technical performance are still lacking. This limits the applicability of CTC analysis in clinic routine. Therefore, in the context of the CANCER-ID consortium, we established a platform to assess technical validity of CTC detection methods in a European multi-center setting using non-small cell lung cancer (NSCLC) as a model.

METHODS

We characterized multiple NSCLC cell lines to define cellular models distinct in their phenotype and molecular characteristics. Standardized tumor-cell-bearing blood samples were prepared at a central laboratory and sent to multiple European laboratories for processing according to standard operating procedures. The data were submitted via an online tool and centrally evaluated. Five CTC-enrichment technologies were tested.

RESULTS

We could identify 2 cytokeratin expressing cell lines with distinct levels of EpCAM expression: NCI-H441 (EpCAMhigh, CKpos) and NCI-H1563 (EpCAMlow, CKpos). Both spiked tumor cell lines were detected by all technologies except for the CellSearch system that failed to enrich EpCAMlow NCI-H1563 cells. Mean recovery rates ranged between 49% and 75% for NCI-H411 and 32% and 76% for NCI-H1563 and significant differences were observed between the tested methods.

CONCLUSIONS

This multi-national proficiency testing of CTC-enrichment technologies has importance in the establishment of guidelines for clinically applicable (pre)analytical workflows and the definition of minimal performance qualification requirements prior to clinical validation of technologies. It will remain in operation beyond the funding period of CANCER-ID in the context of the European Liquid Biopsy Society (ELBS).

Label-Free Enrichment and Molecular Characterization of Viable Circulating Tumor Cells from Diagnostic Leukapheresis Products

INTRODUCTION

Circulating tumor cells (CTCs) may be used to improve cancer diagnosis, prognosis, and treatment. However, because knowledge regarding CTC biology is limited and the numbers of CTCs and CTC-positive cancer patients are low, progress in this field is slow. We addressed this limitation by combining diagnostic leukapheresis (DLA) and microfluidic enrichment to obtain large numbers of viable CTCs from metastasized breast cancer patients.

METHODS

DLA was applied to 9 patients, and 7.5 mL of peripheral blood was drawn. CTCs were enriched with the Parsortix™ system. The quality of CTCs from fresh and cryopreserved DLA products was tested, and CTCs were cultured in vitro. Single uncultured and cultured CTCs were isolated by micromanipulation to determine different parameters, such as genomic aberrations and mutation profiles of selected tumor-associated genes. Expression levels of estrogen receptor and HER2/neu were monitored during in vitro culture.

RESULTS

Viable CTCs from peripheral blood and fresh or frozen DLA products could be enriched. DLA increased the likelihood of successful CTC culture. Cryopreserved DLA products could be stored with minimal CTC loss and no overt reduction in the tumor cell quality and viability during an observation period of up to 3 years. The analyzed parameters did not change during in vitro culture. DLA samples with high CTC numbers and lower ratios of apoptotic CTCs were more likely to grow in culture.

CONCLUSIONS

The increased CTC numbers from fresh or cryopreserved DLA products facilitate multiple functional and molecular analyses and, thus, could improve our knowledge of their biology.

Abstract P3-01-12: Clinical safety of diagnostic leukapheresis as a liquid biopsy to collect circulating tumor cells in primary and metastatic breast cancer patients

Introduction: The enumeration of circulating tumor cells (CTCs) has been shown to be of prognostic relevance for neoadjuvant, adjuvant and metastatic setting of breast cancer in multiple clinical trials. Moreover, the serial determination of CTCs enables therapy monitoring in the metastatic setting. One major caveat is the low number of CTCs detected by established methods which limits the possibility for further evaluation including phenotyping and genotyping. Therefore, the clinical use of CTCs as liquid biopsy for making therapy decisions is still under discussion. Diagnostic leukapheresis (DLA) has been previously established by our research group and implemented in the workflow for isolation and detection of CTCs enabling a reliable detection of CTCs at high frequency. The aim of this clinical study was to assess the safety of leukapheresis in 39 patients with primary and metastatic breast cancer.

Methods: DLA was performed at least 1d before surgery or chemotherapy. A median blood volume of 2.7 L (range, 1.0 L–5.3 L) was processed. Citrate dextrose solution A was used for anticoagulation with ratios ranging from 11:1 to 24:1. Complete blood count as well as measuring blood pressure and heart rate was performed before start of DLA and immediately after DLA. CTCs were enumerated using the CellSearch system. DLA products containing a median number of 1,8x108 MNCs were processed.

Results: 41 patients were eligible for DLA. Only in two patients DLA could not be performed due to technical problems. Thirty-nine patients underwent leukapheresis. Twenty-six patients had non metastatic breast cancer. Thirteen patients were diagnosed with metastatic breast cancer. Severe adverse events including hypotension, nauseas, tingling e.g. resulting in interruption of apheresis were not observed. The DLA did not interfere with the start of chemotherapy or surgery. Complete blood count before and after DLA showed statistic significant but clinically irrelevant decrease in numbers of leukocytes, thrombocytes, hemoglobin and the percentage of hematocrit. In 11/21 DLA samples (52%) of patients with primary breast cancer CTCs were detected. Number of CTCs ranged from 1 to 51. In 11/13 DLA samples (85%) of patients with MBC CTCs were detected. Number of CTCs ranged from 1 to 2913.

Conclusion: Establishing a routine DLA protocol we demonstrated that this procedure is clinically safe and can be implemented into the clinical workflow of breast cancer patient care.

Citation Format: Fehm T, Meier-Stiegen F, Jaeger B, Reinhardt F, Naskou J, Franken A, Neubauer H, Driemel C, Ruckhaeberle E, Niederacher D, Fischer J, Stoecklein N. Clinical safety of diagnostic leukapheresis as a liquid biopsy to collect circulating tumor cells in primary and metastatic breast cancer patients [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-01-12.

Toward a real liquid biopsy in metastatic breast and prostate cancer: Diagnostic LeukApheresis increases CTC yields in a European prospective multicenter study (CTCTrap)

Frequently, the number of circulating tumor cells (CTC) isolated in 7.5 mL of blood is too small to reliably determine tumor heterogeneity and to be representative as a “liquid biopsy”. In the EU FP7 program CTCTrap, we aimed to validate and optimize the recently introduced Diagnostic LeukApheresis (DLA) to screen liters of blood. Here we present the results obtained from 34 metastatic cancer patients subjected to DLA in the participating institutions. About 7.5 mL blood processed with CellSearch® was used as “gold standard” reference. DLAs were obtained from 22 metastatic prostate and 12 metastatic breast cancer patients at four different institutions without any noticeable side effects. DLA samples were prepared and processed with different analysis techniques. Processing DLA using CellSearch resulted in a 0–32 fold increase in CTC yield compared to processing 7.5 mL blood. Filtration of DLA through 5 μm pores microsieves was accompanied by large CTC losses. Leukocyte depletion of 18 mL followed by CellSearch yielded an increase of the number of CTC but a relative decrease in yield (37%) versus CellSearch DLA. In four out of seven patients with 0 CTC detected in 7.5 mL of blood, CTC were detected in DLA (range 1–4 CTC). The CTC obtained through DLA enables molecular characterization of the tumor. CTC enrichment technologies however still need to be improved to isolate all the CTC present in the DLA.

What's new?

Circulating tumor cells (CTC) can mirror tumor heterogeneity but a standard blood sample (7.5 mL) is too small to truly represent the tumor. To increase the yield of CTC, the authors used Diagnostic LeukApheresis in which liters of blood are screened for the presence of CTC in metastatic cancer patients. They report a significant increase in CTC yield and consequently, a better molecular characterization of the tumor, encouraging further research into the use of leukapheresis as “liquid biopsy” in cancer patients.

Isolation of circulating tumor cells from pancreatic cancer by automated filtration

It is now widely recognized that the isolation of circulating tumor cells based on cell surface markers might be hindered by variability in their protein expression. Especially in pancreatic cancer, isolation based only on EpCAM expression has produced very diverse results. Methods that are independent of surface markers and therefore independent of phenotypical changes in the circulating cells might increase CTC recovery also in pancreatic cancer. We compared an EpCAM-dependent (IsoFlux) and a size-dependent (automated Siemens Healthineers filtration device) isolation method for the enrichment of pancreatic cancer CTCs. The recovery rate of the filtration based approach is dramatically superior to the EpCAM-dependent approach especially for cells with low EpCAM-expression (filtration: 52%, EpCAM-dependent: 1%). As storage and shipment of clinical samples is important for centralized analyses, we also evaluated the use of frozen diagnostic leukapheresis (DLA) as source for isolating CTCs and subsequent genetic analysis such as KRAS mutation detection analysis. Using frozen DLA samples of pancreatic cancer patients we detected CTCs in 42% of the samples by automated filtration.

Survivin and XIAP - two potential biological targets in follicular thyroid carcinoma

Follicular thyroid carcinoma's (FTC) overall good prognosis deteriorates if the tumour fails to retain radioactive iodine. Therefore, new druggable targets are in high demand for this subset of patients. Here, we investigated the prognostic and biological role of survivin and XIAP in FTC. Survivin and XIAP expression was investigated in 44 FTC and corresponding non-neoplastic thyroid specimens using tissue microarrays. Inhibition of both inhibitor of apoptosis proteins (IAP) was induced by shRNAs or specific small molecule antagonists and functional changes were investigated in vitro and in vivo. Survivin and XIAP were solely expressed in FTC tissue. Survivin expression correlated with an advanced tumour stage and recurrent disease. In addition, survivin proved to be an independent negative prognostic marker. Survivin or XIAP knockdown caused a significant reduction in cell viability and proliferation, activated caspase3/7 and was associated with a reduced tumour growth in vivo. IAP-targeting compounds induced a decrease of cell viability, proliferation and cell cycle activity accompanied by an increase in apoptosis. Additionally, YM155 a small molecule inhibitor of survivin expression significantly inhibited tumour growth in vivo. Both IAPs demonstrate significant functional implications in the oncogenesis of FTCs and thus prove to be viable targets in patients with advanced FTC.

Abstract 1532: The isolation of CTC from diagnostic leukapheresis

Introduction

At present, the CellSearch system is the only validated method for the detection of circulating tumor cells (CTC) that has been cleared by the U.S. Food and Drug Administration. This system, designed for the enumeration of CTC in 7.5 mL of blood, detects CTC based on their expression of EpCAM and cytokeratins and negativity for CD45. However, the number of CTC that are detected in patients with metastatic carcinomas is in most cases too small to reliably determine tumor heterogeneity and to be representative as a ‘liquid biopsy’. Our aim is to identify and isolate a sufficient number of circulating tumor cells in virtually all metastatic cancer patients to enable their characterization and to represent a real-time liquid biopsy. For this purpose we used Diagnostic LeukApheresis (DLA) to increase the blood volume to be analyzed. We developed several techniques to isolate CTC from DLA to enable a multicenter comparison of CTC detection in DLA products.

Methods

DLAs were performed for ∼1 hour to obtain 40 mL of product containing ∼4 x10⁁9 mononuclear cells representing ∼1 liter of blood. Using CellSearch a maximum of 2 mL of DLA could be processed for EpCAM+ CTC (Fisher et al. doi: 10.1073/pnas.1313594110) and EpCAM- CTC (de Wit et al doi: 10.1038/srep12270). Using filtration through microsieves with 5 μm pores a maximum of 1.0 mL of DLA could be processed. To process 18 mL DLA product protocols were developed for leukocyte depletion using RosetteSep™ (StemCell Technologies, USA) and for EpCAM selection using an anti-EpCAM coated column (Leukocare AG). All enriched cell fractions were stained using CD45 PerCP, Cytokeratins PE and the nuclear dye DAPI, followed by fluorescence microscopy scanning and analysis.

Results

Leukocyte depletion using the RosetteSep™ CTC Enrichment cocktail was first optimized using small sample volumes (1 mL) spiked with cells from cancer cell lines. Depletion of leukocytes ranged from 3.1 to 3.9 logs with an average recovery of spiked cancer cells of 50-60%. Isolation of CTC expressing EpCAM was pursued using anti-EpCAM coated columns and optimized for selection and release of EpCAM expressing cells by passage of cells from cancer cell lines through the column resulting in 34-100% recovery. Both procedures were scaled up to enable processing of 18 mL of DLA. Leukocytes were depleted using RosetteSepTM by 3.1 - 3.9 logs whereas with anti-EpCAM columns only 1.7 - 1.8 logs depletion were reached. Using RosetteSepTM 21% and with the anti-EpCAM coated columns 2% of the tumor cells spiked into 18ml DLA were recovered.

Conclusion

Standard operating procedures were developed to isolate CTC in DLA's from breast, prostate cancer and lung cancer patients for evaluation and comparison in the EU sponsored consortiums CTCTrap (www.utwente.nl/tnw/ctctrap/) and CANCER-ID (www.CANCER-ID.eu). Isolation of EpCAM expressing CTC using the anti-EpCAM coated columns will need further optimization before it can proceed to multicenter comparison.

Citation Format: Kiki C. Andree, Anouk Mentink, Martin Scholz, Roland Kirchner, Rui P. Neves, Christiane Driemel, Rita Lampignano, Hans Neubauer, Dieter Niederacher, Tanja Fehm, Wolfram T. Knoefel, Johannes C. Fischer, Nikolas H. Stoecklein, Leon WMM Terstappen. The isolation of CTC from diagnostic leukapheresis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1532.

Characterization of different CTC subpopulations in non-small cell lung cancer

Circulating tumour cells (CTCs) serve as valuable biomarkers. However, EpCAM positive CTCs are less frequently detected in NSCLC patients compared to other epithelial tumours. First, EpCAM protein expression was analysed in primary and metastatic lung cancer tissue. In both groups 21% of the samples were EpCAM negative. Second, the CellSearch system identified 15% of patients (n = 48) as CTC positive whereas a multiplex RT-PCR for PIK3CA, AKT2, TWIST, and ALDH1 following EGFR, HER2 and EpCAM based enrichment detected CTCs in 29% of the patients. Interestingly, 86% of CTC positive patients were found to express ALDH1. Only 11% of the patients were CTC-positive by both techniques. CTC positivity was associated with patient disease state when assessed by the multiplex RT-PCR assay (p = 0.015). Patients harbouring tumours with an altered EGFR genotype were more frequently CTC-positive compared to patients with EGFR wildtype tumours. In subsets of patients, CTCs were found to express genes involved in resistance to therapy such as HER3 and MET. In conclusion, using multiple targets for CTC capture and identification increases the sensitivity of CTC detection in NSCLC patients, which can be explained by the presence of different CTC subtypes with distinct molecular features.

Serum microRNA profiles as prognostic or predictive markers in the multimodality treatment of patients with gastric cancer

Despite the implementation of multimodality treatment strategies, the persistently poor prognosis of gastric cancer patients is predominantly caused by the lack of predictive markers for response assessment in the neoadjuvant setting, preventing individualized therapy. Therefore, the identification of novel predictive and prognostic markers for application in the multimodality treatment of gastric cancer patients is required. The aim of the present study was to characterize the serum microRNA (miRNA/miR) profile of gastric cancer patients undergoing multimodality therapy to identify possible prognostic and predictive markers. The study consisted of 32 patients with gastric cancer who had undergone either primary surgical resection (n=14) or neoadjuvant therapy followed by surgical resection (n=18). Histopathological regression was defined as a major histopathological response when the resected specimens contained <10% vital residual tumor cells. Intratumoral miRNA was isolated from pre-operative or post-neoadjuvant blood serum samples. Initially, microarray analyses were performed in six of the patients that received neoadjuvant treatment (three responders versus three non-responders), to assess the amplification profile of dysregulated miRNAs. Based on these findings, possible predictive or prognostic markers were validated in all study patients by performing single reverse transcription-polymerase chain reaction (RT-PCR) analysis. Depending on the extent of the histopathological regression, a differential miRNA expression profile was identified in the microarray analyses. Based on the amplification profile, miR-21, miR-29a and miR-221 were selected for additional validation. However, the single RT-PCR measurements of the three selected miRNAs did not exhibit any prognostic or predictive value in the patients treated with primary resection or neoadjuvant therapy and resection. Thus, the current pilot study failed to identify a prognostic or predictive value in selected miRNAs using single RT-PCR measurements, however, the microarray results revealed a differential microRNA expression profile depending on the histopathological regression. The findings of the present study may have been affected by the small sample size.

[Comparison of different stool tests for the detection of cancer of the colon]

BACKGROUND AND OBJECTIVE

Colonoscopy is the gold standard for the diagnosis of colonic neoplasia. Because of the low compliance, the discomfort of bowel preparation and the procedure itself and the (albeit small) risk of perforation or bleeding alternative procedures such as stool tests are being focused on.

PATIENTS AND METHODS

After informed consent stool samples of 116 patients (44 male, 72 female, median age 47 years), scheduled for colonoscopy and 22 patients (17 m, 5 f, 69 y) with known colorectal cancer stool samples were collected. The samples were investigated by three methods: a biochemical (Guajak) test for fecal occult blood, an immunological test for fecal occult blood and a test determining the dimer pyruvat kinase M2 (tumor M2-PK).

RESULTS

Sensitivity for detection of colorectal cancer or polyps was 27 % and 10 % for the biochemical test, 91 % and 19 % for the immunological test and 77 % and 48 % for the M2-PK-test, respectively. Specificity was 89 %, 94 % and 72 %, respectively.

CONCLUSIONS

Both methods for detection of occult blood had a similar specificity. The sensitivity of the immunological test for the detection of colorectal cancer was significantly higher. The M2-PK-test had a markedly lower specificity in diagnosing cancer. Because of the low sensitivity for polyps the usefulness of stool tests is questionable. Reducing incidence and mortality of colorectal cancer should be achieved by colonoscopy, a recommendation that requires specific communication to the public.

The nucleus-encoded HCF107 gene of Arabidopsis provides a link between intercistronic RNA processing and the accumulation of translation-competent psbH transcripts in chloroplasts

To understand the functional significance of RNA processing for the expression of plastome-encoded photosynthesis genes, we investigated the nuclear mutation hcf107 of Arabidopsis. The mutation is represented by two alleles, both of which lead to a defective photosystem II (PSII). In vivo protein labeling, in vitro phosphorylation, and immunoblot experiments revealed that the psbB gene product (CP47) and an 8-kD phosphoprotein, the psbH gene product (PsbH), are absent in mutant plants. PsbH and PsbB are essential requirements for PSII assembly in photosynthetic eukaryotes, and their absence in hcf107 is consistent with the PSII-less mutant phenotype. RNA gel blot hybridizations showed that the hcf107 mutation specifically impairs the accumulation of some but not all oligocistronic psbH transcripts that are released from the pentacistronic psbB-psbT-psbH-petB-petD precursor RNA by intergenic endonucleolytic cleavage. In contrast, neither the levels nor the sizes of psbB-containing RNAs are affected. S1 nuclease protection analyses revealed that psbH RNAs are lacking only where psbH is the leading cistron and that they are processed at position -45 in the 5' leader segment of psbH. These data and additional experiments with the cytochrome b(6)f complex mutant hcf152, which is defective in 3' psbH processing, suggest that only those psbH-containing transcripts that are processed at their -45 5' ends can be translated. Secondary structure analysis of the 5' psbH leader predicted the formation of stable stem loops in the nonprocessed transcripts, which are unfolded by processing at the -45 site. We propose that this unfolding of the psbH leader segment as a result of RNA processing is essential for the translation of the psbH reading frame. We suggest further that HCF107 has dual functions: it is involved in intercistronic processing of the psbH 5' untranslated region or the stabilization of 5' processed psbH RNAs, and concomitantly, it is required for the synthesis of CP47.