Given the vast phenotypic and genetic heterogeneity of acute and chronic myeloid malignancies, hematologists have eagerly awaited the introduction of next-generation sequencing (NGS) into the routine diagnostic armamentarium to enable a more differentiated disease classification, risk stratification, and improved therapeutic decisions. At present, an increasing number of hematologic laboratories are in the process of integrating NGS procedures into the diagnostic algorithms of patients with acute myeloid leukemia (AML), myelodysplastic syndromes (MDS), and myeloproliferative neoplasms (MPNs). Inevitably accompanying such developments, physicians and molecular biologists are facing unexpected challenges regarding the interpretation and implementation of molecular genetic results derived from NGS in myeloid malignancies. This article summarizes typical challenges that may arise in the context of NGS-based analyses at diagnosis and during follow-up of myeloid malignancies.Table 1Challenges accompanying the introduction of massive parallel sequencing in clinical routine diagnostics in hemato-oncologyChallengeBackgroundCurrent and future approachDiscrimination of leukemia-related mutations from polymorphisms or passenger mutationsDriver mutations expected to occur at higher allele frequency in patient samples than passenger mutations; driver mutations more likely to have an impact on protein function than polymorphisms or passenger mutationsOptimization of cancer-specific databases including reporting of rare physiological gene variantsImplementation of novel bioinformatic algorithms based on prediction of functional impactQuantitative and dynamic VAF monitoring (separately and together with other mutations) at follow-upDiscrimination of somatic leukemia-related mutations from CHIPCHIP is presented in ~10% of individuals aged 70 to 80 and in up to 20% in the age group > 80 yearsQuantitative and dynamic VAF monitoring (separately and together with other mutations) at follow-upClarifying the significance of CHIP in the context of myeloid malignanciesDiscrimination of leukemia-related somatic mutations from pathogenic germline alterationsChallenge to differentiate acquired somatic mutations from germline pathogenic variants at diagnosisMutation detection in germline control samples (e.g., skin fibroblasts, saliva) in mutations such as in RUNX1, CEBPAThorough medical family history followed by molecular genetic tests in relatives if necessaryHigh and stable VAF (e.g., 40–50%) at follow-up despite clinical response to treatment may be indicative for germline alterationDiscrimination of true genetic alterations from PCR, sequencing and post-sequencing artifactsMany artefacts are known to arise during NGS library preparation, sequencing and data analysisError correction using molecular identifiers that individually label original input DNA moleculesRefinement of error-correction computational methods in post-sequencing NGS data analysisConfirmation using Sanger sequencingLimited sensitivity of NGS for minimal residual diseas...

Nonsense-mediated mRNA decay (NMD), which is best known for degrading mRNAs with premature termination codons (PTCs), is thought to be triggered by aberrant translation termination at stop codons located in an environment of the mRNP that is devoid of signals necessary for proper termination. In mammals, the cytoplasmic poly(A)-binding protein 1 (PABPC1) has been reported to promote correct termination and therewith antagonize NMD by interacting with the eukaryotic release factors 1 (eRF1) and 3 (eRF3). Using tethering assays in which proteins of interest are recruited as MS2 fusions to a NMD reporter transcript, we show that the three N-terminal RNA recognition motifs (RRMs) of PABPC1 are sufficient to antagonize NMD, while the eRF3-interacting C-terminal domain is dispensable. The RRM1-3 portion of PABPC1 interacts with eukaryotic initiation factor 4G (eIF4G) and tethering of eIF4G to the NMD reporter also suppresses NMD. We identified the interactions of the eIF4G N-terminus with PABPC1 and the eIF4G core domain with eIF3 as two genetically separable features that independently enable tethered eIF4G to inhibit NMD. Collectively, our results reveal a function of PABPC1, eIF4G and eIF3 in translation termination and NMD suppression, and they provide additional evidence for a tight coupling between translation termination and initiation.

The nonsense-mediated mRNA decay (NMD) pathway acts to selectively identify and degrade mRNAs that contain a premature translation termination codon (PTC), and hence reduce the accumulation of potentially toxic truncated proteins. NMD is one of the best studied mRNA quality-control mechanisms in eukaryotes, and it has become clear during recent years that many physiological mRNAs are also NMD substrates, signifying a role for NMD beyond mRNA quality control as a translation-dependent posttranscriptional regulator of gene expression. Despite a great deal of scientific research for over twenty years, the process of NMD is far from being fully understood with regard to its physiological relevance to the cell, the molecular mechanisms that underpin this pathway, all of the factors that are involved, and the exact cellular locations of NMD. This unit details some of the fundamental RNA based approaches taken to examine aspects of NMD, such as creating PTC+ reporter genes, knocking down key NMD factors via RNAi, elucidating the important functions of NMD factors by complementation assays or Tethered Function Assays, and measuring RNA levels by reverse-transcription quantitative PCR.

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