Next-generation nuclear morphology to grade solid tumours
Authors: Glen Kristiansen
Journal: The Lancet Oncology (2018)

A central task for surgical pathologists diagnosing tumours is to reliably assess tumour aggressiveness from morphologic features. This tumour grade guides clinical therapy planning, and specific grading systems exist for all major tumour types. Typical components of grading systems are tissue architecture, degree of resemblance to the respective benign tissue, mitotic activity, necrosis, and aberrances in nuclear morphology (ie, nuclear pleomorphism, variations in size, nucleolar prominence, and chromatin distribution). Flemming1Flemming W Beiträge zur Kenntnis der Zelle und ihrer Lebenserscheinungen.Archiv Mikrosk Anat. 1879; 18: 302-436Crossref Scopus (50) Google Scholar coined the term chromatin in 1879 when describing nuclear structures that could be stained with basic dyes in the interphase, whereas Waldeyer2Waldeyer W Über Karyokinese und ihre Beziehungen zu den Befruchtungsvorgängen.Archiv Mikrosk Anat. 1888; 32: 1-122Crossref Scopus (75) Google Scholar introduced the term chromosome in 1888 when describing mitotic chromatin. Soon after, chromosomal alterations in tumours were described by Bovari,3Bovari T Zur Frage der Entstehung maligner Tumoren. G Fischer, Jena, Germany1914Google Scholar who also hypothesised a causal role of unequal chromatin distribution in carcinogenesis. It is probably the simplicity of this theory that explains its popularity among medical laymen and patients to this day. The dogma of all morphologists, that function follows form, also applies to chromatin structure, which directly affects genomic activity by activating or silencing genes and gene families. In our contemporary concept, chromatin contains the whole eukaryotic genome, rather densely packaged around histones (H2A, H2B, H3, and H4). Post-translational modifications of histones (eg, acetylation, methylation) also determine chromatin compaction, structure, and dynamics, and it is not surprising that mutations in the genes responsible for these regulatory modifications are often seen in cancer.4Morgan MA Shilatifard A Chromatin signatures of cancer.Genes & Dev. 2015; 29: 238-249Crossref PubMed Scopus (144) Google Scholar Advances in sequencing technologies have massively broadened our knowledge of driver oncogenes and their role during tumour progression and have helped promote the development of therapeutic drugs, but a holistic view that integrates gene expression and higher chromatin regulation is yet to be established. Although most pathologists know that chromatin irregularities, especially coarse and clumped chromatin as seen under the light microscope, are typical of higher graded tumours, the diagnostic value of subnuclear morphology has not been unearthed so far. This is probably because the subjective and semiquantitative perception of chromatin coarseness necessitates digital image analysis to enable standardised measurements. DNA cytometry, introduced in 1966, is a simple approach to compare overall DNA content in tumour cells and normal cells.5Sandritter W Carl M Ritter W Cytophotometric measurements of the DNA content of human malignant tumors by means of the Feulgen reaction.Acta cytologica. 1966; 10: 26-30PubMed Google Scholar This technique gained popularity because aberrant DNA content (aneuploidy) was more common in more aggressive tumours and neoplastic lesions than in benign, reactive lesions. Today, most clinicians and pathologists do not use data on DNA ploidy in patient care, even though a re-evaluation of this technique might be warranted.6Danielsen HE Pradhan M Novelli M Revisiting tumour aneuploidy—the place of ploidy assessment in the molecular era.Nat Rev Clin Oncol. 2016; 13: 291-304Crossref PubMed Scopus (94) Google Scholar To classify chromatin patterns with modern image analysis technology is an obvious next step in developing more standardised diagnostic algorithms that could become part of tumour grading in the future. In The Lancet Oncology, Kleppe and colleagues7Kleppe A Albregtsen F Vlatkovic L et al.Chromatin organisation and cancer prognosis: a pan-cancer study.Lancet Oncol. 2018; (published online Feb 2.)http://dx.doi.org/10.1016/S1470-2045(17)30899-9Summary Full Text Full Text PDF PubMed Scopus (49) Google Scholar describe Nucleotyping, an automated method to determine chromatin heterogeneity that reveals relevant prognostic information in various solid tumour types, including colorectal, endometrial, and ovarian carcinoma. Although the preparatory steps are similar to conventional DNA cytometry (eg, dissection of cells, feulgen stain, multiple images, etc), this procedure does not yield overall staining intensity; instead, it gives a measure of chromatin organisation computed by the entropy of pixel grey levels. The resultant so-called chromatin value provides a continuous measure of chromatin disorder in a given tumour that is then dichotomised into homogenous or heterogeneous chromatin groups. For all analysed tumour entities, survival times were consistently shorter in patients with chromatin heterogeneous tumours than in patients with chromatin homogeneous tumours, and the prognostic value of chromatin heterogeneity was independent of other parameters in a Cox analysis. This is an important finding, especially because the technique is applicable to formalin-fixed, paraffin-embedded tissue and could easily be added to any routine workflow in pathology laboratories to complement conventional tumour grading. However, some questions remain. Is Nucleotyping easily transferable to other academic institutes? Is this biomarker robust enough to perform comparably in a decentralised setting? How relevant are the trained personnel who identified the nuclei of interest in terms of interobserver variability of the technique? Pending clarification of these questions, this excellent study serves as an example of next-generation morphology that might enrich the pathologist's armamentarium in the digital future, which is only just beginning. I declare no competing interests. Chromatin organisation and cancer prognosis: a pan-cancer studyThe consistent prognostic prediction of Nucleotyping in different biological and technical circumstances suggests that the marker of chromatin heterogeneity can be reliably assessed in routine clinical practice and could be used to objectively assist decision making in a range of clinical settings. An immediate application would be to identify high-risk patients with stage II colorectal cancer who might have greater absolute benefit from adjuvant chemotherapy. Clinical trials are warranted to evaluate the survival benefit and cost-effectiveness of using Nucleotyping to guide treatment decisions in multiple clinical settings. Full-Text PDF Open Access