Molecular diagnostics, i.e. the detection and analysis of disease-related changes of DNA or RNA, is becoming ever more important for the diagnosis of bone marrow (BM) neoplasms. In modern BM haematopathology, molecular diagnostics should always be part of an integrated diagnostic approach including clinical information, morphology and immunophenotyping. It is the responsibility of the haematopathologist to interpret the information gathered and to produce a final diagnosis. For this purpose, the practising haematopathologist must be familiar with the various molecular techniques needed and possess an in-depth knowledge of their applications for the diagnosis of BM neoplasms. The first part of this chapter focuses on the most important molecular techniques currently used in everyday diagnostics in the modern haematopathological laboratory. The second part highlights the major molecular and genetic aberrations of diagnostic value across the different haematological disease entities. Ideally the haematopathological laboratory should either be able to perform the relevant tests or be in close cooperation with a laboratory performing them for optimal diagnostics. Such cooperations also include multidisciplinary conferences, where clinicians and haematopathologists meet to discuss the diagnoses of afflicted patients.

Aplasia is a pathologic term that is broadly defined as the absence or near-absence of one or more haematopoietic lineages in the bone marrow (BM). Clinically, BM aplasia affecting more than one lineage is referred to as aplastic anaemia (AA), despite the fact that this group of disorders often results in pancytopaenia rather than anaemia alone. Cytopaenias can be seen in a number of different conditions, and new-onset pancytopaenia in children and adults requires an extensive work-up, including a BM core biopsy (BMB) for confirmation of haematopoietic aplasia/hypoplasia and exclusion of an infiltrative marrow process or fibrosis. Bone marrow aplasia develops as a result of injury to multipotent haematopoietic stem cells, which can occur in the context of constitutional (primary aplasia) or acquired (secondary aplasia) disorders (Table 4.1). This chapter will discuss the diagnostic criteria and pathophysiology of specific disorders presenting with aplasia and demonstrate an algorithmic approach to the diagnostic evaluation of patients presenting with this common and non-specific finding (Table 4.2).

The bone marrow (BM) extracellular matrix (ECM) is often inconspicuous, and hence neglected in a normal BM trephine biopsy (BMB), but it has essential functions in the homeostasis of bone and the maturation of haematopoiesis (see Chapter 2). The ECM surrounds the bony trabeculae, which are covered by osteoblasts and osteoclasts, the latter an essential component of the stem cell niche, and embeds the maturing haematopoietic cells. It is rich in hyaluronic acid, type III collagen (reticulin), fibronectin and laminin.

Myeloid and lymphoid neoplasms with eosinophilia (MLNE) and rearrangements of PDGFRA, PDGFRB and FGFR1 were recognized as a standalone category in the 2008 WHO classification. PCM1-JAK2 was added to this family as a new provisional entity in the 2016 WHO classification [1, 2]. The features shared by neoplasms in this category include a common presentation with eosinophilia or hypereosinophilia in peripheral blood and an increased number of eosinophilic forms in bone marrow (BM). Some cases present as acute leukaemia. Some cases may lack hypereosinophilia. The underlying mechanism is the overexpression of an aberrant tyrosine kinase as a result of a fusion gene, or rarely of a mutation, and a diagnosis and classification requires the demonstration of the specific gene fusions. The cell of origin is a mutated pluripotent stem cell that has the potential to involve myeloid, lymphoid or both lineages, concomitantly or sequentially, leading to clinically complex and heterogeneous manifestations. A common scenario is the presentation as a chronic myeloproliferative neoplasm (MPN), usually with eosinophilia followed within a variable time period and depending on the gene fusion involved, by a progression to acute myeloid leukaemia (AML) or mixed phenotype acute leukaemia (usually in the BM), and B- or T-lymphoblastic leukaemia/lymphoma (B-/T-ALL) in BM or in an extramedullary site. Thus it is critical to recognize the clinicopathologic features of these neoplasms, identify the molecular genetic lesions and classify them accordingly. An accurate diagnosis and classification have important therapeutic and prognostic implications.

Plasma cell neoplasms are derived from mature, IG heavy chain class-switched terminally differentiated B-cells, which usually secrete a monoclonal immunoglobulin or M-protein and consist of a homogeneous population of neoplastic plasma cells [1]. Plasma cell myeloma (PCM) is a common malignancy, manifesting itself by bone marrow (BM) infiltration and bone destruction, and therefore representing an entity frequently encountered in BM biopsies(BMB). Its precursor lesion, non-IgM monoclonal gammopathy of unknown significance (MGUS) is a frequent finding in elderly individuals and shows a low, but definite risk for progression to PCM. In this chapter, we will discuss the diagnosis and differential diagnosis of PCM, MGUS and clonal plasma cell disorders with associated paraneoplastic syndromes, including POEMS syndrome and TEMPI syndrome. Amyloidosis is discussed in Chapter 3, and other mature B-cell neoplasms with a clonal plasma cell component in Chapter 15.