Concise Review: Getting to the Core of Inherited Bone Marrow Failures

Uncovering the Genetic Etiology of Inherited Bone Marrow Failure Syndromes Using a Custom-Designed Next-Generation Sequencing Panel

Inherited bone marrow failure syndromes (IBMFS) are a group of heterogeneous disorders that account for ∼30% of pediatric cases of bone marrow failure and are often associated with developmental abnormalities and cancer predisposition. This article reports the laboratory validation and clinical utility of a large-scale, custom-designed next-generation sequencing panel, Children's Hospital of Philadelphia (CHOP) IBMFS panel, for the diagnosis of IBMFS in a cohort of pediatric patients. This panel demonstrated excellent analytic accuracy, with 100% sensitivity, ≥99.99% specificity, and 100% reproducibility on validation samples. In 269 patients with suspected IBMFS, this next-generation sequencing panel was used for identifying single-nucleotide variants, small insertions/deletions, and copy number variations in mosaic or nonmosaic status. Sixty-one pathogenic/likely pathogenic variants (54 single-nucleotide variants/insertions/deletions and 7 copy number variations) and 24 hypomorphic variants were identified, resulting in the molecular diagnosis of IBMFS in 21 cases (7.8%) and exclusion of IBMFS with a diagnosis of a blood disorder in 10 cases (3.7%). Secondary findings, including evidence of early hematologic malignancies and other hereditary cancer-predisposition syndromes, were observed in 9 cases (3.3%). The CHOP IBMFS panel was highly sensitive and specific, with a significant increase in the diagnostic yield of IBMFS. These findings suggest that next-generation sequencing-based panel testing should be a part of routine diagnostics in patients with suspected IBMFS.

Bone Marrow Failure Syndromes, Overlapping Diseases with a Common Cytokine Signature

Bone marrow failure (BMF) syndromes are a heterogenous group of non-malignant hematologic diseases characterized by single- or multi-lineage cytopenia(s) with either inherited or acquired pathogenesis. Aberrant T or B cells or innate immune responses are variously involved in the pathophysiology of BMF, and hematological improvement after standard immunosuppressive or anti-complement therapies is the main indirect evidence of the central role of the immune system in BMF development. As part of this immune derangement, pro-inflammatory cytokines play an important role in shaping the immune responses and in sustaining inflammation during marrow failure. In this review, we summarize current knowledge of cytokine signatures in BMF syndromes.

Our approach to bone marrow biopsies in cytopenia

Unexplained cytopenia is one of the most common indications for performing trephine bone marrow (BM) biopsy (BMB). The histopathological examination in this regard must be seen in the broader context of a multimodal approach in order to reach an as entity-specific as possible diagnosis, considering medical history, physical examination, laboratory data, peripheral blood morphology, BM aspiration smear, flow cytometry results and, if indicated, cytogenetics and molecular genetics. The particular irreplaceability of the histopathological work-up and the expectations to the BMB lie especially in the detection of fibrosing and/or focal processes (e.g. localized islets of blasts) and disorders extrinsic to the BM such as e.g. metastases, thrombotic microangiopathies, granulomatous myelitides etc. We propose a systematic combined histopathological pattern-based and blood count-based approach that can be applied in such circumstances to achieve a precise diagnosis or, at least, a clinically useful differential diagnosis, particularly taking into consideration specific morphologic pitfalls and application of ancillary techniques. Constitutional BM failure syndromes will not be profoundly addressed.

MECOM-associated syndrome: a heterogeneous inherited bone marrow failure syndrome with amegakaryocytic thrombocytopenia

Heterozygous mutations in MECOM (MDS1 and EVI1 complex locus) have been reported to be causative of a rare association of congenital amegakaryocytic thrombocytopenia and radioulnar synostosis. Here we report on 12 patients with congenital hypomegakaryocytic thrombocytopenia caused by MECOM mutations (including 10 novel mutations). The mutations affected different functional domains of the EVI1 protein. The spectrum of phenotypes was much broader than initially reported for the first 3 patients; we found familial as well as sporadic cases, and the clinical spectrum ranged from isolated radioulnar synostosis with no or mild hematological involvement to severe bone marrow failure without obvious skeletal abnormality. The clinical picture included radioulnar synostosis, bone marrow failure, clinodactyly, cardiac and renal malformations, B-cell deficiency, and presenile hearing loss. No single clinical manifestation was detected in all patients affected by MECOM mutations. Radioulnar synostosis and B-cell deficiency were observed only in patients with mutations affecting a short region in the C-terminal zinc finger domain of EVI1. We propose the term MECOM-associated syndrome for this heterogeneous hereditary disease and inclusion of MECOM sequencing in the diagnostic workup of congenital bone marrow failure.

Mutant KLF1 in Adult Anemic Nan Mice Leads to Profound Transcriptome Changes and Disordered Erythropoiesis

Anemic Nan mice carry a mutation (E339D) in the second zinc finger of erythroid transcription factor KLF1. Nan-KLF1 fails to bind a subset of normal KLF1 targets and ectopically binds a large set of genes not normally engaged by KLF1, resulting in a corrupted fetal liver transcriptome. Here, we performed RNAseq using flow cytometric-sorted spleen erythroid precursors from adult Nan and WT littermates rendered anemic by phlebotomy to identify global transcriptome changes specific to the Nan Klf1 mutation as opposed to anemia generally. Mutant Nan-KLF1 leads to extensive and progressive transcriptome corruption in adult spleen erythroid precursors such that stress erythropoiesis is severely compromised. Terminal erythroid differentiation is defective in the bone marrow as well. Principle component analysis reveals two major patterns of differential gene expression predicting that defects in basic cellular processes including translation, cell cycle, and DNA repair could contribute to disordered erythropoiesis and anemia in Nan. Significant erythroid precursor stage specific changes were identified in some of these processes in Nan. Remarkably, however, despite expression changes in large numbers of associated genes, most basic cellular processes were intact in Nan indicating that developing red cells display significant physiological resiliency and establish new homeostatic set points in vivo.

iPSC modeling of severe aplastic anemia reveals impaired differentiation and telomere shortening in blood progenitors

Aplastic Anemia (AA) is a bone marrow failure (BMF) disorder, resulting in bone marrow hypocellularity and peripheral pancytopenia. Severe aplastic anemia (SAA) is a subset of AA defined by a more severe phenotype. Although the immunological nature of SAA pathogenesis is widely accepted, there is an increasing recognition of the role of dysfunctional hematopoietic stem cells in the disease phenotype. While pediatric SAA can be attributable to genetic causes, evidence is evolving on previously unrecognized genetic etiologies in a proportion of adults with SAA. Thus, there is an urgent need to better understand the pathophysiology of SAA, which will help to inform the course of disease progression and treatment options. We have derived induced pluripotent stem cell (iPSC) from three unaffected controls and three SAA patients and have shown that this in vitro model mimics two key features of the disease: (1) the failure to maintain telomere length during the reprogramming process and hematopoietic differentiation resulting in SAA-iPSC and iPSC-derived-hematopoietic progenitors with shorter telomeres than controls; (2) the impaired ability of SAA-iPSC-derived hematopoietic progenitors to give rise to erythroid and myeloid cells. While apoptosis and DNA damage response to replicative stress is similar between the control and SAA-iPSC-derived-hematopoietic progenitors, the latter show impaired proliferation which was not restored by eltrombopag, a drug which has been shown to restore hematopoiesis in SAA patients. Together, our data highlight the utility of patient specific iPSC in providing a disease model for SAA and predicting patient responses to various treatment modalities.

Bone marrow failure syndrome caused by homozygous frameshift mutation in the ERCC6L2 gene

Inherited bone marrow failure syndromes (IBMFS) are group of disorders that lead to inadequate production of blood cells. Mutations in genes involved in telomere maintenance, DNA repair, and the cell cycle cause IBMFS. ERCC6L2 gene mutations have been associated with bone marrow failure that includes developmental delay and microcephaly. We report 2 cases of bone marrow failure with no extra-hematopoietic manifestations in patients from unrelated families with a homozygous truncating mutation in ERCC6L2. Bone marrow failure without developmental delay or microcephaly with ERCC6L2 mutation has not been previously described.