New MECOM variant in a child with severe neonatal cytopenias spontaneously resolving

A novel missense mutation in the MECOM gene in a Chinese boy with radioulnar synostosis with amegakaryocytic thrombocytopenia

Radioulnar synostosis with amegakaryocytic thrombocytopenia (RUSAT) type 2, caused by MDS1 and EVI1 complex locus (MECOM) gene mutations, is a rare inherited bone marrow failure syndrome (IBMFS) with skeletal anomalies, characterized by varying presentation of congenital thrombocytopenia (progressing to pancytopenia), bilateral proximal radioulnar synostosis, and other skeletal abnormalities. Due to limited knowledge and heterogenous manifestations, clinical diagnosis of the disease is challenging. Here we reported a novel MECOM mutation in a Chinese boy with typical clinical features for RUSAT-2. Trio-based whole exome sequencing of buccal swab revealed a novel heterozygous missense mutation in exon 11 of the MECOM gene (chr3:168818673; NM_001105078.3:c.2285G > A). The results strongly suggest that the variant was a germline mutation and disease-causing mutation. The patient received matched unrelated donor hematopoetic stem cell transplantation (HSCT). This finding was not only expanded the pathogenic mutation spectrum of MECOM gene, but also provided key information for clinical diagnosis and treatment of RUSAT-2.

A novel mutation in MECOM affects MPL regulation in vitro and results in thrombocytopenia and bone marrow failure

MECOM-associated syndrome (MECOM-AS) is a rare disease characterized by amegakaryocytic thrombocytopenia, progressive bone marrow failure, pancytopenia and radioulnar synostosis with high penetrance. The clinical phenotype may also include finger malformations, cardiac and renal alterations, hearing loss, B-cell deficiency and predisposition to infections. The syndrome, usually diagnosed in the neonatal period because of severe thrombocytopenia, is caused by mutations in the MECOM gene, encoding for the transcription factor EVI1. The mechanism linking the alteration of EVI1 function and thrombocytopenia is poorly understood. In a paediatric patient affected by severe thrombocytopenia, we identified a novel variant of the MECOM gene (p.P634L), whose effect was tested on pAP-1 enhancer element and promoters of targeted genes showing that the mutation impairs the repressive activity of the transcription factor. Moreover, we demonstrated that EVI1 controls the transcriptional regulation of MPL, a gene whose mutations are responsible for congenital amegakaryocytic thrombocytopenia (CAMT), potentially explaining the partial overlap between MECOM-AS and CAMT.

Mecom mutation related to radioulnar synostosis with amegakaryocytic thrombocytopenia reduces HSPCs in mice

Radioulnar synostosis with amegakaryocytic thrombocytopenia (RUSAT) is an inherited bone marrow failure syndrome characterized by the congenital fusion of the forearm bones. RUSAT is largely caused by missense mutations that are clustered in a specific region of the MDS1 and EVI1 complex locus (MECOM). EVI1, a transcript variant encoded by MECOM, is a zinc finger transcription factor involved in hematopoietic stem cell maintenance that induce leukemic transformation when overexpressed. Mice with exonic deletions in Mecom show reduced hematopoietic stem and progenitor cells (HSPCs). However, the pathogenic roles of RUSAT-associated MECOM mutations in vivo have not yet been elucidated. To investigate the impact of the RUSAT-associated MECOM mutation on the phenotype, we generated knockin mice harboring a point mutation (translated into EVI1 p.H752R and MDS1-EVI1 p.H942R), which corresponds to an EVI1 p.H751R and MDS1-EVI1 p.H939R mutation identified in a patient with RUSAT. Homozygous mutant mice died at embryonic day 10.5 to 11.5. Heterozygous mutant mice (Evi1KI/+ mice) grew normally without radioulnar synostosis. Male Evi1KI/+ mice, aged between 5 and 15 weeks, exhibited lower body weight, and those aged ≥16 weeks showed low platelet counts. Flow cytometric analysis of bone marrow cells revealed a decrease in HSPCs in Evi1KI/+ mice between 8 and 12 weeks. Moreover, Evi1KI/+ mice showed delayed leukocyte and platelet recovery after 5-fluorouracil-induced myelosuppression. These findings suggest that Evi1KI/+ mice recapitulate the bone marrow dysfunction in RUSAT, similar to that caused by loss-of-function Mecom alleles.

MECOM Deficiency: from Bone Marrow Failure to Impaired B-Cell Development

MECOM deficiency is a recently identified inborn error of immunity and inherited bone marrow failure syndrome caused by haploinsufficiency of the hematopoietic transcription factor MECOM. It is unique among inherited bone marrow failure syndromes, many of which present during later childhood or adolescence, because of the early age of onset and severity of the pancytopenia, emphasizing the importance and gene dose dependency of MECOM during hematopoiesis. B-cell lymphopenia and hypogammaglobulinemia have been described in a subset of patients with MECOM deficiency. While the mechanisms underlying the B-cell deficiency are currently unknown, recent work has provided mechanistic insights into the function of MECOM in hematopoietic stem cell (HSC) maintenance. MECOM binds to regulatory enhancers that control the expression of a network of genes essential for HSC maintenance and self-renewal. Heterozygous mutations, as seen in MECOM-deficient bone marrow failure, lead to dysregulated MECOM network expression. Extra-hematopoietic manifestations of MECOM deficiency, including renal and cardiac anomalies, radioulnar synostosis, clinodactyly, and hearing loss, have been reported. Individuals with specific genotypes have some of the systemic manifestations with isolated mild thrombocytopenia or without hematologic abnormalities, highlighting the tissue specificity of mutations in some MECOM domains. Those infants with MECOM-associated bone marrow failure require HSC transplantation for survival. Here, we review the expanding cohort of patient phenotypes and accompanying genotypes resulting in MECOM deficiency, and the proposed mechanisms underlying MECOM regulation of human HSC maintenance and B-cell development.

Emerging bone marrow failure syndromes- new pieces to an unsolved puzzle

Inherited bone marrow failure (BMF) syndromes are genetically diverse - more than 100 genes have been associated with those syndromes and the list is rapidly expanding. Risk assessment and genetic counseling of patients with recently discovered BMF syndromes is inherently difficult as disease mechanisms, penetrance, genotype-phenotype associations, phenotypic heterogeneity, risk of hematologic malignancies and clonal markers of disease progression are unknown or unclear. This review aims to shed light on recently described BMF syndromes with sparse concise data and with an emphasis on those associated with germline variants in ADH5/ALDH2, DNAJC21, ERCC6L2 and MECOM. This will provide important data that may help to individualize and improve care for these patients.

Phenotypic heterogeneity in individuals with MECOM variants in 2 families

MECOM encodes the transcriptional regulators, EVI1 and MDS1-EVI1, from two distinct transcription start sites. EVI1 plays important roles in hematopoiesis and stem cell self-renewal. Recently, our group and others revealed that individuals with MECOM variants present diverse hematological and skeletal defects, including radioulnar synostosis (RUS). In the present study, we analyzed two families suspected with MECOM-associated syndrome. In family 1, a MECOM splicing variant (c.2285+1G>A) was identified in an individual with bone marrow failure (TRS4) without RUS and her mother, who had mild leukocytopenia, thrombocytopenia, and bilateral RUS. A copy neutral loss of heterozygosity decreasing the variant allele frequency was observed in the bone marrow of TRS4 and the peripheral blood leukocytes of her mother. However, TRS4 remained transfusion-dependent. In family 2, a MECOM variant (c.2208-4A>G), which was predicted to cause a cryptic acceptor site that results in a 3-base insertion (an insertion of Ser) in the mRNA, was identified in the proband, with bone marrow failure; this variant was also observed in her brother and father, both of whom have skeletal malformations, but no cytopenia. RT-PCR using leukocytes revealed a transcript with a 3-bp insertion in the proband, her brother, and the father, suggesting that the transcript variant with a 3-bp insertion is independent of blood phenotype. Collectively, these results suggest the presence of intrafamilial clinical heterogeneity in both families with MECOM splicing variants. Somatic genetic event may complicate the understanding of clinical variability among family members.