Syndromic congenital myelofibrosis associated with a loss-of-function variant in RBSN

A novel MPIG6B gene mutation in an adolescent girl with congenital thrombocytopenia and myelofibrosis

The MPIG6B gene, which encodes G6b-B, regulates platelet production, aggregation, and activation. Loss-of-function of G6b-B can cause thrombocytopenia, myelofibrosis, and anemia in both humans and mice. Several pathogenic MPIG6B mutations have been reported, such as c.324C>A (p.C108*), c.61_61+1dup (p.Ala21GlyfsX159), c.149dup (p.Ala52GlyfsX128), G6b c.469G>A (p.Gly157Arg) c.392delC (p.P134Lfs*10), and c523C>T(p.Arg175Ter). We have added to this database by reporting a new homozygous nonsense mutation (c.420T>A(p.Tyr140Ter)) of MPIG6B in a 14-year-old girl who presented with pallor, scattered cutaneous petechia of the limb, thrombocytopenia, anemia and myelofibrosis. This novel MPIG6B gene mutation encodes a shorter mutated G6b-B that does contain the transmembrane region immunoreceptor tyrosine-based inhibitory motif. The patient was effectively treated with allogeneic hematopoietic stem cell transplantation with peripheral stem cells from a matched unrelated donor. Her symptoms and the MPIG6B mutation disappeared after treatment, and she was healthy and had returned to school at the last follow-up.

RABENOSYN separation-of-function mutations uncouple endosomal recycling from lysosomal degradation, causing a distinct Mendelian Disorder

Rabenosyn (RBSN) is a conserved endosomal protein necessary for regulating internalized cargo. Here, we present clinical, genetic, cellular and biochemical evidence that two distinct RBSN missense variants are responsible for a novel Mendelian disorder consisting of progressive muscle weakness, facial dysmorphisms, ophthalmoplegia and intellectual disability. Using exome sequencing, we identified recessively-acting germline alleles p.Arg180Gly and p.Gly183Arg which are both situated in the FYVE domain of RBSN. We find that these variants abrogate binding to its cognate substrate PI3P and thus prevent its translocation to early endosomes. Although the endosomal recycling pathway was unaltered, mutant p.Gly183Arg patient fibroblasts exhibit accumulation of cargo tagged for lysosomal degradation. Our results suggest that these variants are separation-of-function alleles, which cause a delay in endosomal maturation without affecting cargo recycling. We conclude that distinct germline mutations in RBSN cause non-overlapping phenotypes with specific and discrete endolysosomal cellular defects.

Mammalian VPS45 orchestrates trafficking through the endosomal system

Vacuolar protein sorting 45 homolog (VPS45), a member of the Sec1/Munc18 (SM) family, has been implicated in the regulation of endosomal trafficking. VPS45 deficiency in human patients results in congenital neutropenia, bone marrow fibrosis, and extramedullary renal hematopoiesis. Detailed mechanisms of the VPS45 function are unknown. Here, we show an essential role of mammalian VPS45 in maintaining the intracellular organization of endolysosomal vesicles and promoting recycling of cell-surface receptors. Loss of VPS45 causes defective Rab5-to-Rab7 conversion resulting in trapping of cargos in early endosomes and impaired delivery to lysosomes. In this context, we demonstrate aberrant trafficking of the granulocyte colony-stimulating factor receptor in the absence of VPS45. Furthermore, we find that lack of VPS45 in mice is not compatible with embryonic development. Thus, we identify mammalian VPS45 as a critical regulator of trafficking through the endosomal system and early embryogenesis of mice.

Current and future therapies for myelofibrosis

Myelofibrosis is classified as a 'Philadelphia-chromosome negative' clonal myeloproliferative disorder. The heterogeneity of this condition and patient population and array of often challenging clinical manifestations can frequently make therapeutic decisions challenging. Despite many advances in therapy with targeted and combination approaches, following an enhanced understanding of underlying disease pathogenesis, cure only remains achievable with allogeneic stem cell transplant. This option is often limited to a small group of younger transplant-eligible patients with more advanced disease who have both a suitable donor and no or few co-morbidities. In this article, we will discuss up-to-date disease prognostication, common clinical challenges associated with myelofibrosis and both standard and novel therapeutic approaches. Increasingly complex prognostic modelling utilises patient-specific, haematological and genomic parameters to improve the accuracy of risk assessment and predict disease progression. We will also focus on difficult clinical scenarios such as disease-associated anaemia, thrombocytopenia and extremes of age. Future and evolving therapies within this field are highly anticipated and novel JAK inhibitor and non-JAK inhibitor-based therapy will also be discussed, including the new challenge of how to switch from one JAK inhibitor therapy to another.