Submission Details

LUMPING AND SPLITTING CONSIDERATIONS:

Thrombocythemia 2; OMIM: 601977; MONDO: 0011173 Thrombocytopenia, congenital amegakaryocytic; OMIM: 604498; MONDO: 0011469

Per criteria outlined by the ClinGen Lumping and Splitting Working Group, we found differences in molecular mechanisms (loss of function vs gain of function), inheritance pattern (autosomal dominant vs autosomal recessive) and phenotypic variability (thrombocythemia vs thrombocytopenia) between these two phenotypes thus the disorders have been splitted. Somatic variants in MPL are also associated with Myelofibrosis with myeloid metaplasia (OMIM 254450) that we did not curate because somatic.

The MPL gene encodes the receptor for thrombopoietin, a hematopoietic growth factor that regulates the development of megakaryocytes and platelets as well as the self-renewal of haematopoietic stem cells.

MPL was first reported in relation to Thrombocythemia 2 in 2004 (Ding et al., PMID 14764528) in a Japanese family with thrombocythemia inherited in an autosomal-dominant manner. A unique point mutation, p.Ser505Asn, was identified in the transmembrane domain of the MPL gene in all of the 8 members with thrombocythemia.

4 unique variants, all missense, have been reported in humans. The most common variant is p.Ser505Asn, that causes Thrombocythemia 2 with a mechanism of heterozygous gain of function, all the families with this variant show an autosomal dominant inheritance for the disorder. However, for the other three variants, p.Lys39Asn (that is present with high frequency in African Americans), p.Arg102Pro and p.Pro106Leu the inheritance is autosomal dominant with incomplete penetrance because thrombocythemia is present in all the homozygous subjects while heterozygous subjects may have normal or increased platelet count (however less than homozygous).

Evidence supporting this gene-disease relationship includes genetic evidences (case-level data) and experimental evidences (functional evidence of biochemical functions, functional alteration in non-patient cells, non-human model organisms that replicate the disease).

Summary of Case Level Data: 7.2 POINTS Variants in this gene have been reported in at least 11 probands in 8 publications (PMIDs: 28979237, 14764528, 19036112, 28034873, 19608689, 15269348, 25538044, 19713221).

Summary of Experimental Data: 5.5 POINTS This gene-disease association is supported by in vitro functional assays and mouse models.

MPL was found expressed in megakaryocytes and platelets, the cells involved in Thrombocythemia 2. It was not detectable on granulocytic cells, blast cells, and lymphocytes (PMID 7529061). Chen et al. showed that MPL physically interacts with TPO upon immunoprecipitation for TPO and western blotting with an anti- MPL antibody. Moreover, they show that within the Mpl extracellular domain amino acids 102–251 are strongly involved in ligand binding and that mutations in residues Asp235 and Leu239 had the largest effect on decreasing binding efficacy (PMID: 20529857).

Alexander et al. showed in non-patient cells (Ba/F3) transfected with mutant MPL that variants in MPL might contribute to constitutive receptor activation and consequent proliferation of cells expressing this receptor, such as megakaryocytes, inducing thrombocytosis (PMID: 8521814). In Ding et al showed that in Ba/F3 cells the Asn505 mutation induces both autonomous dimerization of Mpl and signal activation (p-MEK-1/2 and p-STAT5) in the absence of TPO (PMID 19483125). Also UT-7 (non-patient) cells expressing the gain-of-function Mpl K39N showed a significant increase in proliferation compared with WT - expressing cells (PMID:29296828).

A mouse model knockout for MPL demonstrated that the expression of MPL is crucial for megakaryocyte and platelet formation, even if it does not directly demonstrate that variants in MPL cause thrombocytosis (PMID 8630375). Favale et al. described a retroviral murine model in which deficient C57/Bl6 Mpl-/- lin- cells were retrovirally transduced with either the human MPLWT or MPL P106L receptor and then injected into lethally irradiated Mpl-/- mice. 4 weeks after transduction MPL P106L mice exhibited thrombocytosis, whereas MPL WT transduced mice did not. TPO levels were high in MPL P106L mice (PMID: 28034873). This mouse model recapitulates Thrombocythemia 2 phenotype.

In summary, MPL is definitively associated with Thrombocythemia 2. This has been repeatedly demonstrated in both the research and clinical diagnostic settings, and has been upheld over time.

This classification was approved by the ClinGen Hemostasis Thrombosis Working Group on 7/22/20 (SOP Version 7).