Submission Details

LUMPING AND SPLITTING CONSIDERATIONS: Thrombocythemia 2; OMIM: 601977; MONDO: 0011173 Thrombocytopenia, congenital amegakaryocytic; OMIM: 604498; MONDO: 0800452

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 Congenital Amegakaryocytic Thrombocytopenia (CAMT) in 1999 (Ihara et al., PMID 10077649) in a Japanese 10 year-old girl with thrombocytopenia and absence of megakaryocytes in the bone marrow. Two heterozygous variants were detected, p.Q186X and c.1499delT, both mutations were predicted to result in a prematurely terminated c-Mpl protein.

Over 30 unique variants have been reported in humans including missense, frameshift, splice-site and nonsense variants.

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

Summary of Case Level Data: 12 POINTS Variants in this gene have been reported in at least 8 CAMT probands in 5 publications (PMIDs 10971406, 11071383, 17666371, 10077649, 11133753). More evidence is available in the literature, but the maximum score for genetic evidence and/or experimental evidence (12 pts.) has been reached. The mechanism for disease is homozygous loss of function.

Summary of Experimental Data: 6 POINTS This gene-disease association is supported by evidences of expression: MPL was found expressed in megakaryocytes and platelets, the cells involved in CAMT. It was not detectable on granulocytic cells, blast cells, and lymphocytes (PMID 7529061). Evidence of protein interaction: 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).

Evidence of functional alteration in patient cells is also present: iPSCs were generate from skin fibroblasts of a patient with CAMT generated few MKs or platelets, even in the presence of TPO, these results were further confirmed by the failure of TPO stimulation to induce phosphorylation of mediators downstream of MPL (pAKT, pERK1/2, pSTAT3, and pSTAT5) (PMID: 23908116).

A mouse model knockout for MPL demonstrated that the expression of MPL is crucial for megakaryocyte and platelet formation, and recapitulated the thrombocytopenia and megakaryocytopenia phenotypes of patients with CAMT (PMID: 8630375). Moreover, a thrombocytopenic zebrafish model of human CAMT was described to have thrombocytopenia and significantly increased bleeding after injury of the tail (PMID:27811851).

Finally, experimental evidence of rescue in patient cells is available: gene editing was able to restore normal function in CD34+ cells from a patient with the W272R variant. Indeed, edited CD34+ displayed similar amounts of surface Mpl proteins as control CD34+ and were able to generate a significantly higher number of megakaryocytic colonies in the presence of Tpo than unedited cells from the patient (PMID: 29296828).

In summary, MPL is definitively associated with CAMT. 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).