Page 36 - 2020_08-Haematologica-web

A.T. Nurden and P. Nurden
adaptor protein ADAP (SLAP-130) for the Fyn tyrosine kinase.88 The bleeding syndrome in these families was mild. As discussed by the authors, studies using mouse Adap-/- models showed that the small platelets had a short survival (also a feature of WAS) and defective aIIbb3 acti- vation, findings confirmed in the patients. Subsequent whole sternum three-dimensional confocal microscopy and intravital two-photon microscopy detected reduced megakaryocyte maturation in Adap-/- mice with signs of ectopic release of platelet-like particles within the bone marrow.89 Biallelic mutations in PTPRJ, encoding CD148, a master regulator of Src family kinases and the most abun- dant membrane-bound tyrosine phosphatase in platelets and megakaryocytes, were next shown to result in non- syndromic microthrombocytopenia in two siblings with mild bleeding.90 Loss of CD148 was accompanied by platelet aggregation defects, particularly in relation to GPVI signaling. The thrombocytopenia was again linked to ectopic platelet release within the bone marrow, a charac- teristic of WAS.
Src, a universal tyrosine kinase
A gain-of-function, heterozygous E727K mutation was identified in SRC by WES in nine members of a large fam- ily; it was linked to thrombocytopenia, a low content of a- granules and reduced platelet function.91 The thrombocy- topenia was syndromic with myelofibrosis, the disease being accompanied by bone defects including mild facial dysmorphia. The marrow showed trilineage dysplasia with immature megakaryocytes poorly able to form pro- platelets. Platelets were variable in size and morphological- ly abnormal; they responded poorly to collagen with reduced GPVI signaling. The gain-of-function mutation was suggested to lift autoinhibition with spontaneous Src activation and upgraded protein tyrosine phosphorylation. The abnormalities were reproduced in a zebrafish model. The mutation may have a feedback on Mpl function.
Mutations affecting ion gradients
Stormorken syndrome was described many years ago as a “multifaceted bleeding syndrome” in which an unusual form of macrothrombocytopenia was associated with reduced platelet survival and upgraded prothrombin con- sumption.92 It was linked to a spontaneous expression of phosphatidylserine on platelets. Platelet function and thrombus formation under flow were affected. Much later it resurfaced as a syndromic macrothrombocytopenia with a range of clinical features including mild anemia, asplenia, myopathy with tubular aggregates, miosis, immune dys- function, ichthyosis and dyslexia.93,94 Bleeding is life-long but is mostly mild. Heterozygous gain-of-function AD variants in STIM1 identified by WES are the cause of the disease. STIM1 is a Ca2+-sensor within the endoplasmic reticulum; on Ca2+-depletion conformational changes induce its coiled-coil 1 domain to elongate and interact with ORAI1, a surface membrane channel that mediates store-operated calcium entry into cells. ORAI1 can be affected in rare cases with miosis and non-progressive myopathy but without significant platelet abnormalities. The mutations make STIM1 (and secondarily ORAI1) con- stitutively active, favoring Ca2+ entry into platelets and pre- sumably megakaryocytes. The York platelet syndrome, with large and morphologically abnormal platelets with giant opaque organelles also had gain-of-function muta- tions in STIM1 thereby expanding the spectrum of
Stormorken syndrome.94 WES and comparison with mouse models led us to identify variants in the TRPM7 gene in two French index cases with macrothrombocytopenia and with a possible link to atrial fibrillation in one family.95 TRPM7 is both a kinase and an ion channel primarily linked to Mg2+ homeostasis; its absence in mice or disease- causing variants in man were both accompanied by cytoskeletal changes in megakaryocytes.
Other miscellaneous defects
Patients in three families with an often strong bleeding history, moderate thrombocytopenia and enlarged platelets were linked by WES to heterozygous mutations within SLFN14 (Schlafen family member 14).96 SLFN14 locates to the nucleus, where it acts as an endoribonuclease and is possibly involved in RNA surveillance. The variants in patients with macrothrombocytopenia localize to the ATPase-AAA-4 domain. The protein is known to associate with ribosomes where it regulates RNA degradation; SLFN14 expression was reduced in the patients and a dom- inant-negative effect was suggested.97 Platelet dense granule content was low and platelet aggregation particularly reduced with collagen, modestly reduced and reversible with ADP but normal with arachidonic acid. In a separate study, severe macrothrombocytopenia but with AR inheri- tance was linked through WES to a homozygous missense mutation in the PRKACG gene encoding the gamma-cat- alytic subunit of the cyclic adenosine monophosphate (cAMP)-dependent protein kinase A.98 Protein kinase A phosphorylates multiple substrates in platelets and in its absence platelet activation and cytoskeletal organization (with extensive degradation of filamin A) were impaired. However the very low platelet count (<10x109/L) precluded platelet aggregation testing. Significantly, megakaryocytes from this patient had a defect in proplatelet formation. Next in this section are AR mutations in G6B (MPIG6B) encoding the megakaryocyte and platelet inhibitory receptor, G6b-B, a critical regulator of hematopoietic lineage differentiation and megakaryocyte function and platelet production.99 The mutations were detected by WES screening of four families with macrothrombocytopenia. One feature was the pres- ence of focal myelofibrosis atypically even in children. Bleeding was moderate and accompanied by anemia and leukocytosis. In a mouse model megakaryocytes were abnormal with reduced proplatelet formation. G6b-B regu- lates the activity of the tyrosine phosphatases Shp1 and Shp2. Nonetheless, platelet aggregation to ADP was only minimally affected.
Severe thrombocytopenia observed in four Spanish fam- ilies was associated with a disorder of ceramide biosynthe- sis with WES linking the disease to AR mutations in KDSR encoding 3-ketodihydrosphingosine reductase, a lipid enzyme that localizes to the endoplasmic reticulum.100 Interestingly, in at least two of these cases the platelet count was normal at birth but fell rapidly to around 30x109/L or lower. Impaired platelet biogenesis was speculated to be due to decreased synthesis of sphingosine-1 phosphate but formal proof of this is lacking. Another disorder in which the platelet count falls after birth is Roifman syndrome in which moderate thrombocytopenia accompanies a variable syndromic disorder characterized by growth retardation, spondylo-epiphyseal dysplasia, cognitive delay and hypogammaglobulinemia.101 Interestingly, the disease is driven by AR variants in the small RNA nuclear gene RNU4ATAC that intervenes in intron splicing. The platelets
2012
haematologica | 2020; 105(8)