This document discusses molecular derangements that can cause primary thrombocytosis, including mutations in the JAK2, MPL, and CALR genes. It notes that JAK2 V617F is present in approximately 50% of patients with essential thrombocythemia or primary myelofibrosis. Pediatric primary thrombocytosis is discussed, which may involve alternative causative mutations and have a more benign clinical course than in adults. Several unanswered questions are posed regarding the natural history and long-term outcomes of primary thrombocytosis in children.

2. • BCR-ABL1 negative MPNs:
• JAK 2 V617F
• JAK-STAT pathway genes
• MPL W515K/L gene
• PRV-1 gene
• Hereditary thrombocytosis:
• TPO gene gain of function mutation (THPO)
• TPO receptor gene cMPL (MPL) mutation
• Other genes
• Congenital amegakaryocytic thrombocytopenia:
• Absence of cMPL expression

3. Thrombocytosis
450-700x109/L : mild thrombocytosis
700-900x109/L : moderate thrombocytosis
> 900x109/L : severe thrombocytosis
> 1000x109/L : extreme thrombocytosis

4. Megakaryocytopoiesis
• TPO (Liver +++, Marrow stroma, kidney).
• IL-3
• SCF
• IL-11
• EPO

5. Physiology
• Platelet count ---- TPO clearance ---- TPO level
• In ITP
• In ET

12. Molecular derangements in primary thrombocytosis
• 1951: interrelatedness of various MPNs
• 2005: mutation in JAK2 gene first identified.
• Mutation in pseudokinase domain of JAK2: JAK 2 V617F
95% of patients with PV, and approximately 50% of patients with ET or PMF.
• JAK-STAT pathway genes --- STAT5, Bcl-xL
• Mutations in JAK2 in exon12
a number of patients with PV and idiopathic erythrocytosis as well.
• MPL W515K/L gene identified in both ET and PMF patients
• PRV-1 gene in numerous patients with PV and ET.
• CALR gene in a significant proportion of JAK2 and MPL wild-type MPN patients.

13. • Allele burden, loss of heterozygosity, and uniparental disomy all may
affect the clinical phenotype in MPN patients.
• Looking at both a mouse model and human samples, Tiedt et al. showed
there was relatively more JAK2V617F mRNA expressed in PV patients and
relatively more wild-type JAK2 mRNA expressed in ET patients.
• Findings of low-penetrance inherited alleles and certain single nucleotide
polymorphisms (SNPs) may also contribute to the resulting phenotype.
• Pardanani and colleagues reported that some JAK2 SNPs were more likely
to be associated with PV or ET.
• Cytogenetic abnormalities: Both deletion of 20q and 13q, and trisomy 9,
have all been identified.

14. Pediatric primary thrombocytosis
• Dame and Sutor reported that ET is over 60 times more common in adults
than in children.
• Lower rate of JAK2V617F mutation.
• The symptoms and sequelae of primary thrombocytosis are more benign in
the pediatric population. However, there have been additional case reports
of pediatric ET patients developing thrombosis.
• The molecular pathogenesis of ET in childhood is not fully aligned with its
adult counterpart.
• PRV-1 expression.
• alternative causative mutations TET2, ASXL1, IDH1, and IDH2
• MPLY252H

15. Unanswered questions that require evaluation
• Is this the same criteria we should utilize in diagnosing children with
primary thrombocytosis?
• Similarly, absence of a causative mutation in a child with a positive
family history does not preclude the diagnosis of a hereditary
thrombocytosis.
• If children with ET do not present with JAK2 or MPL mutation, can
they develop JAK2V61F or MPLW515L at some point during the
course of their illness?
• If so, does it change the clinical course of their disease?

16. • Can reversion to normal occur spontaneously in certain children if
they are/become JAK2-mutant?
• While in JAK2 and MPL negative pediatric ET patients it is likely that
there is another causative mutation, the genetic landscape of ET in
children is unknown.
• Are there situations in which children may have persistently elevated
platelet counts that are not reactive, but also not truly ET?

17. • To find the driver mutation in these patients, should further genetic
evaluation be carried out in additional genes of the relevant
pathways, as was done by Ismael and colleagues? We believe the
answer is “yes”, and we are currently initiating a broader, targeted
analysis in our patients.
• Since we do not know enough about the natural course of primary
thrombocytosis in childhood, is it possible that a number of young
adult patients developed disease earlier and went undetected for many
years?

18. • The only way to answer these questions concerning disease course is
to systematically follow well-characterized young ET patients so we
can learn the natural history of the disease in children.
• Alternatively, diagnosis of primary thrombocytosis in childhood may
mean they will be thrombocythemic for 50 or 60 years instead of 20
or 30 years.
• Does a more prolonged exposure to endothelial damage and
activation and/or platelet dysfunction worsen the later risk of long-
term complications?

19. • Should episodes of thrombosis or hemorrhage be used as treatment
criteria in pediatric patients or should the focus be on prevention of
long-term vascular and organ damage?
• This highlights the need for cooperative trials for pediatric primary
thrombocytosis, so that biological understanding of the disease can
be expanded, treatment strategies standardized, and counseling
families of affected pediatric patients improved.