Primary myelofibrosis is a condition characterized by the buildup of scar tissue (fibrosis) in the bone marrow, the tissue that produces blood cells. Because of the fibrosis, the bone marrow is unable to make enough normal blood cells. The shortage of blood cells causes many of the signs and symptoms of primary myelofibrosis.

Initially, most people with primary myelofibrosis have no signs or symptoms. Eventually, fibrosis can lead to a reduction in the number of red blood cells, white blood cells, and platelets. A shortage of red blood cells (anemia) often causes extreme tiredness (fatigue) or shortness of breath. A loss of white blood cells can lead to an increased number of infections, and a reduction of platelets can cause easy bleeding or bruising.

Because blood cell formation (hematopoiesis) in the bone marrow is disrupted, other organs such as the spleen or liver may begin to produce blood cells. This process, called extramedullary hematopoiesis, often leads to an enlarged spleen (splenomegaly) or an enlarged liver (hepatomegaly). People with splenomegaly may feel pain or fullness in the abdomen, especially below the ribs on the left side. Other common signs and symptoms of primary myelofibrosis include fever, night sweats, and bone pain.

Primary myelofibrosis is most commonly diagnosed in people aged 50 to 80 but can occur at any age.

Primary myelofibrosis is a rare condition that affects approximately 1 in 500,000 people worldwide.

Mutations in the JAK2, MPL, CALR, and TET2 genes are associated with most cases of primary myelofibrosis. The JAK2 and MPL genes provide instructions for making proteins that promote the growth and division (proliferation) of blood cells. The CALR gene provides instructions for making a protein with multiple functions, including ensuring the proper folding of newly formed proteins and maintaining the correct levels of stored calcium in cells. The TET2 gene provides instructions for making a protein whose function is unknown.

The proteins produced from the JAK2 and MPL genes are both part of a signaling pathway called the JAK/STAT pathway, which transmits chemical signals from outside the cell to the cell's nucleus. The protein produced from the MPL gene, called thrombopoietin receptor, turns on (activates) the pathway, and the JAK2 protein transmits signals after activation. Through the JAK/STAT pathway, these two proteins promote the proliferation of blood cells, particularly a type of blood cell known as a megakaryocyte.

Mutations in either the JAK2 gene or the MPL gene that are associated with primary myelofibrosis lead to overactivation of the JAK/STAT pathway. The abnormal activation of JAK/STAT signaling leads to overproduction of abnormal megakaryocytes, and these megakaryocytes stimulate another type of cell to release collagen. Collagen is a protein that normally provides structural support for the cells in the bone marrow. However, in primary myelofibrosis, the excess collagen forms scar tissue in the bone marrow.

Although mutations in the CALR gene and the TET2 gene are relatively common in primary myelofibrosis, it is unclear how these mutations are involved in the development of the condition.

Some people with primary myelofibrosis do not have a mutation in any of the known genes associated with this condition. Researchers are working to identify other genes that may be involved in the condition.

Genetic Testing Information

• Genetic Testing Registry: Primary myelofibrosis

Genetic and Rare Diseases Information Center

• Primary myelofibrosis

Patient Support and Advocacy Resources

• National Organization for Rare Disorders (NORD)

Clinical Trials

• ClinicalTrials.gov

Catalog of Genes and Diseases from OMIM

• MYELOFIBROSIS

Scientific Articles on PubMed

• PubMed

• Chaligne R, Tonetti C, Besancenot R, Roy L, Marty C, Mossuz P, Kiladjian JJ, Socie G, Bordessoule D, Le Bousse-Kerdiles MC, Vainchenker W, Giraudier S. New mutations of MPL in primitive myelofibrosis: only the MPL W515 mutations promote a G1/S-phase transition. Leukemia. 2008 Aug;22(8):1557-66. doi: 10.1038/leu.2008.137. Epub 2008 Jun 5. Citation on PubMed
• Ciurea SO, Merchant D, Mahmud N, Ishii T, Zhao Y, Hu W, Bruno E, Barosi G, Xu M, Hoffman R. Pivotal contributions of megakaryocytes to the biology of idiopathic myelofibrosis. Blood. 2007 Aug 1;110(3):986-93. doi: 10.1182/blood-2006-12-064626. Epub 2007 May 1. Citation on PubMed or Free article on PubMed Central
• Klampfl T, Gisslinger H, Harutyunyan AS, Nivarthi H, Rumi E, Milosevic JD, Them NC, Berg T, Gisslinger B, Pietra D, Chen D, Vladimer GI, Bagienski K, Milanesi C, Casetti IC, Sant'Antonio E, Ferretti V, Elena C, Schischlik F, Cleary C, Six M, Schalling M, Schonegger A, Bock C, Malcovati L, Pascutto C, Superti-Furga G, Cazzola M, Kralovics R. Somatic mutations of calreticulin in myeloproliferative neoplasms. N Engl J Med. 2013 Dec 19;369(25):2379-90. doi: 10.1056/NEJMoa1311347. Epub 2013 Dec 10. Citation on PubMed
• Nangalia J, Massie CE, Baxter EJ, Nice FL, Gundem G, Wedge DC, Avezov E, Li J, Kollmann K, Kent DG, Aziz A, Godfrey AL, Hinton J, Martincorena I, Van Loo P, Jones AV, Guglielmelli P, Tarpey P, Harding HP, Fitzpatrick JD, Goudie CT, Ortmann CA, Loughran SJ, Raine K, Jones DR, Butler AP, Teague JW, O'Meara S, McLaren S, Bianchi M, Silber Y, Dimitropoulou D, Bloxham D, Mudie L, Maddison M, Robinson B, Keohane C, Maclean C, Hill K, Orchard K, Tauro S, Du MQ, Greaves M, Bowen D, Huntly BJP, Harrison CN, Cross NCP, Ron D, Vannucchi AM, Papaemmanuil E, Campbell PJ, Green AR. Somatic CALR mutations in myeloproliferative neoplasms with nonmutated JAK2. N Engl J Med. 2013 Dec 19;369(25):2391-2405. doi: 10.1056/NEJMoa1312542. Epub 2013 Dec 10. Citation on PubMed or Free article on PubMed Central
• Pikman Y, Lee BH, Mercher T, McDowell E, Ebert BL, Gozo M, Cuker A, Wernig G, Moore S, Galinsky I, DeAngelo DJ, Clark JJ, Lee SJ, Golub TR, Wadleigh M, Gilliland DG, Levine RL. MPLW515L is a novel somatic activating mutation in myelofibrosis with myeloid metaplasia. PLoS Med. 2006 Jul;3(7):e270. doi: 10.1371/journal.pmed.0030270. Citation on PubMed or Free article on PubMed Central
• Saint-Martin C, Leroy G, Delhommeau F, Panelatti G, Dupont S, James C, Plo I, Bordessoule D, Chomienne C, Delannoy A, Devidas A, Gardembas-Pain M, Isnard F, Plumelle Y, Bernard O, Vainchenker W, Najman A, Bellanne-Chantelot C; French Group of Familial Myeloproliferative Disorders. Analysis of the ten-eleven translocation 2 (TET2) gene in familial myeloproliferative neoplasms. Blood. 2009 Aug 20;114(8):1628-32. doi: 10.1182/blood-2009-01-197525. Epub 2009 Jun 29. Citation on PubMed
• Tefferi A, Pardanani A, Lim KH, Abdel-Wahab O, Lasho TL, Patel J, Gangat N, Finke CM, Schwager S, Mullally A, Li CY, Hanson CA, Mesa R, Bernard O, Delhommeau F, Vainchenker W, Gilliland DG, Levine RL. TET2 mutations and their clinical correlates in polycythemia vera, essential thrombocythemia and myelofibrosis. Leukemia. 2009 May;23(5):905-11. doi: 10.1038/leu.2009.47. Epub 2009 Mar 5. Citation on PubMed or Free article on PubMed Central
• Tefferi A. Novel mutations and their functional and clinical relevance in myeloproliferative neoplasms: JAK2, MPL, TET2, ASXL1, CBL, IDH and IKZF1. Leukemia. 2010 Jun;24(6):1128-38. doi: 10.1038/leu.2010.69. Epub 2010 Apr 29. Citation on PubMed or Free article on PubMed Central
• Tefferi A. Pathogenesis of myelofibrosis with myeloid metaplasia. J Clin Oncol. 2005 Nov 20;23(33):8520-30. doi: 10.1200/JCO.2004.00.9316. Citation on PubMed