Cardiac hypertrophy associated with myeloproliferative neoplasms in JAK2V617F transgenic mice

Abstract

Background: Myeloproliferative neoplasms (MPNs) are blood malignancies manifested in increased production of red blood cells, white blood cells, and/or platelets. A major molecular lesion associated with the diseases is JAK2V617F, an activation mutation form of tyrosine kinase JAK2. Cardiovascular events represent the leading cause of morbidity and mortality associated MPNs, but the underlying mechanism is not well understood.

Methods: Previously, we generated JAK2V617F transgenic mice which displayed MPN-like phenotypes. In the present study, we further characterized these mice by analyzing the time course of MPN phenotype development and associated cardiac abnormalities. We performed detailed histochemical staining of cardiac sections.

Results: JAK2V617F transgenic mice developed cardiomegaly as a subsequent event of increased blood cell production during the course of MPN phenotype development. The cardiomegaly is manifested in increased ventricular wall thickness and enlarged cardiomyocytes. Trichrome and reticulin staining revealed extensive collagen fibrosis in the heart of JAK2V617F transgenic mice. Thrombosis in the coronary artery and inflammatory cell infiltration into cardiac muscle were also observed in JAK2V617F transgenic mice, and the latter event was accompanied by fibrosis.

Conclusion: JAK2V617F-induced blood disorders have a major impact on heart function and lead to cardiac hypertrophy. JAK2V617F transgenic mice represent an excellent model system to study both hematological malignancies and cardiovascular diseases.

Figure 1

JAK2V617F mice develop cardiomegaly associated with increased blood cell counts. Photos of hearts from typical control and JAK2V617F transgenic mice at age of 40–45 weeks are shown (A). Bar graphs represent average heart mass (B), body mass (C), heart/body ratio (D), red blood cell counts (E), and platelet counts (F) of control (open bar) and JAK2V617F transgenic (closed bar) mice at indicated ages. Error bars denote standard deviation (n ≥10). * P <0.05, **P < 0.01, ***P < 0.001.

Figure 2

Cardiomegaly in JAK2V617F mice is manifested in increased ventricular wall thickness. The top panel shows representative H&E staining of middle transverse cardiac sections from 40–45 week-old control and JAK2V617F transgenic mice (magnification, ×1.6). Data in the bar graph represents mean ± SD (n ≥10). Note that the left ventricular wall thickness was significantly increased in JAK2V617F transgenic mice (P < 0.01).

Figure 3

The size of cardiomyocytes is significantly increased in JAK2V617F transgenic mice. Representative histology of H&E-stained cardiac sections corresponding to the left ventricle free wall of hearts from 40–45 week-old control (A and C) and JAK2V617F (B and D) (magnification, ×100). A and B show myofibers running parallel to the apical-basal axis, and B and D show myofibers running circumferentially. Note that an enlarged nucleus is seen in a hypertrophic myocyte (pointed by an arrow in D). Data in the bar graph (E) represent cross-sectional area measurements of at least 20 cardiomyocytes per animal shown in A and B from 10 control and 10 JAK2V617F mice. Error bars denote SD (n ≥200). P < 0.001.

Figure 4

Masson’s Trichrome stain reveals collagen fibrosis in hearts from JAK2V617F transgenic mice. Representative trichrome stains of heart sections from 40–45 week-old control (A and C) and JAK2V617F transgenic (B and D) mice are shown (magnification, ×40). Note the extensive interstitial (B) and perivascular (D) fibrosis represented by the blue stains. Data in the bar graphs represent quantitative measurements of interstitial (E) and perivascular (F) trichrome staining intensity of at least 5 randomly selected areas from each of 10 control and 10 JAK2V617F transgenic mice. Error bars denote SD (n ≥50). P < 0.001.

Figure 5

Reticulin fibrosis is significantly increased in hearts from JAK2V617F transgenic mice. Representative reticulin stains of heart sections from 40–45 week-old control (A and C) and JAK2V617F transgenic (B and D) mice are shown (magnification, ×40). Note that interstitial reticulin fibrosis is much more intensive in JAK2V617F transgenic mice in comparison with control mice. Data in the bar graph (E) represent quantitative measurements of interstitial reticulin staining intensity of at least 5 randomly selected segments from each of 10 control and 10 JAK2V617F transgenic mice. Error bars denote SD (n ≥50). P < 0.001.

Figure 6

Thrombosis occurs in the coronary artery of JAK2V617F transgenic mice. Trichrome stains of cardiac sections demonstrate a typical thrombus in the coronary artery of 40–45 week-old JAK2V617F transgenic mice.

Figure 7

Inflammatory cell infiltration occurs in JAK2V617F transgenic mouse hearts and is accompanied by fibrosis. Consecutive sections of the heart from a 40–45 week-old JAK2V617F transgenic mouse were subjected to H&E (A), Masson’s trichrome (B), reticulin (C), and toluidine blue staining (D) (magnification, ×40; bar = 50 um). White arrows indicate inflammatory cells, and a black arrow points to mast cells.

Figure 8

Real time PCR assays revealed marginal expression of transgenic JAK2V617F in the mouse heart. Expressions of the endogenous mouse Jak2 gene and the JAK2V617F transgene in the bone marrow (BM), peripheral blood (PB), spleen, and heart of 40-week-old JAK2V617F transgenic mice were determined by real time PCR. Data represent relative mRNA levels (mean ± SD, n = 3) normalized to mouse GAPDH in arbitrary units. *** P < 0.001.