Evidence for an Involvement of Cardiomyocyte Hyperplasia Rather than Hypertrophy in Cardiac Enlargement of Noonan Syndrome Patients

Background: Noonan syndrome (NS) is caused by mutations in genes of the RAS/RAF/MAP kinase pathway, such as RAF1 or PTPN11. Apart from variable multisystemic pathologies patients primarily suffer from hypertrophic cardiomyopathy often leading to heart failure. It has been proposed that cardiac enlargement is not solely caused by cardiomyocyte (CM) hypertrophy but that hyperplasia might also contribute. The aim of this study was to evaluate CM morphometry and numbers in postnatal NS myocardium.

Method: Myocardial tissue from three NS patients carrying RAF1 (aged 5 years) or PTPN11 mutations (aged 9 months and 13 years) was compared to age matched DCM and control hearts. Immunofluorescence was performed to analyze tissue composition, CM size, and cell cycle activity, whereas Western blot was used to evaluate signaling pathways regulating cell growth and proliferation.

Results: Histopathological analyses revealed a higher density of CM nuclei and a higher ratio of CM to non-myocyte nuclei in all NS compared to DCM and control hearts. CM cross sectional area was increased in DCM but unaltered in NS myocardium compared to controls. CM length was reduced in NS hearts resulting in a reduced calculated CM volume compared to both DCM and controls. Based on heart volume derived from MRI the number of CM per heart was estimated which was up to 8× higher in NS compared to DCM hearts (13.3 ± 2.8 × 109 vs. 1.7 ± 0.2 × 109, p < 0.01). Immunofluorescence for the mitosis marker phospho-histone H3 showed markedly increased cell cycle activity of CM and non-myocytes in the 5 years old NS patient carrying a RAF1 mutation (4.66% PHH3 positive nuclei vs. 0.05% in the control). In contrast, mitosis rates were not different in the two NS patients carrying PTPN11 mutations. Western blot analyses revealed activation of the p42/p44 MAP kinase signaling pathway in the heart of the 5 years old NS patient but not in a 13-year-old heart carrying a PTPN11 mutation. The latter showed evidence for activation of the Akt/mTOR pathway.

Conclusion: These data support the idea that cardiomyocyte hyperplasia contributes to heart enlargement in NS patients. The time window of a potential proliferative growth phase seems to differ in NS patients and is not primarily determined by age but possibly dependent on the disease causing mutation. The latter is supported by differential activation of growth and cell cycle regulating signaling pathways in NS hearts with RAF1 versus PTPN11 mutations.

Publication History

Article published online:
28 January 2023