Molecular basis of Noonan syndrome-associated hypertrophic obstructive cardiomyopathy: PTPN11 and RAF1

Both PTPN11 and RAF1 encode proteins that function within the renin-angiotensin system/mitogen-activated protein kinase signalling cascade, and gain-of-function variants in either gene result in constitutive downstream MEK-ERK hyperactivation—the shared molecular driver of cardiomyocyte hypertrophy in Noonan syndrome.

PTPN11 encodes SHP2, a non-receptor protein tyrosine phosphatase that acts upstream of Renin–Angiotensin System (RAS) as a positive regulator of Mitogen-Activated Protein Kinase (MAPK) signalling by dephosphorylating inhibitory tyrosine residues on renin–angiotensin system effectors, thereby amplifying downstream mitogen-activated protein kinase kinase and extracellular signal-regulated kinase activity. ^{Reference Leegaard, Gregersen, Nielsen, Bjerre and Handrup14} Gain-of-function PTPN11 variants—such as p. Gln510Glu in Case 1—result in constitutive SHP2 activation, sustaining elevated phospho-ERK levels in cardiomyocytes. Although hypertrophic cardiomyopathy is less prevalent in PTPN11-related Noonan syndrome (18–30%) than in RAF1-related disease, the severity of obstruction can be comparable when it does occur, as demonstrated by Case 1’s peak left ventricular outflow tract gradient of 130 mmHg despite prior surgical myectomy. The PTPN11 c.1528C>G; p. Gln510Glu variant identified in Case 1 maps to the protein tyrosine phosphatase domain of SHP2, distinct from the N-SH2 and C-SH2 domains in which the majority of Noonan syndrome-associated PTPN11 variants cluster. Protein tyrosine phosphatase-domain variants are less frequently reported than N-SH2 variants but have been shown to similarly relieve the autoinhibited conformation of SHP2, producing constitutive catalytic activation and sustained downstream MEK-ERK signalling. ^{Reference Aoki, Niihori, Inoue and Matsubara7} The p. Gln510Glu variant itself has been reported only sporadically in the Noonan syndrome literature and has not, to our knowledge, been subjected to dedicated functional validation; its classification as gain-of-function is therefore inferred from its domain location and from the clinical phenotype observed in this patient, rather than confirmed by in vitro kinase assay. This represents a limitation in the molecular characterisation of Case 1 and highlights a need for functional studies of less common PTPN11 variants identified in Noonan syndrome-associated hypertrophic obstructive cardiomyopathy.

RAF1 encodes Raf-1 proto-oncogene serine/threonine kinase (MAP3K), which functions directly downstream of renin–angiotensin system and phosphorylates MEK1/2, the immediate upstream activators of ERK1/2. ^{Reference Aoki, Niihori, Inoue and Matsubara7,Reference Razzaque, Nishizawa and Komoike8} The RAF1 c.770C>T; p. Ser257Leu variant identified in Case 2 is a recurrent, well-characterised pathogenic variant clustered in the CR2 domain of RAF1, which has been demonstrated to produce elevated kinase activity, increased mitogen-activated protein kinase kinase and extracellular signal-regulated kinase phosphorylation, and altered downstream signalling in cardiomyocytes consistent with cardiac hypertrophy. ^{Reference Aoki, Niihori, Inoue and Matsubara7,Reference Razzaque, Nishizawa and Komoike8} This variant was first reported by Razzaque et al. in 2007 in four of thirty Noonan syndrome patients, two of whom had severe hypertrophic cardiomyopathy, and has since been documented in numerous unrelated individuals with Noonan syndrome-associated hypertrophic cardiomyopathy in multiple cohorts. ^{Reference Razzaque, Nishizawa and Komoike8} Importantly, hypertrophic cardiomyopathy is present in approximately 95% of patients with pathogenic RAF1 variants—the highest gene-specific hypertrophic cardiomyopathy prevalence in Noonan syndrome—underscoring the direct pathogenic role of RAF1 in myocardial hypertrophy. ^{Reference Aoki, Niihori, Inoue and Matsubara7}

Preclinical evidence directly supports mitogen-activated protein kinase kinase inhibition as a targeted approach for RAF1-mediated hypertrophic cardiomyopathy. Wu et al. demonstrated in a knock-in mouse model of Noonan syndrome carrying the Raf1 L613V variant that postnatal MEK-ERK inhibition ameliorated both the cardiac hypertrophy and morphological phenotype of the disease. ^{Reference Wu, Simpson and Hong15} In vitro studies using induced pluripotent stem cell-derived cardiomyocytes from patients with Noonan syndrome-associated gain-of-function variants confirmed that individual pathogenic variants are both necessary and sufficient to induce cardiomyocyte hypertrophy and that pharmacological mitogen-activated protein kinase kinase inhibition attenuates this effect. ^{Reference Gelb, Roberts and Tartaglia9} These converging preclinical data provide the biological rationale for trametinib’s efficacy in both PTPN11- and RAF1-related Noonan syndrome-associated hypertrophic obstructive cardiomyopathy.

RAF1-specific clinical evidence for trametinib

The RAF1 genotype in Case 2 is particularly well-represented in the published trametinib literature, providing a strong reference framework for the response observed. In the systematic review by De Brouchoven et al. (2025), two of the seventeen reviewed cases carried the RAF1 c.770C>T; p. Ser257Leu variant—precisely the same variant identified in Case 2 of the present report. ^{Reference De Brouchoven, Lorand and Bofferding12} One of these patients, reported by Meisner et al. (2021), was a 20-week-old infant with biventricular hypertrophic cardiomyopathy, pulmonary valve stenosis, and multifocal atrial tachycardia; trametinib at 0.025 mg/kg/day resulted in complete resolution of multifocal atrial tachycardia within 72 hours and significant improvement of biventricular hypertrophic cardiomyopathy over six months. ^{Reference Meisner, Bradley and Russell13} A second RAF1 p. Ser257Leu patient reported by Mussa et al. (2021) was a preterm neonate with severe biventricular obstructive hypertrophic cardiomyopathy who showed transient improvement in cardiac function and reduction in septal thickness on trametinib before dying of pulmonary capillary hemangiomatosis—a cause of death considered unrelated to trametinib. ^{Reference De Brouchoven, Lorand and Bofferding12}

Case 2 extends these observations to an older child with established RAF1-related hypertrophic obstructive cardiomyopathy who had already undergone surgical right ventricular outflow tract intervention and remained on optimised beta-blocker therapy, yet carried a severely elevated peak left ventricular outflow tract gradient of 163 mmHg. The greater than 80% gradient reduction achieved with trametinib over ten weeks in this patient—without dose escalation or adverse effects—represents one of the most substantial reported haemodynamic responses to mitogen-activated protein kinase kinase inhibition in the RAF1 genotypic subgroup and in older paediatric patients.

PTPN11-specific clinical evidence and comparison

While RAF1-related hypertrophic obstructive cardiomyopathy has a well-established response profile to trametinib, evidence for PTPN11-related disease is more limited. Pascarella et al. (2024) described a neonate with PTPN11 p. Phe285Ser-related Noonan syndrome who exhibited reduction in left ventricular posterior wall thickness after two months of trametinib, with no recurrence of chylothorax at 18-month follow-up. ^{Reference Pascarella, Limongelli and De Falco16} Nakano et al. (2022) documented reduction in left ventricular mass and NT-proBNP in a child carrying the same variant. ^{Reference Nakano, Rankin and Annam17} Case 1 extends the PTPN11-specific evidence base by demonstrating a rapid and dramatic reduction in left ventricular outflow tract gradient—from 130 to 40 mmHg within three days—in a 10-year-old with post-surgical hypertrophic obstructive cardiomyopathy, a patient profile not previously described.

The speed of gradient reduction in Case 1 is particularly striking and warrants mechanistic consideration. Structural regression of myocardial hypertrophy is generally a slow process, typically unfolding over months to years. ^{Reference Andelfinger, Marquis and Raboisson11,Reference De Brouchoven, Lorand and Bofferding12} A gradient reduction of this magnitude within three days is more consistent with a functional effect of mitogen-activated protein kinase kinase inhibition on left ventricular outflow tract dynamics—specifically, reduction of dynamic outflow obstruction through decreased myocardial hypercontractility and improved diastolic relaxation—rather than a structural change. This is supported by evidence that extracellular signal-regulated kinase signalling modulates intracellular calcium handling and myofilament calcium sensitivity, both of which contribute to the dynamic nature of left ventricular outflow tract obstruction in hypertrophic cardiomyopathy. ^{Reference Chaput and Andelfinger18}

Dosing, treatment duration, and monitoring

Both patients received trametinib at approximately 0.025 mg/kg/day, consistent with the most commonly used starting dose in the paediatric Noonan syndrome literature ^{Reference De Brouchoven, Lorand and Bofferding12} and with recommendations from paediatric oncology phase I studies. ^{Reference Geoerger, Moertel and Whitlock19} This dosage is lower than that used in oncologic indications—a mechanistically justified distinction, as germline RASopathy mutations generally confer a lower magnitude of pathway hyperactivation than somatic cancer mutations, and the therapeutic goal is normalisation rather than ablation of mitogen-activated protein kinase signalling. ^{Reference De Brouchoven, Lorand and Bofferding12,Reference Chaput and Andelfinger18} In both cases, no dose escalation was required, suggesting that 0.025 mg/kg/day may be an appropriate starting and maintenance dose for hypertrophic obstructive cardiomyopathy in this age group.

The optimal treatment duration for Noonan syndrome-associated hypertrophic obstructive cardiomyopathy remains undefined. Attempts at treatment cessation have been associated with haemodynamic relapse in some reported cases, including recurrence of elevated left ventricular outflow tract gradients and rising NT-proBNP levels necessitating re-treatment. ^{Reference Leegaard, Gregersen, Nielsen, Bjerre and Handrup14,Reference Pascarella, Limongelli and De Falco16} Wolf et al. (2024) reported in a retrospective cohort of 30 infants that, following extended trametinib treatment, sustained cardiac and clinical improvement was maintained after treatment weaning in five patients without relapse. ^{Reference Wolf, Zenker and Boleti20} Based on current evidence, a pragmatic approach involves continuing therapy until sustained cardiac remodelling is documented, typically 3 months to 2 years, followed by gradual weaning with close echocardiographic and biomarker surveillance.

Both patients in this series demonstrate a clinically important pattern of continued improvement over time. With continued, uninterrupted trametinib therapy, both patients demonstrated progressive improvement beyond the initially documented response. In Case 1, the left ventricular outflow tract peak gradient decreased further from 40 mmHg at two months to approximately 35 mmHg at seven to eight months, with interventricular septal thickness decreasing from 19 to 18 mm and posterior wall thickness from 21 to 20 mm over the same interval. In Case 2, the gradient decreased further from 30 to 35 mmHg at ten weeks to approximately 25–30 mmHg at five to six months, with interventricular septal thickness decreasing from 16 to 15 mm and posterior wall thickness from 17 to 16 mm over the same interval. In both cases, the haemodynamic improvement observed acutely within days of treatment initiation was therefore followed by a second, slower phase of incremental functional and structural improvement over subsequent months, consistent with the two-phase model proposed below.

This biphasic pattern of response is, in our view, one of the more clinically informative observations of this report. It suggests that trametinib’s haemodynamic benefit in Noonan syndrome-associated hypertrophic obstructive cardiomyopathy arises through two distinct, temporally separated mechanisms: an early functional effect, evident within days and most plausibly mediated by extracellular signal-regulated kinase-dependent modulation of myocardial contractility and calcium handling (as discussed in Section PTPN11-Specific Clinical Evidence and Comparison), followed by a slower, structural effect over subsequent months as sustained MEK-ERK inhibition permits gradual regression of myocardial hypertrophy itself. The continued incremental gains seen in both patients between the early and extended follow-up assessments are consistent with ongoing structural remodelling rather than a plateaued functional ceiling. This has direct implications for clinical practice and the design of future studies: the full benefit of trametinib in Noonan syndrome-associated hypertrophic obstructive cardiomyopathy may not be apparent from early follow-up alone, and continued therapy over a period of months should be anticipated to yield further gains in both gradient reduction and structural regression, supporting its use as a sustained, disease-modifying therapy rather than a short-course intervention.

Safety and tolerability

Trametinib was well tolerated in both patients. Case 1 experienced no adverse effects at any point during the follow-up period, while Case 2 developed mild, self-limiting cutaneous pruritus that resolved with oral antihistamine therapy and did not require dose modification. This is consistent with the published Noonan syndrome literature, in which non-critical adverse events—most commonly cutaneous toxicity (eczema, acneiform rash), diarrhoea, and gastrointestinal symptoms—were reported in 7 of 17 reviewed cases, predominantly at doses ≥ 0.025 mg/kg/day, and led to dose reduction or discontinuation in only a minority. ^{Reference De Brouchoven, Lorand and Bofferding12} Adverse effects in Noonan syndrome patients receiving trametinib appear less severe than those observed in adult oncology patients, attributed to the lower doses used, the absence of combination chemotherapy, and the fundamental difference in therapeutic intent: in RASopathies, mitogen-activated protein kinase kinase inhibition aims to normalise signalling rather than ablate it. ^{Reference De Brouchoven, Lorand and Bofferding12,Reference Chaput and Andelfinger18}

Three deaths were reported across the published Noonan syndrome trametinib literature; none were considered directly attributable to drug toxicity. ^{Reference De Brouchoven, Lorand and Bofferding12} Nonetheless, given the off-label nature of trametinib in this population, structured monitoring is advisable throughout treatment, incorporating serial echocardiography, NT-proBNP measurement, skin and ophthalmologic examination, and hepatic and haematological bloodwork, as outlined in the protocol proposed by Nakano et al. ^{Reference Nakano, Rankin and Annam17}

Clinical implications

Both patients in this series had undergone prior surgical right ventricular outflow tract intervention and were receiving optimised beta-blocker therapy, yet continued to carry severe, symptomatic left ventricular outflow tract obstruction. This illustrates the persistence of haemodynamically significant residual disease after current standard-of-care strategies and the clinical imperative for molecularly targeted approaches. The response to trametinib in both cases—spanning two distinct genotypes, two different mechanisms of renin-angiotensin system/mitogen-activated protein kinase pathway activation, and different clinical presentations—suggests that mitogen-activated protein kinase kinase inhibition is effective regardless of the specific upstream molecular lesion, as long as it converges on mitogen-activated protein kinase kinase hyperactivation.

The fact that Case 2’s RAF1 genetic result was confirmed only after treatment initiation also highlights an emerging clinical reality: in patients with a strong clinical Noonan syndrome diagnosis and life-limiting hypertrophic obstructive cardiomyopathy, it may be appropriate to initiate trametinib while genetic results are pending, particularly when conventional management has been exhausted. However, we strongly advocate that comprehensive molecular genetic testing be completed in all such patients, both to validate the diagnosis, inform prognosis, and allow genotype-specific therapeutic guidance.

The evidence base for trametinib in Noonan syndrome, while expanding, remains predominantly derived from uncontrolled case reports and series subject to publication bias. Prospective, multicentre clinical trials with standardised dosing protocols, objective haemodynamic endpoints, biomarker assessment, and structured long-term follow-up are urgently required. International registries represent a critical interim measure for systematically accumulating real-world safety and efficacy data across the Noonan syndrome genotypic spectrum.

This report has several limitations that warrant explicit acknowledgement. First, although follow-up has now extended to seven to eight months in Case 1 and five to six months in Case 2 with continued structural and functional improvement, this remains short relative to the months-to-years timeframe over which Noonan syndrome-associated hypertrophic obstructive cardiomyopathy and its treatment typically evolve, and longer-term data will be needed to confirm whether the observed gains are maintained or further extended. Second, both cases are derived from a single-patient, retrospective, off-label treatment context without a control group, and causal attribution of haemodynamic and structural change to trametinib—rather than to natural disease fluctuation or other unmeasured factors—rests on temporal association and biological plausibility rather than controlled comparison. Third, the PTPN11 p. Gln510Glu variant in Case 1 lacks dedicated functional validation in the literature, introducing uncertainty into its molecular classification as gain-of-function. Finally, as a two-patient case report, these findings cannot be generalised to the broader Noonan syndrome population, and prospective multicentre data are required before firm conclusions can be drawn about optimal treatment duration, weaning strategy, or long-term safety.