LZTR1 molecular genetic overlap with clinical implications for Noonan syndrome and schwannomatosis

Nerve enlargement in patients with Noonan syndrome: A retrospective cohort study

Noonan syndrome (NS) is an autosomal dominant condition characterized by facial dysmorphism, congenital heart disease, development delay, growth retardation and lymphatic disease. It is caused by germline pathogenic variants in genes encoding proteins in the Ras/mitogen-activated protein kinase signaling pathway. Nerve enlargement is not generally considered as a feature of NS, although some cases have been reported. High-resolution nerve ultrasound enables detailed anatomical assessment of peripheral nerves and can show enlarged nerves. This retrospective cohort study aims to describe the sonographic findings of patients with NS performed during a 1-year time period. Data on the degree of enlargement, the relation to increasing age, pain in extremities, genotype on the gene level and clinical features were collected. Twenty-nine of 93 patients visiting the NS Center of Expertise of the Radboud University Medical Center Nijmegen underwent high-resolution ultrasound. In 24 patients (83%) nerve enlargement was found. Most of them experienced pain. We observed a weak correlation with increasing age and the degree of nerve enlargement but no association with pain, genotype at the gene level or clinical features. This study shows that patients with NS have a high predisposition for sonographic nerve enlargement and that the majority experience pain.

Preclinical evaluation of CRISPR-based therapies for Noonan syndrome caused by deep-intronic LZTR1 variants

Gene variants in LZTR1 are implicated to cause Noonan syndrome associated with a severe and early-onset hypertrophic cardiomyopathy. Mechanistically, LZTR1 deficiency results in accumulation of RAS GTPases and, as a consequence, in RAS-MAPK signaling hyperactivity, thereby causing the Noonan syndrome-associated phenotype. Despite its epidemiological relevance, pharmacological as well as invasive therapies remain limited. Here, personalized CRISPR-Cas9 gene therapies might offer a novel alternative for a curative treatment in this patient cohort. In this study, by utilizing a patient-specific screening platform based on iPSC-derived cardiomyocytes from two Noonan syndrome patients, we evaluated different clinically translatable therapeutic approaches using small Cas9 orthologs targeting a deep-intronic LZTR1 variant to cure the disease-associated molecular pathology. Despite high editing efficiencies in cardiomyocyte cultures transduced with lentivirus or all-in-one adeno-associated viruses, we observed crucial differences in editing outcomes in proliferative iPSCs vs. non-proliferative cardiomyocytes. While editing in iPSCs rescued the phenotype, the same editing approaches did not robustly restore LZTR1 function in cardiomyocytes, indicating critical differences in the activity of DNA double-strand break repair mechanisms between proliferative and non-proliferative cell types and highlighting the importance of cell type-specific screens for testing CRISPR-Cas9 gene therapies.

Noonan syndrome: Neuroimaging findings and morphometric analysis of the cranium base and posterior fossa in children

BACKGROUND AND PURPOSE

There are a few studies regarding intracranial findings in neonates with Noonan syndrome (NS); however, there are no quantitative analyses in a pediatric population. The aim of this study was to find characteristic intracranial abnormalities and to quantitatively analyze the posterior fossa and cranium base in children with NS.

METHODS

A total of 30 patients (11 females and 19 males, mean age 13.1 ± 4.3 years) were retrospectively identified between July 2017 and June 2022. Twenty-one patients had MRI. Age at MRI examination, sex, genetic mutations, and clinical findings were noted. In patients with MRI, the presence of white matter lesions, basal ganglia lesions, corpus callosum abnormalities, sellar/parasellar lesions, and tonsillar ectopia was noted. For morphometric analysis, cerebellar diameter, vermis and clivus heights, cranial base, tentorial and infratentorial angles, and McRae's and Twining's lines were each measured twice by two radiologists individually.

RESULTS

The most common lesions were focal white matter lesions, followed by abnormalities of the splenium of the corpus callosum. The cerebellar diameter, vermis and clivus heights, Twining's line, and infratentorial angle were significantly smaller; cranial base angle and tentorial angle were significantly larger in NS (p < .05). Interrater and intrarater agreements were the highest for cerebellar diameter and the lowest for tentorial angle measurements.

CONCLUSION

Children with NS had characteristic callosal and tentorial findings and neuroimaging findings similar to other RASopathies. This study also shows that a small posterior fossa and flattening of the cranial base are present in children with NS, which may aid in diagnosis.

Natural history of MRAS-related Noonan syndrome: Evidence of mild adult-onset left ventricular hypertrophy and neuropsychiatric features

Gain of function pathogenic variants in MRAS have been found in a small subset of pediatric subjects presenting with Noonan syndrome (NS) associated with hypertrophic cardiomyopathy (HCM) and moderate to severe intellectual disability. These variants are considered to confer a high-risk for the development of severe HCM with poor prognosis and fatal outcome. We report on the natural history of the first adult subject with NS carrying the recurrent pathogenic p.Thr68Ile amino acid substitution. Different from what had previously been observed, he presented with a mild, late-onset left ventricular hypertrophy, and a constellation of additional symptoms rarely seen in NS. The present case provides evidence that HCM does not represent an obligatory, early-onset and severe complication in subjects with MRAS variants. It also adds new data about late-onset features suggesting that other unexpected complications might be observed in adult subjects providing anticipatory guidance for individuals of all age.

The molecular genetics of RASopathies: An update on novel disease genes and new disorders

Enhanced signaling through RAS and the mitogen-associated protein kinase (MAPK) cascade underlies the RASopathies, a family of clinically related disorders affecting development and growth. In RASopathies, increased RAS-MAPK signaling can result from the upregulated activity of various RAS GTPases, enhanced function of proteins positively controlling RAS function or favoring the efficient transmission of RAS signaling to downstream transducers, functional upregulation of RAS effectors belonging to the MAPK cascade, or inefficient signaling switch-off operated by feedback mechanisms acting at different levels. The massive effort in RASopathy gene discovery performed in the last 20 years has identified more than 20 genes implicated in these disorders. It has also facilitated the characterization of several molecular activating mechanisms that had remained unappreciated due to their minor impact in oncogenesis. Here, we provide an overview on the discoveries collected during the last 5 years that have delivered unexpected insights (e.g., Noonan syndrome as a recessive disease) and allowed to profile new RASopathies, novel disease genes and new molecular circuits contributing to the control of RAS-MAPK signaling.