Noonan syndrome and Noonan-like syndrome with loose anagen hair: rare phenotypes may emerge during follow-up

Introduction

Background

Noonan syndrome (NS; MIM 163950) and Noonan-like syndrome with loose anagen hair (NS/LAH; MIM 607721, 617506) constitute a group of developmental disorders characterized by unique facial features, short stature, and congenital heart defects (1-3). It was first described by a pediatric cardiologist, Dr. Noonan (4). The incidence of NS is estimated to be between 1/2,000 and 1/2,500 (5). There have been 100 reported cases of NS/LAH (6). Both of these diseases are caused by rat sarcoma/mitogen-activated protein kinases (RAS/MAPK) signaling pathway hyperactivity. The RAS/MAPK pathway is a highly conserved cascade of phosphorylation. Germline mutations that are activated are responsible for “RASopathies”, which are rare autosomal dominant or recessive disorders. NS is a representative RASopathy, a class that includes many developmental disorders. To date, more than 20 genes having germline pathogenic variants that have found to be associated with NS (7). PTPN11, SOS1, and RAF1 are the three most common genes; disease-causing variants in these genes account for approximately 65–80% of cases (8,9). The RAS/MAPK pathway represents a signal transduction cascade involved in the processes of cell proliferation, differentiation, survival, and death.

Rationale and knowledge gap

NS and NS/LAH are the most common disorders in this group and are characterized by heart defects, short stature, and typical facial features. PTPN11 has been identified as a major NS gene, and several reports have described PTPN11 mutations in relation to clinical manifestations in NS patients.

Objective

Here, we performed a retrospective review of all genetically confirmed cases of NS and Noonan-like syndrome in a single center in Beijing. Our primary aim was to dissect the molecular findings associated with NS beyond the most common gene, PTPN11. Special attention was given to two patients with mutations in the SHOC2 gene who developed systemic lupus erythematosus (SLE) at an early age. We present this article in accordance with the STROBE reporting checklist (available at https://tp.amegroups.com/article/view/10.21037/tp-24-113/rc).

Methods

Patient selection

We collected retrospective data from the NS population in the Department of Endocrinology at the Children’s Hospital of Capital Institute of Pediatrics from 2017 to 2022. Twenty-five patients had a genetic diagnosis of NS or NS/LAH. All these patients were diagnosed using the NS scoring system developed by van der Burgt et al. (10) or with the presence of a causative mutation for NS. In this study, the manifestations of 25 NS patients were analyzed. This group included 18 males (72%) and 7 females (28%). Patients without genetic confirmation of the clinical diagnosis were excluded. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the Capital Institute of Pediatrics Ethics Committee (No. SHERLL2023049) and informed consent was taken from their parents.

The patients were identified due to clinical features suggestive of a RASopathy. Variants were detected by routine diagnostic testing for known RASopathy genes using targeted gene panel sequencing or whole-exome sequencing of DNA extracted from venous blood samples (hospital laboratory or qualified gene company).

Statistical analysis

The data were analyzed using SPSS Statistics 24.0. Fisher’s test was used to compare the differences between the patients in the two groups. Statistical significance was indicated by a P value of less than 0.05.

Results

Twenty-five patients had genetically confirmed NS or NS/LAH, with a median age of 6.3 years (range, 1–13 years). Data were available for 18 males and 7 females. The clinical findings for 25 individuals are summarized in Table 1. Gene mutations (Figure 1) associated with NS were detected in PTPN11 (n=19, 76%), SHOC2 (n=2, 8%), KRAS (n=1, 4%), LZTR1 (n=1, 4%), BRAF (n=1, 4%), and PPP1CB (n=1, 4%). Table 2 shows the differences between PTPN11 and the other mutations.

Figure 1 Patients’ genetics in our study cohort (n, %).

PTPN11 gene

All the subjects were unrelated sporadic patients. Their birthweights and body lengths were usually normal. Short stature manifested in 100% of patients. The most common congenital heart defect was atrial septal defects (ASDs) (26.8%), followed by pulmonary valve stenosis (PVS) (15.3%). Eleven recurrent pathogenic variants of the PTPN11 gene were identified in 19 patients, with c.922A>G (p.Asn308Asp) being the most common pathogenic variant (5/19, 26.3%), followed by the 1510A>G, 188A>G, 1507G>C and 844A>G mutations in two patients each (10.5% in PTPN11, 8% in total). Among all the mutations, 36.8% (7/19) were located in the N-terminal src-homology 2 (N-SH2) domain, and 63.2% (12/19) were located in the protein tyrosine phosphatase (PTP) domain.

KRAS and BRAF

We identified a 2-year-old boy harboring a KRAS variant (c.173C>T, p.Thr58Ile) (reported by Schubbert et al.) (11) and a 5-year-old girl harboring an NRAS variant (c.1799T>G, p.Val600Gly). Both kids exhibited slow intellectual development, typical facial features, and short stature. The girl also had hearing loss. There were no ectodermal abnormalities, cutaneous defects, papillomas, or deep palmar or plantar creases.

LZTR1 gene

Two compound heterogeneous variants, c.651+1G>T and c.1943-256C>T, in the LZTR1 gene were identified in one male patient with the atypical facial appearance of NS, a short stature (−3.0 standard deviation), ASD and cryptorchidism. Family screening for the variants revealed that each parent was a carrier of one of these variants, but neither the mother nor the father was clinically affected. The c.1943-256C>T variant has been previously reported in NS (12), and the c.651+1G>T variant (13) has been reported to be associated with congenital heart disease and increased cancer risk.

NS/LAH

SHOC2 gene

Two patients harbored the SHOC2 mutation c.4A>G (p.Ser2Gly) (2,3), and no other variants causing monogenic SLE were found. The first patient was admitted to the hospital because of growth restriction and cognitive impairment at 2 years old. He also urinated frequently. Physical examination revealed the typical facial manifestations of NS, squinting, a head circumference of 50 cm, loose hair and cryptorchidism. Cardiac ultrasound results indicated ASD. Routine urine analysis did not reveal a urinary tract infection, and bladder ultrasound and radiography indicated thickening of the bladder wall, a large bladder diverticulum and dilated lower ureter. Posterior urethral valvulotomy was performed under a cystoscope. During the follow-up, the patient began to receive growth hormone therapy at age 4.5 years, and his height increased by almost 17 cm for 20 months and then stopped. At 7.5 years, he developed thrombocytopenia and recurrent fever. No malar butterfly rash was found. He was diagnosed with SLE based on antinuclear antibody (ANA) positivity (1:1,000+), American College of Rheumatology (ACR) score (5/11), and Systemic Lupus International Collaborating Clinics (SLICC) score (7/11). After high-dose methylprednisolone treatment, hydroxychloroquine and mycophenolate mofetil were used as maintenance therapies.

The second patient was a 2-year-old girl with growth restriction. She was diagnosed with hypertrophic cardiomyopathy (HCM) (interventricular septum: 6 mm; left ventricular posterior wall: 6.6 mm). An electrocardiogram indicated changes in the T wave. Despite her typical facial features, loose hair, and large head circumference, no further etiological examination was conducted. When she was 6 years old, she was found to have hemolytic anemia, thrombocytopenia, pericardial effusion, pleural effusion, and albuminuria. Her height was −3.8 standard deviation, and her head circumference was 54 cm. Examination showed clubbing of her fingers and heart murmur in the auscultation area of the pulmonary valve (interventricular septum: 8.9 mm). She was also diagnosed with SLE based on ANA positivity (1:1,000+), ACR score (5/11), and SLICC score (7/11). After high-dose methylprednisolone treatment, the dosage was gradually reduced for maintenance.

PPP1CB gene

One boy had a de novo mutation in PPP1CB and presented with nystagmus, squinting, and a tall forehead. He had short stature, absolute macrocephaly, and loose anagen hair. Cardiac ultrasound revealed a patent ductus arteriosus (PDA). The patient was very hyperactive and excited. He carried a mutation in PPP1CB, c.146C>G; p.Pro49Arg. This variant has been previously reported (14).

Discussion

Key findings

In this study, we analyzed a small cohort of patients with short stature and NS caused by sporadic gene mutations. PTPN11 (76%) and SHOC2 (8%) were the most common mutations in NS, whereas the RAF1, KRAS, BRAF, LZTR1, and PPP1CB mutations were less common. We found, though very rare, SHOC2-related NS is associated with a particularly risk of SLE.

Strengths and limitations

Amino acid substitutions at p.Ser2Gly have been shown to lead to a gain of function in SHOC2 and increase RAS/MAPK pathway activation (15). The almost invariant occurrence of the c.4A>G missense change in SHOC2 is mirrored by the relatively homogeneous clinical phenotype of NS/LAH 1 (15,16). In this study, the two patients with SHOC2 mutations presented with SLE at 6 years and 7.5 years old. Autoimmune diseases and autoantibodies are frequently present in patients with RASopathies; subclinical thyroiditis occurs more often, but SLE combined with NS is very rare (17-21). Three other patients with comorbid SLE have been reported, with ages of 13, 13, and 24 years (19-21) (Table 3). The C.4A>G variant in the SHOC2 gene may be a hotspot and a male dominant variant (male:female =4:1). However, this could be coincidental because of the small sample size. Interestingly, all five patients had serositis, but only one patient had classical malar rash. All these findings suggest that SLE caused by mutations in the SHOC2 gene is different from classical SLE. We reported the two youngest patients with the SHOC2 variant who developed SLE. Other cases reported the age of onset of SLE from adolescence to adulthood. Patients with SHOC2 gene variants are more prone to SLE than those with pathogenic variants in other RASopathies. Therefore, attention should be given to monitoring for SLE during long-term follow-up. However, the etiology of this disease remains to be determined.

Comparison with similar researches

A host of studies have shown that RAS/MAPK signaling may affect the peripheral tolerance to prevent autoimmune destruction by self-reactive T cells (22-25). RAS-guanosine triphosphate (GTP) activates several effectors, and increased activation of extracellular-signal-regulated kinase (ERK) and/or PI3K in lymphoid cells could explain this disease (26). Although monogenic SLE is very rare, 3–10% of early-onset SLE cases are caused by recognized damaging variants (27,28), Our two patients suffered very early-onset SLE and typical NS/LAH, and genetic tests confirmed our suspicions of monogenic SLE. Continuing surveillance is of paramount importance in NS patients. The male patient also had a posterior urethral valve; this deformity has not been previously reported in NS/LAH patients, and our findings have enriched the spectrum of manifestations of NS/LAH.

Explanations of findings

Gain-of-function mutations in PTPN11 account for approximately 40–50% of all cases of NS (29,30). In our cohort, 76% of all NS cases were caused by PTPN11 mutations, which is a relatively high percentage compared with that in previous studies. This may be because all of our patients were genetically diagnosed rather than clinically diagnosed. These mutations affect residues located within or close to the interacting surfaces of the PTP and N-SH2 domains. SOS1 (10–20%) and RAF1 (3–17%) (31,32), which are commonly reported mutations, were not found in our cohort.

Implications and actions needed

Genotype-phenotype analysis revealed that congenital heart defects, cryptorchidism, ocular abnormalities, and thoracic anomalies were more prevalent among the group of patients with other mutations than among those with PTPN11 mutations, although these differences were not significant. We observed a significantly greater incidence of other complicating diseases among patients with other mutations than among patients with NS with the PTPN11 mutation. Some features that are considered very suggestive of NS, such as lymphatic abnormalities (33), were not frequently found in our population. The non-PTPN11 mutations tend to affect the backbone of the RAS/MAPK cascade, and this tendency for mutations to affect the central or lower part of the cascade may lead to more severe conditions (6). However, our analyses of genotype-phenotype correlations were underpowered due to the small sample size.

Conclusions

We confirmed the previously observed PTPN11 mutation 922A>G (Asn308Asp), a hotspot mutation in Chinese patients with NS. The development of second-generation sequencing technology has enabled the identification of common and rare genes that cause NS. We conclude that SHOC2 variants are a rare cause of NS combined with SLE. The role of the SHOC2 gene in SLE occurrence needs to be further explored. Larger cohorts are needed to obtain a more accurate clinical definition of each mutation.

Acknowledgments

We would like to thank all the patients and their families.

Funding: The study was supported by the Research Foundation of Capital Institute of Pediatrics (No. LCYJ-2023-27).

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://tp.amegroups.com/article/view/10.21037/tp-24-113/rc

Data Sharing Statement: Available at https://tp.amegroups.com/article/view/10.21037/tp-24-113/dss

Peer Review File: Available at https://tp.amegroups.com/article/view/10.21037/tp-24-113/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tp.amegroups.com/article/view/10.21037/tp-24-113/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the Capital Institute of Pediatrics Ethics Committee (No. SHERLL2023049) and informed consent was taken from their parents.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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