SMAD4 mutations causing Myhre syndrome are under positive selection in the male germline.

While it is widely thought that de novo mutations (DNMs) occur randomly, we previously showed that some DNMs are enriched because they are positively selected in the testes of aging men. These "selfish" mutations cause disorders with a shared presentation of features, including exclusive paternal origin, significant increase of the father's age, and high apparent germline mutation rate. To date, all known selfish mutations cluster within the components of the RTK-RAS-MAPK signaling pathway, a critical modulator of testicular homeostasis. Here, we demonstrate the selfish nature of the SMAD4 DNMs causing Myhre syndrome (MYHRS). By analyzing 16 informative trios, we show that MYHRS-causing DNMs originated on the paternally derived allele in all cases. We document a statistically significant epidemiological paternal age effect of 6.3 years excess for fathers of MYHRS probands. We developed an ultra-sensitive assay to quantify spontaneous MYHRS-causing SMAD4 variants in sperm and show that pathogenic variants at codon 500 are found at elevated level in sperm of most men and exhibit a strong positive correlation with donor's age, indicative of a high apparent germline mutation rate. Finally, we performed in vitro assays to validate the peculiar functional behavior of the clonally selected DNMs and explored the basis of the pathophysiology of the different SMAD4 sperm-enriched variants. Taken together, these data provide compelling evidence that SMAD4, a gene operating outside the canonical RAS-MAPK signaling pathway, is associated with selfish spermatogonial selection and raises the possibility that other genes/pathways are under positive selection in the aging human testis.

Gain-of-function variants in SMAD4 compromise respiratory epithelial function.

BACKGROUND: Myhre syndrome is an exceedingly rare yet increasingly diagnosed genetic disorder arising from germline variants in the SMAD4 gene. Its core manifestation is the progression of stiffness and fibrosis across multiple organs. Individuals with Myhre syndrome exhibit a propensity for upper respiratory tract remodeling and infections. The molecular and cellular mechanisms underlying this phenotype remain unclear. OBJECTIVE: We sought to investigate how SMAD4 pathogenic variants associated with Myhre syndrome affect SMAD4 protein levels, activation, and physiological functions in patient-derived nasal epithelial cells. METHODS: Clinical observations were conducted on a cohort of 47 patients recruited at Massachusetts General Hospital from 2016 to 2023. Nasal epithelial basal cells were isolated and cultured from inferior turbinate brushings of healthy subjects (n = 8) and patients with Myhre syndrome (n = 3; SMAD4-Ile500Val, Arg496Cys, and Ile500Thr). Transcriptomic analysis and functional assays were performed to assess SMAD4 levels, transcriptional activity, and epithelial cell host defense functions, including cell proliferation, mucociliary differentiation, and bacterial elimination. RESULTS: Clinical observations revealed a prevalent history of otitis media and sinusitis among most individuals with Myhre syndrome. Analyses of nasal epithelial cells indicated that SMAD4 mutations do not alter SMAD4 protein stability or upstream regulatory SMAD phosphorylation but enhance signaling transcriptional activity, supporting a gain-of-function mechanism, likely attributable to increased protein-protein interaction of the SMAD complex. Consequently, Myhre syndrome nasal basal cells exhibit reduced potential in cell proliferation and mucociliary differentiation. Furthermore, Myhre syndrome nasal epithelia are impaired in bacterial killing. CONCLUSIONS: Compromised innate immunity originating from epithelial cells in Myhre syndrome may contribute to increased susceptibility to upper respiratory tract infections.

Emergence of the natural history of Myhre syndrome: 47 patients evaluated in the Massachusetts General Hospital Myhre Syndrome Clinic (2016-2023).

Myhre syndrome is an increasingly diagnosed ultrarare condition caused by recurrent germline autosomal dominant de novo variants in SMAD4. Detailed multispecialty evaluations performed at the Massachusetts General Hospital (MGH) Myhre Syndrome Clinic (2016-2023) and by collaborating specialists have facilitated deep phenotyping, genotyping and natural history analysis. Of 47 patients (four previously reported), most (81%) patients returned to MGH at least once. For patients followed for at least 5 years, symptom progression was observed in all. 55% were female and 9% were older than 18 years at diagnosis. Pathogenic variants in SMAD4 involved protein residues p.Ile500Val (49%), p.Ile500Thr (11%), p.Ile500Leu (2%), and p.Arg496Cys (38%). Individuals with the SMAD4 variant p.Arg496Cys were less likely to have hearing loss, growth restriction, and aortic hypoplasia than the other variant groups. Those with the p.Ile500Thr variant had moderate/severe aortic hypoplasia in three patients (60%), however, the small number (n = 5) prevented statistical comparison with the other variants. Two deaths reported in this cohort involved complex cardiovascular disease and airway stenosis, respectively. We provide a foundation for ongoing natural history studies and emphasize the need for evidence-based guidelines in anticipation of disease-specific therapies.

An additional patient with SMAD4-Juvenile Polyposis-Hereditary hemorrhagic telangiectasia and connective tissue abnormalities: SMAD4 loss-of-function and gain-of-function pathogenic variants result in contrasting phenotypes.

Loss-of-function pathogenic variants in somatic and germline cells in SMAD4 may cause cancer and juvenile polyposis-Hereditary Hemorrhagic Telangiectasia (SMAD4-JP-HHT), respectively. In a similar manner, gain-of-function somatic and germline pathogenic variants in SMAD4 can cause various forms of cancer as well as Myhre syndrome. The different SMAD4 molecular mechanisms result in contrasting clinical phenotypes demonstrated by SMAD4-JP-HHT and Myhre syndrome. We report an additional patient with SMAD4-JP-HHT and aortopathy, and expand the phenotype to include severe valvulopathy, cutaneous, ophthalmologic, and musculoskeletal features consistent with an inherited disorder of connective tissue. We compared this 70-year-old man with SMAD4-JP-HHT to 18 additional literature cases, and also compared patients with SMAD4-JP-HHT to those with Myhre syndrome. In contrast to aorta dilation, hypermobility, and loose skin in SMAD4-JP-HHT, Myhre syndrome has aorta hypoplasia, stiff joints, and firm skin representing an intriguing phenotypic contrast, which can be attributed to different molecular mechanisms involving SMAD4. We remind clinicians about the possibility of significant cardiac valvulopathy and aortopathy, as well as connective tissue disease in SMAD4-JP-HHT. Additional patients and longer follow-up will help determine if more intensive surveillance improves care amongst these patients.

Myhre syndrome in adulthood: clinical variability and emerging genotype-phenotype correlations.

Myhre syndrome (MS, MIM 139210) is a rare multisystemic disorder caused by recurrent pathogenic missense variants in SMAD4. The clinical features have been mainly documented in childhood and comprise variable neurocognitive development, recognizable craniofacial features, a short stature with a pseudo-muscular build, hearing loss, thickened skin, joint limitations, diverse cardiovascular and airway manifestations, and increased fibrosis often following trauma or surgery. In contrast, adults with MS are underreported obscuring potential clinical variability. Here, we describe 24 adults with MS, including 17 diagnosed after the age of 18 years old, and we review the literature on adults with MS. Overall, our cohort shows a milder phenotype as well as lower mortality rates compared to what has been published in literature. Individuals with a codon 500 variant in SMAD4 present with a more pronounced neurodevelopmental and systemic phenotype. However, in contrast to the literature, we observe cardiovascular abnormalities in individuals with the p.(Arg496Cys) variant. In addition, we describe scoliosis as a new manifestation and we report fertility in two additional males with the p.(Arg496Cys). In conclusion, our study contributes novel insights into the clinical variability of MS and underscores the importance of variant-specific considerations, and we provide recommendations for the management of MS in adulthood.