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.

中文翻译：

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导致 Myhre 综合征的 SMAD4 突变在雄性种系中处于正向选择


虽然人们普遍认为从头突变 （DNM） 是随机发生的，但我们之前表明，一些 DNM 是富集的，因为它们在老年男性的睾丸中被正向选择。这些 “自私” 突变导致具有共同特征的疾病，包括完全父系起源、父亲年龄的显著增加以及高表观种系突变率。迄今为止，所有已知的自私突变都聚集在 RTK-RAS-MAPK 信号通路的组成部分中，RTK-RAS-MAPK信号通路是睾丸稳态的关键调节剂。在这里，我们证明了导致 Myhre 综合征 （MYHRS） 的 SMAD4 DNMs 的自私性质。通过分析 16 个信息丰富的三重奏，我们表明在所有情况下，导致 MYHRS 的 DNM 都起源于父系衍生的等位基因。我们记录了 MYHRS 先证者父亲的父辈年龄效应，具有统计学意义，超过了 6.3 岁。我们开发了一种超灵敏的测定法来量化精子中自发性引起 MYHRS 的 SMAD4 变异，并显示密码子 500 处的致病性变异在大多数男性的精子中水平升高，并且与供体的年龄呈很强的正相关，表明表观种系突变率高。最后，我们进行了 体外测定以验证克隆选择的 DNM 的特殊功能行为，并探索了不同 SMAD4 精子富集变体的病理生理学基础。综上所述，这些数据提供了令人信服的证据，表明 SMAD4 是一种在经典 RAS-MAPK 信号通路之外起作用的基因，与自私的精原细胞选择有关，并增加了其他基因/通路在衰老的人类睾丸中处于正选择下的可能性。