期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2022
卷号：119
期号：6
DOI：10.1073/pnas.2105228119
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
The Rab5 GTPase functions in early endosome (EE) fusion in the endocytic pathway. Here, we propose that RAB5B also has a noncanonical vesicular fusion function in the regulated secretion pathway that produces mature surfactant proteins SP-B and SP-C in the lung. This function was revealed from investigation of a proband with interstitial lung disease suggestive of a surfactant dysfunction disorder who carried a de novo Asp136His variant in the
RAB5B gene. Our modeling in
C. elegans provided information on the genetic and cell biological mechanism, and analyses of proband and normal lung biopsies suggested a function for RAB5B and EEs in surfactant protein processing/trafficking. This work indicates that
RAB5B p.Asp136His causes a surfactant dysfunction disorder.
Pathogenic variants in surfactant proteins SP-B and SP-C cause surfactant deficiency and interstitial lung disease. Surfactant proteins are synthesized as precursors (proSP-B, proSP-C), trafficked, and processed via a vesicular-regulated secretion pathway; however, control of vesicular trafficking events is not fully understood. Through the Undiagnosed Diseases Network, we evaluated a child with interstitial lung disease suggestive of surfactant deficiency. Variants in known surfactant dysfunction disorder genes were not found in trio exome sequencing. Instead, a de novo heterozygous variant in
RAB5B was identified in the Ras/Rab GTPases family nucleotide binding domain, p.Asp136His. Functional studies were performed in
Caenorhabditis elegans by knocking the proband variant into the conserved position (Asp135) of the ortholog,
rab-5. Genetic analysis demonstrated that
rab-5[Asp135His] is damaging, producing a strong dominant negative gene product.
rab-5[Asp135His] heterozygotes were also defective in endocytosis and early endosome (EE) fusion. Immunostaining studies of the proband’s lung biopsy revealed that RAB5B and EE marker EEA1 were significantly reduced in alveolar type II cells and that mature SP-B and SP-C were significantly reduced, while proSP-B and proSP-C were normal. Furthermore, staining normal lung showed colocalization of RAB5B and EEA1 with proSP-B and proSP-C. These findings indicate that dominant negative–acting RAB5B Asp136His and EE dysfunction cause a defect in processing/trafficking to produce mature SP-B and SP-C, resulting in interstitial lung disease, and that RAB5B and EEs normally function in the surfactant secretion pathway. Together, the data suggest a noncanonical function for RAB5B and identify
RAB5B p.Asp136His as a genetic mechanism for a surfactant dysfunction disorder.
