Genotype-Phenotype Relationship in a Large Cohort of Osteogenesis Imperfecta Patients with COL1A1 Mutations Revealed by a New Scoring System
Osteogenesis imperfecta (OI) is a rare heritable bone disorder characterized by decreased bone mineral density (BMD), impaired bone strength, recurrent fractures, and progressive bone deformity. It affects approximately 1 in 15,000 to 20,000 births. OI is traditionally classified into four types (I, II, III, and IV), ranging from mild to severe forms. However, the clinical heterogeneity of OI makes it challenging to quantify its severity using the traditional classification system. The pathogenesis of OI is closely linked to mutations in the COL1A1 and COL1A2 genes, which encode the alpha chains of type I collagen, the most abundant bone matrix protein. These mutations lead to either qualitative or quantitative defects in type I collagen, resulting in the clinical manifestations of OI.
This study aimed to investigate the spectrum of COL1A1 mutations in a large cohort of Chinese patients with OI and to quantitatively assess the genotype-phenotype relationship using a newly developed clinical scoring system. The study included 161 patients diagnosed with OI at the Peking Union Medical College Hospital between January 2010 and December 2017. The patients’ COL1A1 mutations were identified using next-generation sequencing and confirmed by Sanger sequencing. A clinical scoring system was developed to evaluate the severity of OI, and statistical analyses were performed to explore the relationship between genotypes and phenotypes.
The study identified a diverse spectrum of COL1A1 mutations, including missense mutations (32.9%), nonsense mutations (16.8%), splice-site mutations (24.2%), frameshift mutations (24.8%), and whole-gene deletions (1.2%). Among these, 38 mutations were novel, expanding the known pathogenic spectrum of COL1A1. The mutations resulted in 53 patients carrying qualitative defects and 67 patients carrying quantitative defects in type I collagen. Patients with qualitative mutations exhibited more severe phenotypes, including shorter stature, lower BMD, higher fracture frequency, more bone deformities, vertebral compressive fractures, limited movement, and dentinogenesis imperfecta (DI), compared to those with quantitative mutations.
The clinical scoring system developed in this study included eight criteria: number of fractures, fracture frequency, vertebral compressive fractures, scoliosis, bowing, height Z-score, BMD Z-score, and limited movement. Higher scores indicated more severe clinical manifestations. The study found that patients with qualitative mutations had significantly higher clinical scores (12.2 ± 5.3) than those with quantitative mutations (7.4 ± 2.4), indicating more severe disease. Additionally, patients with qualitative mutations had lower serum alkaline phosphatase (ALP) levels, suggesting impaired osteoblast function.
The study also explored the relationship between specific mutations and clinical features. For example, glycine substitutions in the triple helix of the alpha-1 chain of type I collagen were common in patients with qualitative mutations. Among these, serine substitutions were the most frequent (57.1%), followed by arginine (19.0%), alanine (7.1%), and others. The study found that patients with glycine substitutions before the 79th amino acid in the triple helix did not present with DI, providing valuable insights into the genotype-phenotype correlation.
The study’s findings highlight the importance of distinguishing between qualitative and quantitative mutations in COL1A1 for predicting the clinical severity of OI. Qualitative mutations, which result in structural abnormalities in type I collagen, are associated with more severe bone lesions compared to quantitative mutations, which reduce the amount of type I procollagen synthesis by half. This distinction is crucial for genetic counseling and prognosis prediction in patients with OI.
The study also identified several hotspot mutations in COL1A1, such as c.769G>A (p.Gly257Arg) and c.1678G>A (p.Gly560Ser), which had been reported multiple times in previous studies. These findings contribute to a better understanding of the genetic basis of OI and may aid in the development of targeted therapies.
In addition to genetic analysis, the study evaluated the clinical characteristics of patients with OI. Patients with type III OI, the most severe form, had the earliest onset of symptoms (median age of 0.5 years) and the highest clinical scores (16.0 ± 3.9). In contrast, patients with type I OI, the mildest form, had the lowest clinical scores (6.6 ± 2.0). The study also found that blue sclera was a common feature in all types of OI, while hearing impairment was rare in pediatric patients.
The study’s limitations include its cross-sectional design, which prevents the assessment of the long-term efficacy of treatments such as bisphosphonates. Additionally, functional experiments were not performed to verify the effects of splice-site mutations on protein structure. Future studies should explore the impact of COL1A1 mutations on bone microstructure and biomechanical properties.
In conclusion, this study provides a comprehensive analysis of the genotype-phenotype relationship in a large cohort of Chinese patients with OI. The identification of 38 novel mutations in COL1A1 expands the known pathogenic spectrum of the disease. The clinical scoring system developed in this study offers a quantitative method for assessing the severity of OI, which is valuable for genetic counseling and prognosis prediction. The findings underscore the importance of distinguishing between qualitative and quantitative mutations in COL1A1 for understanding the clinical manifestations of OI and guiding treatment strategies.
doi.org/10.1097/CM9.0000000000000013
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