Co-Pathogenic Role of BRCA1 and OBSCN Deletions in Chinese Familial Breast Cancer: A Case Report

Introduction

The International Agency for Research on Cancer (IARC) under the World Health Organization (WHO) released global cancer statistics for 2023. The latest data reveals that breast cancer (BC) remains the most common malignancy among women, with 298 000 new cases, accounting for 31% of all cancers among females, and 43 000 deaths, making it the second leading cause of cancer-related mortality after lung cancer, based on United States statistics. These figures underscore the severe threat breast cancer poses to human health [1].

Breast cancer development is influenced by numerous factors, including environmental and genetic components, such as age, diet, body mass index, reproductive history, common oncogenes, breast density, and family history [2]. Family history a well-known factor significantly affecting breast cancer risk, with approximately 5% to 10% of breast cancer cases being familial, involving the inheritance of germline mutations in susceptibility genes across generations [3]. To date, research has identified pathogenic germline mutations in more than 10 susceptibility genes associated with hereditary breast cancer. Based on international research advancements, BRCA1, BRCA2, TP53, and PALB2 are currently recognized as high-penetrance breast cancer susceptibility genes, with mutations in these genes increasing breast cancer risk by at least 5-fold. Moderately penetrant genes, such as CHEK2 and ATM, are associated with 2- to 4-fold increased risk [4]. However, each familial case has a unique genetic profile, contributing to the heterogeneity of breast cancer and explaining why a one-size-fits-all treatment approach is often suboptimal.

NGS (next generation sequencing) technology has significant advantages such as high throughput, high sensitivity, low cost, and rapid detection. It can sequence many DNA or RNA molecules at a time, accurately detect low-frequency mutations, and is widely used in genomics, clinical diagnosis and other fields. The utilization of NGS emerged many years ago in analyzing the germline mutation patterns of breast cancer, demonstrating that BRCA1/2 were the most prevalent hereditary breast cancer mutations [5]. While current screening focuses on BRCA1/2 genes, their study demonstrated that expanding the detection to non-BRCA1/2 genes could increase the identification of hereditary breast cancer cases by 22.2%. Therefore, personalized gene testing using NGS for each family is a precise approach and the most effective diagnostic and therapeutic method for patients.

In this study, we selected a breast cancer family from Liaocheng, China, and employed WES to sequence blood samples from genetically related individuals within the family. In addition to BRCA1, HMMR, and OBSCN were also identified within this pedigree as potential genetic contributors to breast cancer pathogenesis. HMMR (Hyaluronan-Mediated Motility Receptor) is a critical oncogene implicated in the pathogenesis of multiple malignancies. Emerging evidence demonstrates that HMMR expression is governed by multifaceted regulatory mechanisms operating across diverse molecular levels. In breast cancer, research has elucidated the complex regulatory network controlling HMMR expression, thereby offering mechanistic insights into its functional role in breast cancer progression. The OBSCN gene encodes the large cytoskeletal protein Obscurin (720–870 kDa), predominantly expressed in cardiac and skeletal muscle tissues, where it plays a crucial role in myofibril assembly, cellular adhesion, and signal transduction. Notably, in breast cancer, downregulation of Obscurin has been implicated in facilitating epithelial-mesenchymal transition (EMT), augmenting cell migration and invasion, and conferring increased resistance to chemotherapeutic agents.

Most previous studies have indicated single-gene-centered genetic patterns. The present study innovatively proposes oligogenic inheritance patterns. Concurrent phylogenetic alterations in a family can increase the degree of variation in disease penetrance, as these alterations can interact with the existing genetic background in the family and affect expression of the disease. However, given that penetrance variations are influenced by a variety of complex factors, including genetic factors (eg, gene polymorphisms, modified genes), environmental factors, and the interaction between genes and the environment. Phylogenetic alteration is just one possible factor, but not the only determining one. Even if there are concurrent phylogenetic alterations, due to the interference of other factors, its ability to explain the variability within and outside the family may be limited.

This study aimed to identify the pathogenic gene responsible for the disease in this family, providing insights into the germline factors associated with breast cancer development. Overall, these analyses may identify key molecular targets that could be utilized for genetic screening, diagnostic efforts, and the treatment of individuals at risk for familial breast cancer.

Case Report

FAMILY CHARACTERISTICS:

This study evaluated 32 members of a family, of whom 29 are currently living and 3 have died. Two individuals from the first generation and 1 from the second generation died from unknown causes. Blood samples were collected from 8 family members, including breast cancer patients III-8, III-13, and III-14, as well as healthy individuals II-2, II-3, II-6, III-10, III-14, and III-16 (Figure 1). According to the patient, none of the non-affected carriers (eg, II-2, II-6, III-10) had any other abnormal clinical indicators; therefore, there was no impact on the determination of hereditary breast cancer. These samples were used for WES analysis. The Ethics Review Committee of Liaocheng Hospital approved this study, and all participants provided informed consent prior to blood sample collection. The proband, III-14 (aged 37), was pathologically diagnosed with invasive ductal carcinoma, WHO grade 3. Immunohistochemistry (IHC) results were ER (−), PR (−), HER-2 (1+), Ki-67 (20%+), CK5/6 (−), P63 (scattered +), E-Cadherin (+), AR (−), and GCDFP-15 (−), leading to a diagnosis of triple-negative breast cancer (TNBC) (Figure 2). The breast ultrasound examination revealed a solid hypoechoic nodule in the right breast (10 o’clock position, 2.2×1.6×1.7 cm), BI-RADS category 4b, right axillary lymph node visualized (Figure 3).

IDENTIFICATION OF PATHOGENIC GENES IN THE FAMILY:

WES analysis of the collected samples identified heterozygous deletions in 3 genes: BRCA1, HMMR, and OBSCN. Further analysis, considering the clinical characteristics of the patients, revealed that those with breast cancer had concurrent heterozygous deletions in BRCA1 and OBSCN, while individuals with isolated heterozygous deletions of BRCA1, HMMR, or OBSCN, or with other genetic profiles, showed no abnormal clinical features (Table 2). III-8 has breast cancer, and the test results show a co-deletion of BRCA1 and OBSCN. The situation in Section III-13 was the same as that in Section III-8. These 2 individuals had the closest genetic relationship with the proband, and the results of their genetic sequencing are largely consistent. III-10 was normal and only showed a heterozygous deletion of HMMR. These findings suggest that the co-occurrence of heterozygous deletions in BRCA1 and OBSCN was the underlying cause of breast cancer in this family. To validate the mutations identified in these 3 genes, we designed specific primers for the mutation sites and performed Sanger sequencing. The Sanger sequencing chromatograms for the proband and a control individual without mutations were compared, showing complete concordance between the Sanger sequencing results and the bioinformatic analysis (Figure 4).

SEQUENCE AND STRUCTURAL ANALYSIS OF BRCA1 AND OBSCN GENE VARIANTS:

DNAMAN software was used to perform amino acid sequence homology comparisons, revealing that the BRCA1: 1841Ser and OBSCN: 18525Thr and 18526Gly residues are highly conserved across multiple species. The OBSCN variant identified in this study has not been previously reported in major genomic databases (eg, gnomAD, ClinVar), underscoring its novelty. This variant is predicted to cause a major structural disruption, based on computational analyses (Figure 5A, 5B). Protein structural analysis showed that a serine-to-valine substitution at position 1841 in the BRCA1 protein results in a premature stop codon, terminating further amino acid synthesis (Figure 5C, 5D). Additionally, due to the deletion of 2 bases, the OBSCN protein undergoes substantial structural changes beyond the 6122nd amino acid; therefore, the detailed structure is not depicted here. While the observational evidence (segregation pattern, evolutionary conservation, and predicted truncation) supports the association of this variant with the phenotype, the mechanistic inference regarding structural disruption remains speculative and requires further experimental validation.

Discussion

Breast cancer is the most prevalent malignancy among females globally. Approximately 10% of breast cancer cases are attributed to pathogenic germline mutations in established susceptibility genes, classified as hereditary breast cancer. This form of malignancy frequently demonstrates familial aggregation, with 2 or more primary cases of breast and/or ovarian cancer occurring among first- to third-degree relatives, thus it is termed familial breast cancer. Familial breast cancer exhibits unique pathogenic mechanisms and distinct clinicopathological characteristics compared to sporadic breast cancer. Key features include a higher incidence among multiple family members, earlier disease onset, and an elevated likelihood of contralateral or bilateral breast malignancies. Furthermore, affected individuals face an increased risk of developing other associated cancers. As a result, preventive measures, diagnostic protocols, and therapeutic strategies for familial breast cancer necessitate tailored approaches that differ from those employed in sporadic cases [6].

Through comprehensive sequencing analysis of a selected familial cohort, we identified 3 distinct mutation sites. By integrating these genomic findings with Sanger sequencing validation and clinical phenotypic data, we established that compound heterozygous deletions of BRCA1 and OBSCN were the genetic predisposition factors in this familial case. BRCA1, a well-characterized tumor suppressor gene, has been extensively documented as a breast cancer susceptibility locus, with its pathogenic variants strongly associated with early disease onset, familial aggregation patterns, and an elevated incidence of triple-negative breast cancer (TNBC) [7]. The germline nature of these mutations suggests potential vertical transmission within the pedigree, placing first-degree relatives at significant risk of inheriting these deleterious variants. From a population health perspective, pathogenic BRCA1/2 variants confer substantially increased lifetime risks for multiple malignancies. Current clinical guidelines recommend that carriers of pathogenic or likely pathogenic BRCA variants adhere to rigorous surveillance protocols and enhanced preventive measures [8–10].

The OBSCN gene encodes a large, multi-domain cytoskeletal protein initially discovered in striated muscle, where it plays a crucial role in structural organization and contraction. The human OBSCN gene spans 150 kb on chromosome 1 and produces at least 4 isoforms through extensive splicing. OBSCN mutations have been observed across various cancer types [11]. The giant cytoskeletal protein encoded by OBSCN (720 to 870 kDa) is associated with breast cancer susceptibility and progression [12]. OBSCN interacts with several cancer-related genes involved in breast tumorigenesis and may regulate disease progression and metastasis. Molecular characteristics of OBSCN, such as copy number alterations and gene expression levels, can serve as tools for identifying breast cancer development and metastasis [13]. The mutation type OBSCN (NM_001386125.1: c.18523+2_18523+3delTG) has not been included or reported in the database (COSMIC and ClinVar). During our analysis of the structure of the OBSCN mutant protein, we discovered that the absence of 2 base pairs led to significant variations in the protein structure. This type of mutation is extremely rare and there are no relevant reports. However, through sequencing and verification of relevant samples, we did indeed find that some patients did have this mutation of OBSCN (eg, III-14). By searching the literature and known databases, we found that the reported mutation types of OBSCN are mostly caused by single-nucleotide mutations. Although this mutation type has only been identified in this family at present, no functional assays or validation with a larger sample have been conducted. Functional assays need to be further studied. Validation with a larger sample also requires time accumulation. However, based on the current results, it is possible to infer the pathogenesis of this family.

While HMMR was not established as a pathogenic variant in this breast cancer pedigree, the literature supports its classification as a breast cancer susceptibility gene associated with elevated risk [14,15]. Functionally, HMMR demonstrates a biological relationship with BRCA1, and both genes are implicated in the regulation of epithelial cell polarization, supporting HMMR’s candidacy as a breast cancer-associated gene [16,17]. Notably, despite the detection of HMMR variants in certain family members, no clinical manifestations of disease were observed, suggesting that HMMR’s potential contribution to breast cancer pathogenesis is context-dependent and not a principal determinant in this family.

Oligogenic inheritance explains how the combination of multiple intermediate-effect genes jointly affects disease risk, rather than the decisive role of a single gene. Modified gene theory explains how modified genes regulate the expression and penetrance of diseases by influencing the expression, function, or intracellular environment of major genes. Some variations of OBSCN occur more frequently in breast cancer patients than in healthy controls and are more common in BRCA1 mutation carriers, supporting its role as a synergistic or modifying factor.

In summary, our findings demonstrate an association between BRCA1 and OBSCN mutations and breast cancer susceptibility within this familial cohort. BRCA1–OBSCN interaction is a putative oligogenic or modifying-gene model, not definitive proof of dual-gene causation. Further investigation is required to delineate the precise mechanistic roles of these genetic variants in the pathogenesis of familial breast cancer. These results contribute novel insights into the genetic architecture of hereditary breast cancer and establish a foundation for subsequent research on inherited cancer predisposition and early detection strategies in high-risk populations.

Conclusions

Our study, using WES, bioinformatics analysis, and Sanger validation, suggests that the heterozygous deletion of BRCA1 and OBSCN was the pathogenic driving gene for this family. This case suggests that concurrent germline alterations in BRCA1 and OBSCN can together underlie breast cancer susceptibility within a single family, highlighting a potential oligogenic model of hereditary breast cancer. While BRCA1 remains the primary driver, rare coexisting variants in cytoskeletal or signaling genes such as OBSCN can modify disease expression and penetrance, underscoring the value of comprehensive genomic analysis in familial cancer risk assessment. These results can provide genetic counseling and guidance recommendations for the family’s breast cancer cases, and can further enrich the data on breast cancer in China.