Gastric adenocarcinoma is one of the most common malignant tumours, characterized by unclear symptoms in early stages, poor prognosis and a high metastasis rate (1). In previous decades, with the continuous improvements in surgical operations, radiotherapy, chemotherapy and other treatments, the incidence and mortality rates of gastric cancer have steadily decreased (2). However, gastric cancer is still the fifth most common malignant tumor and the fourth leading cause of cancer-associated death worldwide (3). The high incidence and mortality rates of gastric adenocarcinoma cannot be ignored worldwide and the prognosis of advanced gastric adenocarcinoma remains unsatisfactory. Therefore, the exploration for new treatment strategies and targets for gastric adenocarcinoma have become the primary focuses of oncology research.

In recent years, notable progress has been made in the study of tumour genomics, which has provided an opportunity to gain a deeper understanding of the molecular mechanisms underlying cancer occurrence and development. The Fanconi anaemia complementation group D2 (FANCD2) protein is key in DNA repair and is involved in homologous recombination repair (HRR) process (4). Mutation of the FANCD2 gene has been associated with the occurrence of a variety of tumours, including gastric adenocarcinoma, hereditary bone marrow failure (BMF), spontaneous tumours and hereditary breast and ovarian cancer predisposition syndromes (5). However, further research is warranted to explore the molecular mechanism of FANCD2 and its interaction with tumours.

The purpose of the present case report was to explore the role of FANCD2 gene mutations in the carcinogenesis and development of gastric adenocarcinoma. The present report highlighted a case of poorly differentiated gastric adenocarcinoma caused by a FANCD2 gene mutation and conducted an in-depth analysis through clinical pathology, tumour genomics and bioinformatics analyses. The results provided more information on the relationship between FANCD2 gene mutations and the occurrence and development of gastric adenocarcinoma, and may potentially manifest novel ideas and methods for early diagnosis, personalized treatment and prognostic evaluation of gastric adenocarcinoma in the future.

The patient is a 32-year-old man who presented with vomiting after eating beginning in November 2022, admitted to Shandong Provincial Hosptial (Jinan, China). The vomit was mostly gastric fluid with some gastric contents. The patient complained that within the following week intermittent acid reflux was accompanied by vomiting of gastric juices without vomiting of blood, gastroparesis or black stools. Therefore, 8 days later, the patient underwent an electron gastroscopy examination at Dezhou Municipal Hospital (Jinan, China). The examination indicated an ulcer of ~4.5 cm in diameter on the side of the lesser curvature of the gastric sinus, with congestion and erosion at the margins and white moss on the bottom (Fig. 1A), and a biopsy was taken during examination. The pathological results of the biopsy demonstrated that the gastric mucosa conformed to poorly differentiated adenocarcinoma, some cells appeared as signet-ring cells Helicobacter pylori (+++), staining was performed following our team's established protocol (6).

The patient came to Shandong Provincial Hospital (Jinan, China) for further diagnosis and treatment 2 days later. After admission, gastroscopy, lower abdominal enhanced CT and blood tumor marker detection were completed, the enhanced abdominal CT (November 2022) suggested the following: i) There was slight thickening and abnormal enhancement of the gastric wall in the gastric sinus (Fig. 1B). This finding was consistent with the CT findings of cancer (considered T2 stage), which suggested a tumour; and ii) there were no obvious abnormalities in the chest or lower abdomen.

Subsequently, 9 days later, after important organ dysfunction, extensive abdominal adhesions, coagulation dysfunction, and cardiovascular and cerebrovascular disease were excluded, laparoscopic radical gastrectomy for gastric cancer (distal gastrectomy plus gastrojejunal anastomosis plus Braun anastomosis plus D2 lymph node dissection) was performed under general anaesthesia (November 2022). During the operation, limited thickening of the gastric wall was observed in the gastric sinus without reaching the periphery. There were no enlarged lymph nodes in the perigastric area. The patient was followed up via outpatient visits every half year and the last follow-up was in May 2024. Postoperative pathology with staining performed following our team's established protocol (6) revealed ulcerative poorly differentiated adenocarcinoma (lateral to the lesser curvature of the gastric antrum), Lauren type: Diffuse type. The area was 2.5×2.5 cm and the cut area 2.5×0.5 cm (Fig. 1C). The lesion invaded the deep muscular layer, with focal involvement of nerves and no clear intravascular tumour thrombus was observed. No cancer was detected in ‘upper tangent’, ‘lower tangent’ or lymph node dissection groups. Based on the 8th gastric cancer guidelines formulated with reference to the National Comprehensive Cancer Network (NCCN) Clinical Practice Guidelines in Oncology, the postoperative pathological stage was pT₂N₀M_x (https://www.nccnchina.org.cn/guide/index) (7). After obtaining informed consent from the patient, the tissue samples obtained during surgery were sent for genetic examination. Using adjacent non-cancerous tissue as the control sample, DNA was extracted using an AllPrep DNA Mini Kit (cat. no. 80204; Qiagen GmbH) and Sure Select Human All Exon V6 (cat. no. 5190-8863; Aligent Technologies, Inc.) according to the manufacturer's instructions from both the adjacent non-cancerous tissue and the surgically resected tumour tissue. The Agilent 2100 Bioanalyzer (Agilent, Technologies, Inc.) was used for DNA quality check, and libraries were loaded at 20 pM and measured with an Invitrogen Qubit (Thermo Fisher Scientific, Inc.) for sequencing. The extracted DNA underwent paired-end sequencing using NovaSeq 6000 SP reagent kit (cat. no. 2002746; Illumina, Inc.) and whole-exome sequencing technology on the NovaSeq 6000 platform (Illumina, Inc.) with a read length of 250bp, which covered all exonic regions of the genes, as well as partial intronic and promoter regions. The obtained FASTQ sequencing data were quality controlled and cleaned via Trimmomatic 0.39 (http://www.usadellab.org/cms/?page=trimmomatic) before being mapped to the hg19 human reference genome using BWA 0.7.17-r1188 (https://bio-bwa.sourceforge.net/). GATK 4.3.0.0 (https://gatk.broadinstitute.org/hc/en-us) was employed to identify all possible variations, including single-nucleotide variants, small and large insertions and deletions in DNA, amplifications and deletions of gene copy numbers, as well as gene fusions and rearrangements, among other forms of genetic variations. By comparing the differences between the tumour tissue and the control sample, mutations exclusive to the tumour cells were considered somatic mutations, while those also present in the control sample were regarded as germline mutations. The genetic testing results demonstrated that all exons of 65 genes contained 1 heritable embryogenic pathogenic variant, namely the Insertion and Deletion (InDel) of exon 42 of FANCD2. Specifically, codon 1,378 of the FANCD2 gene, asparagine, is changed to serine and is terminally encoded at codon 5 of a new reading frame, notated as FANCD2 N1378Sfs*5 variant. It has been predicted that the N1378Sfs*5 variant may cause nonsense mutation-mediated mRNA degradation process of FANCD2 in cells, which may result in a functional deletion of the FANCD2 gene (Fig. 2). For immunohistochemical staining for FANCD2 that was subsequently performed on the postoperative tumour tissue sections of the patient, formalin (10%) was applied as the fixative at room temperature for 24 h, followed by embedding in paraffin wax. The sections were cut to a thickness of 5 µm. For permeabilization, 0.3% Triton X-100 was applied for 10 min at room temperature. The blocking was performed with 5% normal goat serum (cat. no. ZLI-9021; ZSGB-BIO) for 30 min at room temperature. The primary antibody used was FANCD2 (cat. no. ab108928; Abcam), diluted at 1:50, and incubated at 4°C overnight. The secondary antibody was from the goat enhanced polymer detection system (cat. no. PV-9003; ZSGB-BIO) used according to the manufacturer's instructions, which includes a polymer horseradish peroxidase and 3,3′-diaminobenzidine substrate for color development. The secondary antibody was incubated at room temperature for 30 min. Light microscopy was used for imaging. Immunohistochemical staining confirmed that the FANCD2 in the patient was a nonsense mutation, which resulted in reduced FANCD2 protein expression levels (Fig. 1D).

The present case report described a 32-year-old male patient with poorly differentiated gastric adenocarcinoma characterized by a mutation in the FANCD2 gene. FANCD2 is a type of DNA damage repair protein that serves a key role in cells by forming complexes with SLX4, FAN1 and other proteins to repair DNA damage (14). Currently, publicly available reports suggest that FANC family mutations can lead to genomic instability and cancer in humans, among which FANCD2 serves an important role in breast cancer, pancreatic cancer, colorectal cancer, glioma and other malignant tumours (15–17). In breast cancer, combined with the results of pathological immunohistochemical staining after surgery, tumour tissue with overexpression of FANCD2 is positively associated with high grade, high proliferation (high Ki-67), HER2 positivity, overexpression of P53 and other classical pathological indicators, which indicate that high levels of FANCD2 may independently predict poor prognosis in patients with breast cancer and, therefore, FANCD2 may be a potential indicator for breast cancer screening (17). In a previous study on pancreatic cancer, ubiquitinated FANCD2 was found to be capable of combining with Fan1 nuclease to affect the process of DNA inter-strand crosslinks (ICLs) and thus leads to genetic instability, which is key in the occurrence, development and prognosis of pancreatic cancer (4). In gastric adenocarcinoma, abnormal expression or mutation of FANCD2 is often associated with malignancy and poor prognosis of the tumour (5,17).

The FANCD2 gene mutation (N1378Sfs*5 exon 42) found in the patient from the present case is an InDel mutation, which involves the transformation of asparagine to serine in the 1,378th codon of the FANCD2 gene and the termination of coding in the 5th codon of the new reading box (13,18–20). It has been predicted that the InDel mutation may cause nonsense mutation-mediated mRNA degradation in the cell, which may lead to functional loss of the FANCD2 gene. The FANCD2 N1378Sfs*5 variant was not included in the normal population database. The N1378Sfs*5 variant was found to be a heritable embryonic pathogenic variant of FANCD2, which further confirmed that the mutation is the fundamental cause of the disease.

Mutations in the FANCD2 gene in gastric cancer are associated with multiple molecular mechanisms. The main possible mechanisms are described below.

The FANC family is mainly involved in DNA repair, especially in the process of inter-strand crosslink (21). In addition to ICLs, the FANC family is also key in the stabilization of the DNA replication forks (22). The FANCD2 gene encodes the FANCD2 protein, which is a core member of the FA pathway. FANCD2 is a DNA damage repair protein involved in the HRR pathway. The FA core complex ubiquitinates the FANCD2-FANCI heterodimer (21), which promotes the recruitment of DNA repair effectors to chromatin damage sites and regulates DNA damage and mitosis (5). When FANCD2 is mutated, it can lead to impaired DNA damage repair ability, which makes cells unable to properly repair DNA double-strand breaks or other types of DNA damage and thereby increases the risk of cell mutation and cancer (14). FANCD2 is also involved in the regulation of apoptosis and survival signalling pathways (23). FANCD2 serves an important role in cell cycle regulation. FANCD2 interacts with cell cycle checkpoint proteins such as RAD51 and breast cancer gene 1 to ensure that cells undergo DNA replication and chromosome separation at the right time (5). Under normal circumstances, FANCD2 can promote apoptosis by activating the P53 signalling pathway (24,25). When FANCD2 is mutated, it may lead to disordered cell cycle regulation and dysregulation of apoptosis signalling pathways, which results in abnormal chromosomal allocation and cell proliferation (5). This may enhance the ability of cells to evade apoptosis and achieve unlimited proliferation.

A previous study has reported that FANCD2 is also associated with immune response regulation (26). In some cases, mutations in FANCD2 may affect the immunogenicity of tumour cells, which enables them to evade immune surveillance or resist immunotherapy (27). Studies have demonstrated that the upregulation of FANCD2 can alleviate the ferroptosis in the glioblastoma cells to promote the resistance to temozolomide. By contrast, knock down of FANCD2 can increase reactive oxygen species levels and inhibit cell survival. These findings suggested that FANCD2 may be a new target for tumour treatment.

In gastric adenocarcinoma, several gene mutations and up- or downregulation can affect the processes of cell growth, differentiation and apoptosis (26). Among them, mutations in DNA repair genes are particularly important. They can affect the ability of cells to repair DNA damage, which lead to insufficient apoptosis and the occurrence of tumours. In the present case report, the FANCD2 gene was identified as a novel gene mutation associated with gastric adenocarcinoma. However, it is important to note that the present case report lacks both in vivo and in vitro experimental validation. The lack of these experimental validations is a notable limitation of the present case report and highlights the need for further research in this area. A deeper understanding of these mechanisms can provide novel ideas and directions for the diagnosis and treatment of gastric adenocarcinoma, enable the development of new treatment strategies for FANCD2 mutations and improve the survival rate and quality of life of patients with gastric cancer.

In summary, the present case report described a case of poorly differentiated gastric adenocarcinoma caused by a mutation in the FANCD2 gene. The InDel mutation in exon 42 of FANCD2, which was mutated to N1378Ss*5, may have contributed to the development of an imprinted cell carcinoma in the gastric antrum in the male patient of the present case study at a young age. This mutation leads to protein instability and functional abnormalities, which thereby promotes the occurrence and development of gastric cancer. To the best of our knowledge, this first identified mutation in FANCD2 is a hereditary germline mutation, which could potentially be a cause of familial gastric cancer. This finding has enhanced the understanding of the molecular mechanisms of gastric adenocarcinoma. The development of specific anti-gastric cancer drugs that target germline mutations in exon 42 of FANCD2 is necessary and urgent.

A previous study has demonstrated the potential role of FANCD2 gene mutations in poorly differentiated gastric adenocarcinomas (26), which provides important clues for further research. For a deeper understanding of these findings and their clinical application, future research should explore how FANCD2 gene mutations affect key biological processes such as DNA repair, cell cycle regulation and apoptosis to elucidate their molecular mechanisms in the development of gastric adenocarcinoma development.

Through collaboration with other health care organizations, further data on patients carrying the FANCD2 gene mutation should be collected to verify whether it is a common cause of gastric adenocarcinoma and to explore its association with clinical features and prognosis. Given that FANCD2 mutations may be a risk factors for familial gastric cancer, genetic counselling and early screening programs should be developed, especially for those with a related family history. Based on the understanding of FANCD2 dysfunction, new drug targets should be sought to develop new therapies or evaluate the efficacy of existing treatments such as PARP inhibitors. In addition, long-term follow-up of similar patients to monitor disease progression and treatment efficacy can help to develop personalized treatment plans. Comprehensive genomic analysis using bioinformatics technology to identify key factors and pathways interacting with FANCD2 and to construct a complete map of the disease network. The study found a novel FANCD2 mutation in gastric adenocarcinoma, highlighting its role in DNA repair. This mutation has clinical significance for diagnosis, prognosis and personalized treatment strategies. These studies enhance our understanding of the FANCD2 gene and its role in the pathogenesis of gastric adenocarcinoma and promote the development of novel diagnostic and therapeutic approaches, which ultimately improves patient survival and their quality of life.