Breast cancer is a type of cancer that originates in the cells of the breast [1]. It usually begins as a small confined tumor that can spread to other parts of the body if not treated early [2]. It encompasses various types of carcinomas including ductal and lobular carcinomas. Common symptoms of breast cancer include a lump or mass of the tumor in the breast area, changes in breast size or shape, skin changes (dimpling), nipple changes (inversion or discharge), and persistent breast discomfort [3, 4].

According to Globocan Cancer Data Statistics 2022, breast cancer is the second most diagnosed cancer worldwide, accounting for one in every four cancer cases and one in every six cancer deaths. The registered number of breast cancer statistics in global cancer data for 2022 is around 2,308,897 cases and 656,84 mortalities worldwide [5]. Several risk factors contribute to the development of breast cancer, including environmental factors, hormonal influences, genetic factors, and lifestyle choices (alcohol consumption, obesity, and lack of physical activity) [6].

Genetic mutations play a crucial role in the etiology of breast cancer. Some common genetic mutations associated with breast cancer include BRCA1, BRCA2, TP53, PALB2, and ATM [7]. Alongside these extensively studied genes, recent studies have associated genetic mutations within ESR1, HER1, and HER2 genes in breast cancer [8].

The estrogen receptor1 (ESR1) gene encodes estrogen receptor α (ERα), a key protein expressed in most breast cancers and serves as an important biomarker in tumors [9]. ERα regulates breast cancer risk by affecting estrogen metabolic pathways, and is essential for normal breast epithelial growth and differentiation. Variations in ESR1, particularly single nucleotide polymorphisms (SNPs), have been studied exclusively to understand their association with breast cancer [10]. The two most studied polymorphisms in the ESR1 gene are rs2234693 and rs2046210 [11, 12].

The rs2234693 polymorphism, located in intron one, is a well-characterized SNP associated with an increased risk of breast cancer and other estrogen-related diseases [13]. This variant may affect breast cancer risk by altering transcription factor-binding sites and affecting alternative splicing of the ESR1 gene, thereby altering gene expression [14]. ESR1 and rs2046210 are located at the 6q25.1 locus on chromosome 6, approximately 29 kb upstream of ESR1 [15]. Its proximity suggests a potential role in modulating ESR1 expression, possibly influencing the proliferation of breast epithelial cells, and contributing to breast tumorigenesis [16].

HER1, also known as ErbB1 or EGFR, is a transmembrane receptor protein involved in important cellular processes such as cell division, growth promotion, apoptosis inhibition, and angiogenesis stimulation [17]. Activation occurs when ligands, such as transforming growth factor alpha (TGF alpha), bind to its extracellular domain, leading to autophosphorylation of tyrosine kinase residues in the intracellular domain. This activation initiates a cascade of downstream signals, including the MAPK and PI3K pathways, which regulate cell proliferation and survival [18]. SNP rs11543848, also known as HER1 R497K, involves a single nucleotide substitution (142285G > A) at codon 521 of the extracellular subdomain IV of the EGFR gene, leading to an arginine (R) to lysine (K) change [19]. This genetic variation may affect EGFR-mediated signaling pathways, which may further affect cancer susceptibility and treatment responses, including implications for breast cancer research [20].

HER2 is a proto-oncogene located on chromosome 17q12 that encodes a 185 kDa transmembrane glycoprotein involved in regulating cell growth and differentiation [21]. Unlike other members of this family, HER2 is constitutively active and independent of ligand binding [22]. Dimerization of HER3 or HER4 triggers transphosphorylation of the intracellular tyrosine kinase domain, thereby initiating pathways such as MAPK and PI3K [23]. Overexpression or amplification of HER2 occurs in 20–25% of breast cancers, correlating with aggressive tumor behavior, increased metastasis, and therapy insensitivity [24]. The rs1136201 SNP, known as I655V, causes a valine-to-isoleucine substitution in the transmembrane domain of the HER2 protein [25]. This polymorphism in the transmembrane region of HER2 is associated with a higher risk of breast cancer [26]. This amino acid change promotes enhanced dimerization, tyrosine kinase activity, and autophosphorylation, thereby initiating cell transformation [21].

This study aimed to investigate the association of ESR1 (rs2234693 and rs2046210), HER1 (rs11543848), and HER2 (rs1136201) polymorphisms with breast cancer risk in the Pashtun population. Previous studies have identified genetic markers for this demographic [27, 28]. However, the purpose of this research was to provide strong validation and deep insight through a comprehensive analysis of larger datasets.