After the molecular characterization of breast tumors published by Perou et al. (23), differences in the distribution of intrinsic subtypes by ethnic groups have been widely reported (8, 24). It is well known that African American (AA) women are more likely to develop TN breast cancer, whereas NHW women have higher odds for luminal-like subtypes (25). Regarding HER2-positive tumors, its distribution among populations is less clear ( Tables 1 , 2 ).

Few studies have reported differences in ERBB2 gene expression and its molecular features by population groups. Kan et al. (53) conducted a study to evaluate molecular differences between an Asian cohort of women from Korea (n=187) with breast cancer, and a group of Caucasians (n=745) and AAs (n=158) from The Cancer Genome Atlas (TCGA) database, and showed a higher frequency of somatic alterations (mutations and copy number variations (CNVs)) in ERBB2 in the Asian cohort (20% vs. 9.1%, respectively), along with a higher proportion of HER2-enriched (8% vs 2.9%, respectively) and luminal/HER2 subtypes (14.4% vs. 10.4%, respectively) in Asian women, compared to the TCGA group (Caucasians + AAs). Similarly, Pan et al. (12) reported a higher level of amplifications in the ERBB2 region (17q12) in a cohort of 560 Asian women from Malaysia, along with a higher prevalence of the HER2-enriched molecular subtype assessed by PAM50 in Asian women compared to a group of Caucasians from the TCGA (23.3% vs 9.9%, respectively).

In terms of gene expression, Grunda et al. (54) reported a higher expression of the ERBB2 gene in Caucasians when compared to AA breast cancer patients (ERBB2 mean fold change (FC) in Caucasians: 1.61 vs AAs: 0.63, p=0.012), and other genes associated with prognosis (ESR1 and GATA3), disease progression (HSPB1 and SERPINA3) and response to chemotherapy (CLDN7 and DLC1). Even though the epidemiological studies described above usually report higher prevalence of HER2-positive tumors among AAs compared to NHWs, it is possible that Grunda’s et al. (54) conflicting results are related to the small sample size of the study (AAs= 11, Caucasians=11). It is also worth mentioning that the later study only assessed gene expression, whereas prevalence studies reported above are based on HER2 protein expression. Therefore, it is possible that several post-transcriptional mechanisms might be exerting an effect on ERBB2/HER2 expression, this way contributing to the reported differences between these population groups (55, 56).

Self-reported ethnicity comprehends a heterogeneous group with mixed ancestries (57). This has drawn attention to the importance of analyzing breast cancer molecular features by genomic estimated race or genetic ancestry. This estimation is based on the analysis of ancestry-informative markers (AIMs), which capture differences in allele frequencies across major continental populations. This approach allows ancestry to be quantified as an objective and continuous variable, enabling researchers to assess how varying proportions of European, Indigenous American (IA), African, and Asian ancestry may correlate with disease phenotypes (44).

This approach was applied in a recent study by Miyashita et al. (58), which assessed the genetic ancestry of 3,433 breast cancer patients from the Tempus Database. The study differentiated African ancestry patients (>20% African, <10% IA ancestry, and a combined African plus European likelihood >70%) from European ancestry patients (>80% European and <10% IA ancestry). Interestingly, the study found a higher enrichment of several hallmark and oncogenic gene signature sets in the European cohort compared to African ancestry patients, including the ERBB2 gene set ERBB2_UP.V1_UP, which refers to a group of genes defined in the Gene Set Enrichment Analysis (GSEA) database that are consistently upregulated in the context of ERBB2 pathway activation. However, this finding was only observed in stage IV tumors with an HR+/HER2- subtype. Although the enrichment of this gene set does not necessarily indicate that the ERBB2 gene itself is expressed, it reflects the upregulation of genes associated with ERBB2 activation and signaling. In this context, these findings strongly suggest the presence of important differences in the tumor biology of breast cancer patients, potentially driven by their genetic ancestry.

A study conducted in Latinas from Colombia by Serrano-Gomez et al. (16) assessed the association between genetic ancestry and gene expression in 42 women with luminal breast tumors. When patients were stratified according to the average fraction of IA ancestry (low IA <36%, high IA ≥ 36%), the high IA ancestry group showed a higher expression of ERBB2, along with other genes located at the HER2 amplicon, such as GRB7 and MIEN1. In line with these results, Marker et al. (21) reported an association between the HER2-enriched subtype and the IA ancestry fraction in a cohort of 1,312 Peruvian women with breast cancer. They reported that the odds of presenting a HER2-enriched subtype increased by a factor of 1.30 per every 10% increase in IA ancestry (p=0.004) after adjusting for age at diagnosis, African ancestry fraction and height. This association was replicated in other sets of patients from Mexico (OR =1.20; 95% CI, 0.90–1.59) and Colombia (OR = 1.28; 95% CI, 1.03–1.60). In the same way, a separate study in Colombian women that included breast cancer patients from different health institutions around the country also reported a suggestive association between the IA ancestry and both HER2-enriched (OR =1.18; 95% CI, 0.50 - 2.67) and luminal B/HER2 subtypes (OR =1.61; 95% CI, 0.84 - 3.09), after adjusting for health institution, age at diagnosis, and clinical stage (59). This positions genetic ancestry as a potentially valuable tool in the clinical setting, as women with a higher contribution of IA ancestry may exhibit more frequent ERBB2 overexpression, potentially making them more responsive to HER2-low targeted therapies, such as antibody-drug conjugates (ADCs) like T-DM1 and T-DXd.

Even though only a few studies have assessed ERBB2 differential expression between population groups, the results gathered so far suggest that there are differences in gene expression by genetic ancestry that may account for differences in the biology, prognosis, and outcomes of breast cancer between population groups (52). For instance, epidemiological studies comparing population groups have reported that Latina women are more likely to present clinical characteristics associated with poor prognosis, such as being diagnosed at advanced stages (60). In line with this, a higher risk of breast cancer mortality has also been reported for Latina women compared to NHW women (61), which may ultimately be related to a higher frequency of more aggressive subtypes, like HER2-positive tumors. It is possible that variations in the distribution of the HER2-positive subtypes by ethnicity could be partly related to the presence of genetic variants.

Genetic variants are defined as specific changes in a genomic region that are partly responsible for phenotype differences reported between population groups, such as differences in susceptibility to various diseases, including cancer (62). Genetic variants or SNPs in ERBB2 have been associated with breast cancer risk (63–65), therapy response and resistance to anti-HER2 treatments (66, 67). Given that most of the genetic variants reported so far in ERBB2 are located at the transmembrane domain coding region, their main effect is related to changes in HER2 protein activity (68). In addition to this, other studies have found differences in HER2 expression according to the ERBB2 SNPs genotype ( Table 3 ).

Su et al. (71) evaluated the expression of the HER2 protein according to the SNP rs1058808 genotype, a genetic variant (C>G) located at the ERBB2 gene residue 1170 that encodes for either a proline (C allele) or alanine (G allele) at the C-terminal region of the HER2 protein (69). This study that included 303 Chinese women reported that patients with the C/G and G/G germline genotypes had a higher frequency of HER2-positive tumors, compared to patients with the C/C genotype (C/G: 58.6% and G/G: 26.3% vs. C/C: 15.2%, p = 0.007). This difference was also statistically significant under the dominant model (C/G + G/G: 84.8% vs. C/C: 15.2%, p = 0.003). Nonetheless, opposite results were reported in a European population, in a study that included 361 breast cancer patients from the United Kingdom. They analyzed germline and tumor genotype of numerous SNPs, including the rs1058808 and HER2 expression by IHC and found a significantly higher proportion of HER2 positive tumors in the proline carriers (either C/G or C/C), compared to the alanine carriers (G/G genotype) (56% vs. 43%, respectively, p=0.015) (17). These contradictory results might be partly explained by the differences in the rs1058808-G minor allele frequency (MAF) between both populations (1000Genomes project (1KGP): Asians G=0.4018 vs Europeans G=0.6730) (72), coupled with the flip-flop phenomenon, where the same allele confers risk in one population but is protective in another (73), although it is also possible that sample size differences between both studies might contribute as well to these conflicting results. On the other hand, these studies only assessed the differences in the proportion of HER2 positive cases according to rs1058808 genotype, therefore, it is likely that other biological factors, meaning additional genetic variants beyond rs1058808, might also contribute to the expression of the HER2 protein in these diverse populations.

The same genetic variant (rs1058808) was also evaluated in another study by Han W et al. (74) in 90 breast cancer women from South Korea. The association with HER2 expression was assessed for the rs1058808 SNP and at the same time, for five other SNPs at the ERBB2 gene, all together as a haplotype. They reported that patients carrying the germline haplotype configuration I, which includes the rs1058808-C allele, were 1.5 times more likely to develop tumors with HER2 overexpression, compared to patients without this specific genotype (OR=1.5; 95% CI, 1.11-2.16, p= 0.009). This haplotype also included the rs1136201 SNP (I655V), a genetic variation (A>G) that leads to an isoleucine (I)-to-valine (V) substitution at codon 655 within the transmembrane domain of the ERBB2 gene (63). This haplotype containing the rs1058808-C and rs1136201-A alleles, was found associated with HER2 overexpression among Korean breast cancer patients (74). According to the 1KGP, both SNPs alleles are considerably more frequent among East Asians, compared to other populations such as Europeans (East Asians: rs1058808-C=0.571 and rs1136201-A=0.8760 vs. Europeans: rs1058808-C= 0.33232and rs1136201-A=0.7545) (72, 75). In that sense, these SNPs genotypes might contribute to the higher prevalence of the HER2-positive tumors reported above among Asian populations when compared to Europeans. Several meta-analyses have reported an association between the rs1136201 genotype and breast cancer risk (63, 76), nonetheless, the impact of this genetic variation on the expression of its protein remains unclear.

Other types of genetic variants that localize at non-coding gene sequences have been reported to affect gene expression (70, 77). One of these genetic variants, the rs2517956, a 2 Kb upstream variant (G>A) of the ERBB2 gene (78), was genotyped in 303 breast cancer patients from China, and evaluated according to the HER2 protein expression by IHC. This study found a higher proportion of HER2 positive tumors among patients with the A/A germline genotype, compared to homozygous G/G breast cancer patients (26.3% vs 15.2%, respectively, p=0.008) (1). However, the highest proportion of HER2-positive tumors was actually found among cases with the heterozygous A/G genotype (58.6%), suggesting this association might have an alternative underlying mechanism to the additive model. Additionally, it is worth noting that the overall rs2517956 allele’s frequency reported on the 1KGP (78) is relatively uniform across population, suggesting that this variant is unlikely to contribute to differences in HER2 expression in breast tumors among ethnic groups.

An important effort to elucidate potential genetic variants that confer risk for HER2-positive breast tumors was made by the French National Cancer Institute through a case-case Genome-Wide Association Study (GWAS) in over 8,703 women (79). They identified a SNP (rs68130068) located at chromosome 2 within an intronic region, which was potentially associated with HER2-positive breast tumors. Even though this SNP genotype (C>T) does not affect the sequence of any nearby gene, they reported that patients with the homozygous genotype for the minor allele (T/T) were 88% more likely to have HER2-positive breast tumors (OR=1.88; 95% CI, 1.33-2.65, p=0.00033), compared to patients with the C/C genotype. Heterozygous (C/T) patients also showed higher risk for HER2-positive tumors when compared to the C/C group (OR=1.20; 95% CI, 1.07-1.34, p=0.0013). Nonetheless, when the genome-wide association was tested, the rs68130068 SNP only reached a borderline level of significance (p=3.6x10^-6).

Even though the mechanism by which the rs68130068 variant and others that are located at non-codifying genome sequences might contribute to the tumor phenotype is not entirely understood, it has been hypothesized that these SNPs regulate gene expression through several allele-specific mechanisms according to the distance from the regulated gene (80). SNPs that map close are referred to as cis- expression quantitative trait loci (eQTLs), whereas SNPs that map far from the regulated genes are referred to as trans-eQTLs (81). In that sense, it is possible that these eQTLs, whose allele frequencies vary across specific populations or ancestry groups, may influence the transcription of ERBB2 or other genes upregulated in HER2-positive tumors. This could provide a potential biological mechanism underlying the disparities observed between populations for this particular breast cancer subtype.

Other studies have also reported differences in HER2 expression in breast tumors according to the genotype of SNPs located at other genes with roles related to gene regulation (FOXP3, NOTCH3) (82, 83), hormone signaling (ESR1 and CYP19A1) (84–86) and proliferation/survival control (WISP-1, CASP8, KRAS, TGFBR2, VEGF-A and CCND3) (87–92). Possibly, the impact of these SNPs on the HER2 expression arises from their regulatory effects on the gene in which they are located, indirectly influencing the expression of other genes and proteins, such as ERBB2/HER2. In this context, differences in the genotype frequencies of these SNPs across populations may partially explain the disparities reported in HER2-positive breast tumors among ethnic groups. Further research is needed to deepen our understanding of health disparities between populations and to shed light on the underlying biology of HER2 expression in breast tumors.