Polygenic Breast Cancer Risk Scores Strive to Overcome Racial Bias

Nancy A. Melville

August 27, 2021

The potential of polygenic risk scores (PRSs) to become key components in the assessment of individual risk for disease in the clinical setting is inching closer to fruition; however, the technology is plagued by one glaring omission of most existing PRSs – the lack of applicability to those of non-European ancestry.

Polygenic risk scores predict an individual's risk of disease based on common genetic variants identified in large genomewide association studies (GWASs). They have gained ground in research, as well as in the unregulated realm of the direct-to-consumer market where they are sold as add-ons to DNA ancestry kits such as 23andMe and MyHeritage.com.

While the risk scores show strong validation in estimating risk among people of European descent, their striking caveat is the lack of applicability to other ancestries, particularly African, and their use in practice outside of clinical trials is discouraged in National Comprehensive Cancer Network guidelines.

Study Underscores Need for Ethnically Diverse Datasets

In a recent study published in JAMA Network Open, researchers evaluated the use of polygenic risk scores' models in a clinical setting. Researchers tested 7 PRSs models for breast cancer risk against the medical records data of 39,591 women of European, African, and Latinx ancestry.

The PRSs models – all used only for research purposes – included three models involving European ancestry cohorts, two from Latinx cohorts, and two from women African descent.

After adjusting for factors including age, breast cancer family history, and ancestry, the PRSs from women with European ancestry highly corresponded to breast cancer risk, with a mean odds ratio of 1.46 per standard deviation increase in the score.

PRSs were also generalized relatively well among women of Latinx ancestry with a mean OR of 1.31. The authors noted that association is likely caused by Latinx individuals in the United States having a greater proportion of European ancestry than individuals with African ancestry. Importantly, however, the effect size was lower for women of African ancestry with a highest OR of 1.19 per standard deviation.

In the highest percentiles of breast cancer risk, women of European descent had odds ratio as high as 2.19-2.48, suggesting a statistically significant association with overall breast cancer risk. No statistically significant associations were found among women of Latinx and African-ancestry.

The PRSs models were smaller for women of non-European ancestry and included fewer genetic variants for women of non-European ancestry were notably smaller and hence reflected fewer genetic variants. Of the two risk scores involving African ancestry, the Women's Health Initiative for Women with African ancestry risk score had just 75 variants, while the African diaspora study (ROOT) had 34 variants, compared with 3,820 and 5,218 in the two largest European ancestry PRSs, the Breast Cancer Association Consortium and the UK Biobank, respectively.

"These results highlight the need to improve representation of diverse population groups, particularly women with African ancestry, in genomic research cohorts," the authors wrote.

First author, Cong Liu, PhD, of Columbia University Irving Medical Center, New York, said that efforts are underway to improve the inclusivity in the Electronic Medical Records and Genomics network data set used in this study.

"Until well-developed and validated PRSs for women with non-European ancestry become available, the current PRSs based on cohorts with European ancestry could be adapted for Latinx women, but not women with African ancestry until additional data sets become available in this important and high-risk group," Liu and colleagues wrote.

In a commentary published with the study, Payal D. Shah, MD, of the Basser Center for BRCA at the University of Pennsylvania, Philadelphia, said that PRSs are "disproportionately applicable to patients with European ancestry and are insufficiently vetted and developed in other populations. If an instrument exists that has clinical utility in informing effective cancer risk mitigation strategies, then we must strive to ensure that it is available and applicable to all."

Higher Morality Among African American Women

While American Cancer Society data shows women with African ancestry generally have incidence rates of breast cancer similar to White women, they have significantly higher mortality from the disease in part because of later-stage diagnosis and health care barriers.

Anne Marie McCarthy, PhD, of the University of Pennsylvania, and Katrina Armstrong, MD, of Harvard Medical School, Boston, wrote in the Journal of the National Cancer Institute that African American women "have 42% higher breast cancer mortality than white women, despite having lower disease incidence, and are more likely to be diagnosed with triple-negative breast cancer, which has poorer prognosis than other molecular subtypes."

McCarthy and Armstrong wrote that African American women are chronically underrepresented in breast cancer studies. And as such, it is impossible to know the extent of the prevalence of mutations and risk.

Failing to address the lack of diversity in genomic studies may worsen health disparities for women with African ancestry, Liu and colleagues wrote. The higher mortality "underscores the urgent need to increase diversity in genomic studies so that future clinical applications of the PRS do not exacerbate existing health disparities. These results highlight the need to improve representation of diverse population groups, particularly women with African ancestry, in genomic research cohorts."

Potential PRS Benefits Underscore Need to Eliminate Bias

The potentially important benefits of PRSs as risk prediction tools used in combination with family history, reproductive history and other factors, should provide strong incentive to push for improvement, Shah wrote.

For instance, if an individual is estrogen receptor positive and shows elevations in breast cancer risk on a reliable PRS, "this may inform antiestrogen chemoprevention strategies," she wrote.

A risk score could furthermore influence the age at which breast cancer screening should begin or factor into whether a patient should also receive surveillance breast MRI.

Importantly, PRSs could also add to other risk factors to provide more precise risk estimates and inform management of women with a pathogenic variant in a breast cancer risk predisposition gene, Shah wrote.

Confluence Project

Among the most promising developments in research is the National Cancer Institute's Confluence Project, a large research resource aiming to include approximately 300,000 breast cancer cases and 300,000 controls of different races/ethnicities, utilizing the confluence of existing GWAS and new genomewide genotyping data.

Having started enrollment in 2018, the project is approaching implementation, said Montserrat García-Closas, MD, MPH, DrPH, deputy director of cancer epidemiology and genetics with the National Cancer Institute.

"We expect genotyping to be completed by the end of 2022 and for the data to be made available to the research community soon after that," she said.

Among the project's key objectives are the development of PRSs to be integrated with known risk factors to provide a personalized risk assessment for breast cancer, overall and by ancestral subtype.

"We plan to apply novel methods to derive multiancestry PRS that will account for differences and similarities in genetic architecture across ethnic/racial groups to develop breast cancer PRSs that can be applied in multiethnic/racial populations," she said.

NCI is working with investigators in Africa, Central and South America, and Asia, and reaching out to non-European organizations such as AORTIC for studies of African populations.

Direct-to-Consumer Global PRS

In the commercial PRS market, efforts to address diversity shortcomings are also gaining momentum, with Myriad Genetics touting a first-of-its kind "global PRS."

The PRS, a recalibrated version the company's riskScore PRS, sold as part of its Myriad myRisk Hereditary Cancer test, will reportedly apply to all ethnicities in estimating an individual's 5-year and lifetime risk of breast cancer.

study presented in June at the American Society of Clinical Oncology meeting, describes the development of the model with the use of three large ancestry-specific PRSs based on African American, Asian, and European cohorts, with the system including a total of 149 single-nucleotide polymorphisms, including 93 well established for breast cancer and 56 that are ancestry specific.

In validation of the data in an independent cohort of 62,707 individuals, the global PRS was strongly associated with breast cancer in the full combined validation cohort as well as in all three of the ancestry subcohorts.

However, the effect size among women with African ancestry was still the lowest of all of the groups, with a mean OR of 1.24 per standard deviation, versus the highest rate of mixed ancestry (OR, 1.59).

According to senior author Holly Pederson, MD, director of medical breast services at the Cleveland Clinic, the applicability of the PRS to women with African ancestry is expected to further improve as additional data become available.

"The discriminatory power in women of African descent was significantly improved but still suboptimal," she said. "The need for more data, particularly in Black women, is challenging not only because there is likely more diversity in the genomic landscape of women of African descent, but also because the barriers created by historical, cultural, institutional and interpersonal dynamics result in the paucity of this data."

"We must be committed to ending bias resulting in health care disparities," Pederson said. She noted that the global PRS is nevertheless "still clinically useful in Black women," and recommended that clinicians be up front with patients on the status of the research challenges.

"As with any clinical shared decision-making conversation between a patient and her provider, it is important for Black women to know that data is limited in the African American population, particularly given the vast genomic diversity of the African continent," she said. "This model, as models that have gone before it, will improve with additional data, particularly in this population."

Commercial PRSs May Benefit Research

While the commercial marketing of PRSs in a direct-to-consumer fashion have raised some concerns, such as how individuals respond to their risk scores, there could be important benefits as well, commented Megan C. Roberts, PhD.

"There may be an opportunity to learn from these companies about how to engage diverse communities in genomic testing," said Roberts, an assistant professor and director of implementation science in precision health and society at the University of North Carolina at Chapel Hill. "Moreover, the data they collect from their customers often can be used for research purposes as well."

In a recent perspective, Roberts and colleagues addressed the role of health disparities in PRSs. She'll be joining international precision public health researchers in October in hosting a free virtual conference at UNC on the topic.

"There is a huge need to improve racial and ethnic diversity in our genomic datasets," Roberts said. "Without this, we will not be able to return on the promise of precision medicine and prevention for improving the health of our whole population."

Pederson disclosed that she is a consultant for Myriad Genetics.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

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