How does genetic testing inform treatment choices for breast cancer?

When you’re diagnosed with breast cancer, your doctors consider many factors to choose the best treatment. One increasingly important piece of information is your genetic makeup. Certain inherited gene changes can influence which treatments are most likely to work, what side effects you might experience, and which surgical options might reduce your future cancer risk.

This isn’t only about predicting risk before cancer develops. Genetic testing after a breast cancer diagnosis can guide real-world treatment decisions, from whether you’ll benefit from specific chemotherapy drugs to whether newer targeted therapies might be suitable.

What genetic changes affect breast cancer treatment?

The most well-known genes linked to breast cancer treatment are BRCA1 and BRCA2. These genes normally help repair damaged DNA in your cells. When you inherit a harmful change (called a pathogenic variant or mutation) in one of these genes, your cells are less able to fix DNA damage, making cancer more likely.

But BRCA genes aren’t the only ones that matter. Other genes involved in DNA repair—such as PALB2, ATM, CHEK2, and TP53—can also influence treatment choices. Some affect how well your body processes certain drugs, while others may make tumours more sensitive to particular therapies.

International guidelines now recommend genetic counselling and testing for many breast cancer patients, particularly those diagnosed at younger ages or with specific tumour types. Testing eligibility criteria vary, but many guidelines are becoming less restrictive, recognising that genetic information can benefit a wider range of patients.

How genetic results guide chemotherapy and targeted therapy

Chemotherapy decisions

If you have a BRCA1 or BRCA2 mutation, your tumour may respond differently to certain chemotherapy drugs compared to cancers without these changes. Some research suggests these tumours may be particularly sensitive to platinum-based chemotherapy drugs (such as carboplatin), though this remains an active area of study.

Genetic testing can also reveal how your body processes specific drugs. For example, the gene CYP2D6 affects how you metabolise tamoxifen, a common hormone therapy. Some people metabolise it poorly, which may reduce its effectiveness. Knowing your CYP2D6 status might help your doctor choose the most suitable hormone treatment.

PARP inhibitors

One of the clearest examples of genetics guiding treatment is the use of PARP inhibitors. These targeted drugs work by blocking an enzyme (PARP) that helps cells repair DNA. In people with BRCA1 or BRCA2 mutations, cancer cells are already struggling with DNA repair. Adding a PARP inhibitor overwhelms them, causing the cancer cells to die while leaving normal cells relatively unharmed.

PARP inhibitors are now approved for certain breast cancers with BRCA mutations, particularly HER2-negative advanced breast cancer. They’re generally given as tablets and have different side effects to traditional chemotherapy.

Tumour testing vs inherited testing

It’s worth noting that testing can happen in two ways:

• Germline testing (from a blood or saliva sample) looks for inherited gene changes you were born with, present in every cell of your body
• Tumour testing (from a biopsy of the cancer itself) looks for gene changes that occurred only in the cancer cells

Both types can inform treatment, but germline testing also tells you whether you inherited a change that might affect your family members or your risk of other cancers.

Surgical decisions and risk reduction

Genetic test results can also influence surgical choices, both for the original breast cancer and for risk reduction.

At initial surgery

If you’re found to have a BRCA1 or BRCA2 mutation, you face a higher risk of:

• A new cancer in the same breast (if you have breast-conserving surgery)
• Cancer in the opposite breast

Some women in this situation choose more extensive surgery, such as mastectomy (removal of the whole breast) rather than lumpectomy, or bilateral mastectomy (removing both breasts) to reduce future risk. These are personal decisions made with your surgical team, balancing cancer risk against quality of life and personal preferences.

Risk-reducing surgery later

Even after your initial breast cancer treatment is complete, knowing you carry a BRCA mutation means you have options to reduce your risk of future cancers:

• Risk-reducing mastectomy of the opposite breast (if you haven’t already had bilateral surgery)
• Risk-reducing removal of the ovaries and fallopian tubes (called salpingo-oophorectomy), which also lowers breast cancer risk by reducing hormone levels

Guidelines recommend discussing these options with genetic counsellors and specialists. Surgery to prevent future cancer is a significant decision that depends on your age, whether you’ve completed your family, and your personal risk tolerance.

The role of genetic counselling

Before and after genetic testing, genetic counselling is an important part of the process. International guidelines consistently recommend genetic counselling at multiple points in your care, though the exact format varies.

A genetic counsellor will:

• Explain which genes are most relevant to your situation
• Discuss what test results can and cannot tell you
• Help you understand how results might affect your treatment and future care
• Explore implications for family members
• Support you in making decisions that fit your values and circumstances

In the UK, specialist genetic counselling is recommended before testing for inherited cancer risk. Guidelines emphasise that testing should happen after appropriate counselling, not as a standalone blood test.

What you can do at Jeen

Jeen offers genetic testing for inherited cancer risk, with specialist support built in. Depending on how broad a look you want at your cancer risk, we offer:

• BRCA Testing: a 10-gene panel covering BRCA1, BRCA2 and the most actionable genes in the BRCA pathway, aligned with NHS Genomic Medicine Service standards. This panel focuses on hereditary breast, ovarian, prostate and related cancer predisposition. Results are available within 17-21 working days of the laboratory receiving your sample, and 45-minute specialist genetic counselling is included with every test.

• Breast Cancer Risk Screening: a 13-gene panel focused on hereditary breast cancer predisposition, with genetic counselling included.

• Cancer Risk Screening: a comprehensive 50-gene panel covering a wider range of hereditary cancers, with 45-minute specialist genetic counselling included.

All tests use an at-home buccal swab (cheek swab), so there’s no need for a blood draw unless you prefer one. An in-clinic blood draw is available for an additional £40. Variant interpretation aligns with NHS Genomic Medicine Service standards, and genetic counselling with a specialist is included in the test price to help you understand your results and next steps.

The NHS offers genetic testing through clinical genetics services, typically for people who meet specific family-history or clinical criteria. Jeen’s panels offer broader gene coverage and direct access without a GP referral, while maintaining the same interpretation standards. If you’re already under NHS cancer care, your oncologist may arrange testing through the NHS pathway—speak to your team about what’s available to you.