Germline Mutation — Inherited Genetic Variation

A germline mutation is a constitutional genetic variant present from conception throughout an individual's entire body. Because it exists in gametes, it can be passed from parent to child in hereditary patterns, most commonly autosomal dominant or autosomal recessive. When a germline mutation inactivates a critical gene responsible for DNA repair or cell-cycle checkpoint control, cellular homeostasis is compromised.

According to Knudson's classic 'two-hit' hypothesis, individuals who inherit a germline alteration in a tumour-suppressor gene possess one non-functional allele in all tissues. If a secondary, somatic mutation subsequently disables the remaining normal allele within a specific cell, malignant transformation can occur, leading to earlier-onset or bilateral tumours compared to sporadic malignancies.

Identifying a germline mutation fundamentally alters clinical management. For an individual without cancer, knowledge of a high-penetrance mutation enables early, tailored surveillance—such as starting colonoscopies at a younger age or undergoing annual breast MRIs—and informs decisions about preventive medications or surgeries. For individuals already diagnosed with cancer, germline status can influence surgical planning and therapeutic selection, whilst prompting cascade testing for biological relatives who may unknowingly share the same elevated risk.

Because germline mutations are constitutional, they are tested using any normal nucleated tissue, most commonly peripheral blood lymphocytes or buccal epithelial cells obtained via a saliva kit. Genomic DNA is extracted and analysed using targeted polymerase chain reaction, multi-gene next-generation sequencing panels, or whole-exome sequencing. Laboratories classify variants based on international consensus guidelines as pathogenic, likely pathogenic, uncertain, likely benign, or benign.

Germline mutations underlie several well-defined hereditary syndromes. BRCA1 and BRCA2 alterations predispose to breast, ovarian, prostate, and pancreatic cancers. Mismatch repair gene mutations (MLH1, MSH2, MSH6, PMS2) cause Lynch syndrome, increasing risks for colorectal, endometrial, and other gastrointestinal malignancies. Other genes with prominent germline mutations include TP53 (Li-Fraumeni syndrome), APC (familial adenomatous polyposis), PTEN (Cowden syndrome), and RET (multiple endocrine neoplasia).

Detecting a germline mutation directly guides therapeutic options. Patients with germline BRCA1/2 mutations exhibit heightened sensitivity to platinum-based chemotherapies and poly (ADP-ribose) polymerase (PARP) inhibitors across ovarian, breast, pancreatic, and prostate malignancies. Germline mismatch repair mutations in Lynch syndrome predict durable responses to immune checkpoint inhibitors such as pembrolizumab. Furthermore, surgical strategies frequently incorporate risk-reducing contralateral or total organ removal rather than organ-sparing techniques.