RET Gene Fusions and Mutations

Overview

The RET gene (REarranged during Transfection), located on chromosome 10q11.2, encodes a receptor tyrosine kinase that plays a critical role in cell proliferation, differentiation, survival, and migration. Under normal conditions, RET interacts with ligands to regulate vital cellular processes. However, when altered, it contributes to several cancers and genetic syndromes.

RET is activated by two primary mechanisms in cancer:

1. RET Gene Fusions – Occur due to aberrant DNA repair, where the RET gene fuses with another gene (commonly KIF5B, CCDC6, or NCOA4). These fusions cause constitutive kinase activation, driving oncogenesis in cancers like non-small cell lung cancer (NSCLC) (1–2% of cases) and papillary thyroid cancer (10–20% of cases).
2. RET Point Mutations – Single DNA base changes in RET that can lead to inherited syndromes or sporadic cancers.

RET alterations are linked to diseases including medullary thyroid carcinoma (MTC), pheochromocytoma, Hirschsprung’s disease, familial medullary thyroid cancer (FMTC), and multiple endocrine neoplasia type 2 (MEN2).

Symptoms

The presence of RET gene fusions or mutations does not cause direct symptoms but manifests through the associated cancers and syndromes:

1. Thyroid Cancer / MTC: Neck lump, hoarseness, difficulty swallowing, and hormonal imbalances.
2. NSCLC (Lung Cancer): Persistent cough, chest pain, hemoptysis, shortness of breath, and weight loss.
3. Pheochromocytoma: High blood pressure, palpitations, sweating, and headaches.
4. Hirschsprung’s Disease: Severe constipation, abdominal swelling, and delayed passage of stool in newborns.
5. MEN2 Syndromes: Multiple endocrine tumors, mucosal neuromas, hyperparathyroidism, and systemic effects depending on the organs involved.

These clinical features usually trigger molecular diagnostic testing for RET alterations.

Causes

RET-related cancers and syndromes arise due to genetic alterations in the RET gene:

1. RET Gene Fusions:
   1. Fusion of RET with partner genes like KIF5B or CCDC6 leads to continuous kinase activation.
   2. These fusions act as oncogenic drivers, especially in lung and thyroid cancers.
2. RET Point Mutations:
   1. Gain-of-function mutations: Lead to uncontrolled RET activation, commonly causing medullary thyroid carcinoma (MTC).
   2. Loss-of-function mutations: Disrupt normal function, associated with Hirschsprung’s disease.
3. Inherited Germline Mutations:
   1. Cause hereditary syndromes such as MEN2A, MEN2B, and FMTC, often linked to thyroid and adrenal tumors.
4. Somatic Mutations:
   1. Sporadically acquired during life, leading to cancers without a family history.

Risk Factors

Individuals at higher risk for RET fusion and mutation-related conditions include:

1. Genetic Predisposition:
   1. Families with MEN2, FMTC, or Hirschsprung’s disease.
   2. Inherited germline RET mutations increase susceptibility.
2. Cancer Patients:
   1. Those with NSCLC or papillary thyroid carcinoma are more likely to harbor RET fusions.
3. Age and Family History:
   1. Younger individuals may present with inherited forms like MEN2, while adults often show sporadic cancers.
4. Co-existing Conditions:
   1. Patients with adrenal tumors, endocrine abnormalities, or gastrointestinal malformations.
5. Geographic and Diagnostic Factors:
   1. Patients in areas with limited access to molecular testing may face delayed diagnosis and poorer outcomes.

Prevention

RET gene fusions and mutations themselves cannot be prevented, but early detection, genetic counseling, and targeted therapy can reduce disease burden and improve prognosis:

1. Diagnostic Testing:
   1. PCR and FISH for detecting RET gene fusions.
   2. Bone marrow aspiration (minimum 2.5 mL) and blood samples in EDTA tubes (6 mL) are required for analysis.
   3. Plasma for circulating tumor DNA (ctDNA) must be collected and transported within 8 hours at ambient temperature.
2. Genetic Counseling:
   1. Families with hereditary RET mutations should undergo genetic testing and counseling to identify carriers early.
3. Targeted Therapy:
   1. Multi-target tyrosine kinase inhibitors (TKIs) such as vandetanib, lenvatinib, cabozantinib, and sorafenib are used in early stages.
   2. Selective RET inhibitors like selpercatinib and pralsetinib (FDA-approved in 2020) show high efficacy and intracranial activity.
   3. Continuous monitoring is essential as resistance may develop.
4. Preventive Health Practices:
   1. Regular cancer screenings in high-risk groups.
   2. Early treatment of thyroid nodules and adrenal abnormalities in genetically predisposed patients.