Mutant HABP2 Causes Non-Medullary Thyroid Cancer

Modeling of mutant (blue) and wildtype (pink) HABP2 (534 position indicated in yellow) showed that the glutamate substitution changed the protein’s structure disrupting the active site and substrate accessibility.

Modeling of mutant (blue) and wildtype (pink) HABP2 (534 position indicated in yellow) showed that the glutamate substitution changed the protein’s structure disrupting the active site and substrate accessibility.

The thyroid is a butterfly-shaped gland that lies at the base of the throat in front of the windpipe. A member of the endocrine system, the thyroid secretes hormones to regulate heart rate, blood pressure, temperature, and metabolism. Cancer of the thyroid is the most common endocrine cancer and the eighth most common cancer in the U.S. An estimated 63,450 Americans will be diagnosed with thyroid cancer this year. The vast majority is of follicular cell origin, and the remaining cancer originates from parafollicular cells, so called medullary thyroid cancer.

Familial non-medullary thyroid cancer (FNMTC) accounts for three to nine percent of thyroid cancer cases and has a dominant inheritance pattern. FNMTC can be syndromic, meaning it is associated with additional cancers and known susceptibility genes, or nonsyndromic, where it is the only cancer. Most FNMTCs are nonsyndromic. These cancers are clinically defined by the presence of thyroid cancer in two or more first-degree relatives in the absence of other predisposing or environmental factors and are more aggressive than sporadic thyroid cancer. Because a number of candidate chromosome sites and genes have been reported, FNMTC development may be due to multiple different alterations.

To further investigate the genetic landscape of FNMTC, Electron Kebebew, M.D., Chief of CCR’s Endocrine Oncology Branch (EOB), and his colleagues have a clinical protocol in which kindreds with more than 200 family members have undergone evaluations. Sudheer Kumar Gara, Ph.D., a Postdoctoral Fellow in the EOB, has been investigating one of the largest kindreds and performed high-throughput DNA sequencing on blood samples from family members with FNMTC. At the time of the initial evaluation, three family members had thyroid cancer. Screening ultrasound identified thyroid neoplasms in four more members, and two of eight individuals in the youngest generation had abnormal ultrasounds showing small thyroid nodules. One affected family member had a previously-removed atypical mole, but none had a history of other primary malignancies, and the researchers detected no other benign tumors, supporting a classification of nonsyndromic FNMTC.

Comparing the DNA sequence from affected family members and unaffected spouses, the investigators identified two single-site variants present only in the affected members. One variant in the HABP2 gene was found in all seven affected family members and resulted in an amino acid change from glycine to glutamate at position 534 (G534E), which is located in a highly-conserved protease domain. Modeling the structure of the protease domain revealed that the glutamate substitution disrupted the enzyme active site and prevented substrate access.

When the scientists examined HABP2 expression in the familial cases, they found high levels of HABP2 protein in the tumors but none in normal thyroid tissue. HABP2 mRNA levels mirrored protein expression in these samples. In contrast, only 25 percent of 12 sporadic thyroid cancers showed any HABP2 expression. The increased expression of the G534E variant in the affected family members’ tumors suggested it was potentially pathogenic but was less common in sporadic cases of thyroid cancer.

By further sequencing the HABP2 coding regions, the researchers found that the variant was heterozygous in germline and tumor DNA. They failed to identify additional changes in the HABP2 coding sequence; HABP2 copy number variations; somatic mutations in BRAF, KRAS, NRAS, or RET oncogenes; or other known rearrangements, suggesting the HABP2 variant functions alone in FNMTC. Analysis of The Cancer Genome Atlas (TCGA) database revealed that up to six percent of cancers have somatic mutations in HABP2.

The investigators next asked what effect normal and variant HABP2 had on cell behavior. Loss of HABP2 increased colony formation and cell migration, suggesting it might act as a tumor suppressor. In support of this idea, overexpression of HABP2 reduced both colony numbers and migration. In contrast, overexpressing the G534E variant enhanced colony formation, increased cell migration, and promoted the formation of foci, all hallmarks of cancer. Expressing equal amounts of normal HABP2 and the G534E variant in the same cells still led to increased numbers of foci, indicating that the variant has a dominant-negative effect on the tumor suppressor function of normal HABP2. The scientists also looked at data for 423 patients with papillary thyroid cancer in TCGA’s database and found that nearly five percent carried the G534E HABP2 variant compared to 0.7 percent in population databases where disease status is not known.

Together, these results indicate that HABP2 is a susceptibility gene for FNMTC that can promote cancer on its own. The G534E variant was found in all affected family members, was expressed in thyroid tumor but not normal tissue, and inhibited normal HABP2 tumor suppressor function in cell studies. Future studies will need to address whether HABP2 can play a similar role in other types of cancer, how exactly the G534E variant participates in cancer initiation and progression, and the clinical implications for carriers of the variant in the family studied here as well as in the general population.

Summary Posted: 08/2015

Reference

Gara SK, Jia L, Merino MJ, Agarwal SK, Zhang L, Cam M, Patel D, Kebebew E. Germline <em>HABP2</em> Mutation Causing Familial Non-Medullary Thyroid Cancer. NEJM. July 30, 2015 PubMed Link