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A complex nine base pair deletion in RET exon 11 common in sporadic medullary thyroid carcinoma
M Alemi1, S D Lucas, J F Sällström
1Department of Pathology, University of Uppsala, Sweden.
Abstract:
Genetic alteration of the RET proto-oncogene is associated with multiple endocrine neoplasia type 2A and 2B (MEN 2A and MEN 2B), familial medullary thyroid carcinoma (FMTC) and Hirschprung's disease. Oncogenically activated RET has also been demonstrated in sporadic medullary thyroid tumors, which in some cases show somatic missense mutations. We have recently described a complex 9 bp deletion in RET exon 11 in a single case of sporadic MTC. In order to determine the prevalence of this mutation among sporadic MTC tumors, we have now analysed 15 cases and five normal controls by PCR-based nonradioactive single-strand conformational polymorphism analysis (PCR-SSCP) and fragment size analysis of exon 11. DNA was extracted from microdissected tumor tissue or normal cells and subjected to nested PCR prior to analysis. A markedly divergent SSCP pattern and a PCR fragment 9 bp shorter than normal were demonstrated in 14 of the 15 MTC tumors. Sequencing revealed the deletion of nine bases encompassing a key cysteine at codon 634, often altered in MEN 2A. Four lymphocyte controls and normal thyroid tissue from one patient failed to show the deletion. Several factors in the DNA sequence environment immediately surrounding the deletions, including an extended inverted repeat, several direct repeats and a so-called symmetric element suggest that the deletional events may be non-random.
Insights
A common 9 bp deletion in RET exon 11 was found in 14 of 15 sporadic medullary thyroid carcinoma (MTC) tumors. This genetic alteration, affecting codon 634, suggests a non-random mutational event in MTC development.
Area of Science:
- Oncology
- Genetics
- Molecular Biology
Background:
- RET proto-oncogene alterations are linked to Multiple Endocrine Neoplasia (MEN) types 2A and 2B, familial medullary thyroid carcinoma (FMTC), and Hirschsprung's disease.
- Oncogenic RET activation occurs in sporadic medullary thyroid tumors, sometimes due to somatic mutations.
- A specific 9 base pair (bp) deletion in RET exon 11 was previously identified in one sporadic MTC case.
Purpose of the Study:
- To determine the prevalence of the 9 bp RET exon 11 deletion in sporadic medullary thyroid carcinoma (MTC).
- To investigate the potential non-random nature of this specific deletion in MTC.
Main Methods:
- Analysis of 15 sporadic MTC tumors and five normal controls using Polymerase Chain Reaction-based nonradioactive Single-Strand Conformational Polymorphism (PCR-SSCP) and fragment size analysis of RET exon 11.
- DNA extraction from microdissected tumor tissue and normal cells, followed by nested PCR.
- DNA sequencing to confirm the deletion and its location.
Main Results:
- A 9 bp deletion in RET exon 11 was detected in 14 out of 15 (93%) sporadic MTC tumors analyzed.
- The deletion encompasses codon 634, a site frequently altered in MEN 2A.
- Normal controls (lymphocytes and thyroid tissue) did not exhibit this deletion.
- Sequence analysis surrounding the deletion revealed structural elements (inverted repeats, direct repeats, symmetric element) suggesting a non-random mutational mechanism.
Conclusions:
- The 9 bp deletion in RET exon 11 is a frequent event in sporadic medullary thyroid carcinoma (MTC).
- This specific deletion, affecting a key cysteine residue, is likely a non-random mutational event contributing to MTC pathogenesis.
- Further research into the mechanisms driving this deletion is warranted.