Next-generation sequencing for GNAS gene aids in intramuscular myxoma diagnosis

Using next-generation sequencing (NGS) to test for mutations in the GNAS gene offers high sensitivity for the confirmation of intramuscular myxoma, according to a recent study.

“[T]he diagnosis of intramuscular myxoma is usually reached by performing several tests to exclude entities that bear morphological similarities, especially in difficult settings such as small biopsies and in cases of cellular variants,” researchers said. “Our findings show the usefulness of NGS GNAS mutation analysis in the diagnosis of intramuscular myxoma.”

Ten patients (mean age, 62 years; five women) with intramuscular myxoma provided four biopsies and six resection specimens that were sent for NGS. Majority of the tumours were located in the lower extremities, with one being localized to the calf; two patients had lesions in the upper extremities. All patients had a minimum tumour cell fraction of 10 percent. [Ann Diag Pathol 2019;43:151409]

NGS detected GNAS mutations in eight of the 10 samples, in which the relative frequencies of mutant alleles ranged from 5 percent to 28 percent.

The most common mutation detected was the substitution of arginine with histidine at the 201st position, which was confirmed in five cases. In the remaining three samples, arginine at the same position was instead substituted with cysteine. One sample yielded low-quality DNA that precluded NGS analysis and in another, NGS found no putative mutation, though the initial diagnosis was upheld after a pathological review.

Taken together, the resulting sensitivity of NGS for the diagnosis of intramuscular myxoma was 88 percent.

The researchers subsequently performed CD34 immunohistochemistry on all the samples and found a diffuse and strong expression pattern of the protein marker, validating literature findings and further confirming the sensitivity of CD34 as a marker for intramuscular myxoma. [Diagn Pathol 2018;13:52; J Med Case Reports 2014;8:441]

“Of course, CD34 is expressed by a variety of tumours and is not specific,” the researchers explained, “but the absence of CD34 virtually excludes the possibility of an intramuscular myxoma, which can serve as an ancillary diagnostic tool.”

CD34 appears to have been largely overlooked in previous investigations of intramuscular myxoma, they added. This may be due to samples being “contaminated” with other tumours that morphologically resemble intramuscular myxoma but are actually of a different lineage. In turn, this may have confounded and diluted potential positive tests.

In the present study, the researchers sought to identify new and sensitive markers for the diagnosis of intramuscular myxoma. Previous studies have implicated gain-of-function of function substitutions, which appear to be absent in similar malignancies, but none have used NGS technology. [J Cell Mol Med 2009;13:1291-1301; Mod Pathol 2009;22:718-724]

“Thanks to the high accuracy of NGS for detecting GNAS mutations, a clean series was ensured, leading to the uncovering of CD34 as a sensitive marker,” the researchers said. “Given the recent widespread application of NGS testing, our study rationale is applicable for several other tumour types.”