Reduced Expression of Schwannomin/Merlin in Human Sporadic Meningiomas

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Abstract
The neurofibromatosis type 2 gene is frequently mutated in sporadic meningiomas. The protein product of the neurofibromatosis type 2 gene is called schwannomin or merlin. Its expression in leptomeningeal cells from which meningiomas are derived and the characteristics of mutated forms in meningiomas, to our knowledge, have not been previously studied. Immunoblotting and immunoprecipitation experiments with two specific antibodies were used to determine the size and subcellular distribution of schwannomin/merlin in rabbit and human brain tissue and established human leptomeningeal LTAg2B cells. Immunoblotting was used to determine the expression level of schwannomin/merlin in 14 human sporadic meningiomas. Both antibodies detect a protein of approximately 66 kDa, which is predominantly expressed in the Triton X-100-insoluble fraction of the brain and LTAg2B cells. The levels of schwannomin/merlin were severely reduced in eight tumors (57%) when compared with the expression levels in the human brain, LTAg2B cells, and the remaining six meningiomas. All six tumors with the normal schwannomin/merlin expression were of meningotheliomatous type. In contrast, all other histological types and one meningotheliomatous tumor with psammoma bodies were deficient in the 66-kDa schwannomin/merlin. Although nonsense mutations leading to premature stop codons are common in the neurofibromatosis type 2 gene in meningiomas, we found no evidence of truncated schwannomin/merlin forms in the tumors analyzed. The absence of complete schwannomin/merlin in almost 60% of primary sporadic meningiomas seems to be an important factor in meningioma tumorigenesis. The development of meningotheliomatous meningiomas is probably linked to alterations in other oncogenes or tumor suppressor genes.