Photoreceptor cell differentiation in retinoblastoma demonstrated by a new immunohistochemical marker mucin-like glycoprotein associated with photoreceptor cells (MLGAPC)

Photoreceptor cell differentiation in retinoblastoma demonstrated by a new immunohistochemical marker mucin‐like glycoprotein associated with photoreceptor cells (MLGAPC) Aims: For further understanding of specific differentiation in retinoblastoma, we studied the expression of newly detected mucin‐like glycoprotein associated with photoreceptor cells (MLGAPC), which is specific for photoreceptor cells of retina and analogous to interphotoreceptor matrix proteoglycan‐1 (IMPG1). Methods and results: Surgically enucleated retinoblastomas (n=21; undifferentiated type, n=15, differentiated type, n=6) were immunohistochemically studied with a polyclonal antibody against MLGAPC, and 17/21 cases (81%) showed positive staining of tumour cells. We classified various staining patterns and structures into four groups: type 1 showing a granular intracellular scattered staining pattern with round small cells; type 2 showing a reticular staining pattern between spindle‐shaped tumour cells; type 3 showing radiating staining from the centre of Homer–Wright rosettes; type 4 showing ring‐shaped, radiating and granular staining associated with Flexner–Wintersteiner rosettes. Eleven of 15 undifferentiated retinoblastomas (73%) showed type 1 or 2, and all the six differentiated cases showed type 3 or 4. Image analysis of immunostaining revealed an increase in MLGAPC‐positive area from 0.48% in undifferentiated cases to 1.60% in differentiated cases, and a negative correlation was shown between mitotic frequency and MLGAPC‐positive area. Conclusions: This study proved MLGAPC as a valuable marker of retinoblastoma, and that photoreceptor differentiation takes place even in `undifferentiated' retinoblastoma.