An immunohistochemical analysis of P-glycoprotein in retinoblastoma

Sir,

Retinoblastoma is the most frequent primary intraocular malignancy in children.[1] Even with the use of multidrug chemotherapy regimens, drug resistance is a major problem while managing this vulnerable group of patients. Multidrug resistance (MDR), associated with expression of P-glycoprotein (P-gp), is one of the most known mechanisms of chemoresistance.[2,3] P-gp is a 170-kDa plasma membrane glycoprotein, which is known to cause an increased efflux of cytotoxic drugs from the cell. In other systemic malignancies, overexpression of P-gp has been associated with chemoresistance and poor prognosis.[2] We analyzed P-gp expression in retinoblastoma specimens enucleated either primarily or after neoadjuvant chemotherapy by immunohistochemistry and correlated this with the histopathological findings.

All eyes enucleated over a 1-year period from January 2010 to December 2010; enucleated either as a primary treatment (group I) or after three to five cycles of standard regimen vincristine, etoposide, and carboplatin (group II) were included in the study. Formalin fixed, paraffin-embedded sections from the enucleated sections were stained with hematoxylin and eosin for studying histopathological features (tumor differentiation and high-risk features). P-gp immunostaining was done and analyzed semiquantitatively as negative, low, or high expression [Figure 1]. Fischer's exact and chi-square tests were used for statistical analysis. Statistical significance was set at a P-value of less than 0.05.

Figure 1.

Photomicrograph showing strong P-gp immunoreactivity in a well- differentiated retinoblastoma (immunostaining × 400)

There were a total of 49 eyes, 37 in group I (mean age 2.48 ± 1.27 years) and 12 in group II (mean age 3.21 ± 1.34); histopathological findings of both the groups are summarized in Table 1. In group I, there were 27% (10 of 37) eyes with positive P-gp expression, of which 5.4% (2 of 37) were high expressions. In group II, among 58.3% (7 of 12) positive cases, P-gp was highly expressed in 41.7% (5 of 12). The difference in the P-gp expression in the two groups was statistically significant (P = 0.0042). Among the well differentiated (WD) tumors (4 of 49), 50% (2 of 4) had high expression and among the well differentiated (PD) tumors (45 of 49), 11.1% (5 of 45) had a high P-gp expression. There was no statistically significant difference with P-gp expression in relation to tumor differentiation, laterality, and presence of high-risk histopathological features.

Table 1.

Histopathology and P-gp expression in Groups I and II

From India, there has been one other study in which Krishnakumar et al. analyzed P-gp expression by imunohistochemistry prior to chemotherapy and studied the long-term follow-up.[4] In our study we found that 58.3% (7 of 12) retinoblastoma cases enucleated after chemotherapy had high expression of P-gp as compared with 27.0% (10 of 37) primarily enucleated cases. This significant difference in P-gp expression indicates that chemotherapy might have selected resistant clones of cells expressing high P-gp.

Though there have been a few studies studying the expression of P-gp in retinoblastoma and its relation to drug resistance, these were retrospective studies and the practical implication have not yet been well considered.[4–6] However, the high P-gp expression in chemotreated eyes in our study and in few other studies support that chemotherapy may have some role in high expression of P-gp in retinoblastoma. These findings could have important implications in understanding the mechanism of chemoresistance in retinoblastoma patients, and warrant further prospective investigation in a larger patient population.