PURPOSE: Transcripts of mucins 1, 4, and 5AC have been identified in human conjunctival tissue. Of these, only MUC5AC has been localized to goblet cells. MUC2 is a goblet cell mucin originally identified in the intestinal mucosa. The presence of MUC2 transcripts and levels of MUC2 and MUC5AC transcripts in normal human conjunctiva, as determined by quantitative polymerase chain reaction (PCR), is reported.
METHODS: RNA from conjunctivae of six donors (3 men, 3 women, 44 to 69 years, all white) was isolated and subjected to competitive reverse transcription-PCR. Internal standards, which are dsDNA molecules with ends complementary to a given primer pair but containing nonhomologous central sequences, were prepared for each gene assayed. Titration of a constant amount of cDNA against serial dilutions of the internal standard allowed quantification of the template cDNA. MUC2 and MUC5AC levels were compared to levels of the "housekeeping" gene, beta2-microglobulin (beta2M). The identity of PCR products was confirmed by sequencing.
RESULTS: In the six individual samples tested, beta2M mRNA is expressed, on average, at approximately 10(-20) moles per sample (1 microg RNA) or approximately 63.5x10(4) molecules. The mRNA encoding MUC5AC, a relatively abundant ocular mucin, exists at levels 10-fold lower than beta2M. In contrast to previous reports of MUC2 mRNA being absent at the ocular surface, these results show that MUC2 transcripts are present and expressed at levels 5900-fold lower than for MUC5AC. Apparently, MUC2 transcripts exist on the order of only approximately 100 to 1000 molecules/microg of RNA in the analyzed samples.
CONCLUSIONS: MUC2 transcripts are present in human conjunctival tissue and their abundance is much lower than that of MUC5AC. This is the first application of competitive PCR to the quantitative analysis of mucin expression in human ocular tissue. The sensitivity of this method allows the detection of MUC2 transcripts that were not detected by Northern blot analysis or in situ hybridization in previous studies. It also makes possible the comparison of relative levels of expression for ocular mucins.
