MOLECULAR CHARACTERIZATION OF FAMILIAL ADENOMATOUS POLYPOSIS IN THE REPUBLIC OF MACEDONIA
Stefanovska A-M1, Josifovski T2, Panovski M2, Jasar D3, Zografski G3, Efremov GD1, Dimovski AJ1,4,*
*Corresponding Author: Professor Dr. Aleksandar J. Dimovski, Faculty of Pharmacy, Vodnjanska 17, 1000 Skopje, Republic of Macedonia; Tel: +389-2-311-9694; Fax: +389-2-312-3054; E-mail: adimovski@baba.ff.ukim.edu.mk
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DISCUSSION

The incidence and prevalence of FAP in the Republic of Macedonia, calculated on the basis of patients detected over a 10 year period, are significantly lower than those in other European populations (0.25 x 10–6 vs. 1.85 x 10–6 and 2 x 10–6 vs. 32 x 10–6, respectively) [1,18]. Familial adenomatous polyposis is responsible for less than 0.1% of colorectal cancers in the Republic of Macedonia, which is lower than reported for the United States and populations of Western Europe [2]. It is most probable that these figures reflect the actual frequency of FAP in the Republic of Macedonia. A similar incidence has recently been reported for Denmark, indicating that the proportion of cases with colorectal cancer attributable to the FAP syndrome is much lower than the orriginal estimates [20].

      Our detailed molecular analyses did not reveal point mutations or small deletions/insertions at the 5’ end of the APC gene (codons 150 and 1600), where more than 90% of reported mutations have been found in patients with multiple polyposis [3,21].The single molecular defect that was detected, was a large deletion in the APC gene in two brothers (patients #4a and #4b) who had the classical form of the disease. A deletion in the APC gene region was also indicated in patient #6 (homozygous for all intragenic APC polymorphisms) and in patient #3 (abnormal transcript with the deletion of exon 14), but could not be confirmed due to lack of sufficient family members for linkage analysis and of DNA material for mapping, respectively. These results indicate that APC deletions are relatively common in FAP patients, and support the suggestion that patients in whom no mutation is detected by the standard approach [PTT, DGGE, single strand conformation polymorphysm (SSCP)] should be tested for an APC gene deletion [22,23]. Because of the size of the APC gene (more than 150 kb) and the objective difficulty of performing linkage analyses when informative family members are not available, fluorescent in situ hybridisation (FISH) with an APC gene-specific probe [24] and/or real-time PCR amplification for the detection of allelic imbalance in the APC locus [4], should be considered as methods of choice for routine testing of APC gene deletions.

      Only 27 large deletions associated with FAP have been described, accounting for about 10% of all APC gene molecular defects [2]. Of these, nine occurred in patients with various degrees of mental impairment [25,26]. Since patients #4a and #4b had an APC gene deletion but did not show any signs of a mental handicap, mental impairment in patients with FAP may be caused by loss of sequences outside the region flanked by the D5S82 and D5S2055 markers.

      In patients with the attenuated form of the disease (patients #1, #2 and #5) no APC gene mutations were found, despite the extensive DNA testing. It is worth noting that patient #1 probably belongs within the HNPCC group of patients, since all of her neoplastic tissues exhibited the MSI phenotype. Although her familial history could not be obtained (she was an orphan without any siblings or offspring), the clinical features of the disease (small number of polyps, one of which was a villous adenoma early occurrence and right-sided localization of the carcinoma) does not exclude the HNPCC syndrome [7]. Because overlap of the attenuated FAP with HNPCC has been also reported by other investigators [7,27], MSI analysis should be performed in all patients suspected of this syndrome.

      The clinical features and APC gene analysis indicate that patients #2 and #5 may belong to “multiple adenoma patients”. Much has still to be learned about the etiopathogenesis of this condition [3].

      The reason for the appearance of aberrant APC gene transcripts in peripheral blood of patients #1 and #2 is not clear. Such aberrant transcripts were not detected in the peripheral blood of 20 normal individuals and 10 colorectal cancer patients who were subsequently analyzed. One possible explanation is that a clone of malignant colonic epithelial cells with a somatic deletion of part of the APC gene is present in the mononuclear cells used for mRNA extraction. Another is that these transcripts arose from aberrant splicing of the primary transcript of the APC gene. Alternative splicing of individual exons is responsible for the creation of APC mRNA isoforms that lack part of exon 9, exons 7, 10 or 14 have been described [28-31]. However, no aberrant transcripts that lack several exons have been reported to date. This implies that the aberrant transcripts detected in patients #1 and #2 have a role in the etiopathogenesis of the multiple polyposis phenotype either through mutational or epigenic alterations of the splice site selection apparatus in colonic epithelial cells.




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