Abstract

Methods

RESULTS

Considering the two entire groups of adenomas (table 1 ), a significant difference was observed in location, size, and histology of HNPCC adenomas compared with sporadic adenomas (p=0.018, p<0.001, and p=0.010, respectively). HNPCC adenomas had a proximal propensity (50% v 26% of sporadic adenomas). The adenomas showed a wide range of sizes in both groups. The median size of HNPCC adenomas was 2.0 mm (range 0.5–34) while sporadic adenomas had a median size of 5.5 mm (range 0.5–45). HNPCC adenomas were more often tubular in comparison with sporadic adenomas. Even though the HNPCC adenomas were more often small and tubular, they were similarly dysplastic as the larger and more often villous sporadic adenomas.

Table 1

Characteristics of hereditary non-polyposis colorectal cancer (HNPCC) and sporadic adenomas

	HNPCC adenomas		Sporadic adenomas
	n	(%)	n	(%)	p Value
Location
Proximal	47	(50)	39	(26)
Distal	29	(30)	72	(47)	0.018
Rectum	19	(20)	41	(27)
Size
<5 mm	70	(70)	67	(44)	<0.001
≥5 mm	30	(30)	85	(56)
Type
Tubular	79	(79)	97	(64)	0.010
Tubulovillous	21	(21)	55	(36)
Dysplasia
Low grade	68	(68)	114	(76)	0.567
High grade	32	(32)	38	(25)

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The proximal propensity was more evident in highly dysplastic HNPCC adenomas: high grade dysplastic HNPCC adenomas were more often (55%) located proximal to the splenic flexure while only six (15%) of the high grade sporadic polyps were right sided (p<0.001).

In the proximal colon, HNPCC adenomas were smaller in comparison with sporadic polyps (p=0.025) but they were more often highly dysplastic (36% v 13%; p<0.020) (fig 1 ). When comparing size as well as dysplasia at one location (tables 2, 3 ), the difference between the HNPCC and sporadic adenomas was clearly apparent. Larger HNPCC polyps (≥5 mm) in particular, in the proximal colon, were more often highly dysplastic (p<0.001): in fact, all HNPCC adenomas ≥5 mm were highly dysplastic. In the sporadic lesions this was not observed: only 25% of the large sporadic polyps at this site had high grade dysplasia.

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Figure 1

Distribution of proximal hereditary non-polyposis colorectal cancer (HNPCC) and sporadic adenomas by size and dysplasia (the horizontal lines indicate median size).

Table 2

Dysplasia of hereditary non-polyposis colorectal cancer (HNPCC) adenomas by location and size

	Proximal (n=47)		Distal (n=29)		Rectum (n=19)
Dysplasia	Low	High	Low	High	Low	High
	30	17	25	4	9	10
Size
<5 mm	30	6	17	2	8	3
≥5 mm	0	11	8	2	2	6

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Difference between ≥5 mm adenomas in the proximal and distal colon and between the proximal colon and rectum: p=0.009 and p=0.07, respectively.

Table 3

Dysplasia of sporadic adenomas by location and size

	Proximal (n=39)		Distal (n=72)		Rectum (n=41)
Dysplasia	Low	High	Low	High	Low	High
	34	5	45	27	33	8
Size
<5 mm	19	2	22	6	17	1
≥5 mm	15	3	23	21	16	7

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Difference between ≥5 mm adenomas in the proximal and distal colon and between the proximal colon and rectum were not significant.

In the rectum, large HNPCC adenomas were also more often highly dysplastic in comparison with sporadic adenomas ≥5 mm. However, this difference was only borderline significant (p=0.084). HNPCC adenomas in the distal colon were smaller than distal sporadic lesions (66% v 39% <5 mm; p=0.047), mostly tubular (72% v 59%; NS), and nearly always low grade dysplastic (86% v 63%; p=0.020).

A significant difference was not only observed between the HNPCC and sporadic groups but also between the large HNPCC adenomas at the three locations. The large proximal HNPCC adenomas were more often highly dysplastic (100% were highly dysplastic) than HNPCC adenomas in the distal colon (22%; p=0.001) whereas a borderline significant difference was detected between proximal and rectal adenomas (75% highly dysplastic; p=0.07). The large HNPCC adenomas in the rectum were also significantly more often highly dysplastic in comparison with the HNPCC adenomas in the distal colon (p<0.03).

Eleven adenomas belonged to patients with a proven MLH1 mutation while 11 adenomas were removed from patients with a proven MSH2 mutation. Three adenomas were from two subjects who had not undergone genetic testing but a MLH1 mutation had been detected in their families. In total, 15 adenomas, eight low grade and seven high grade dysplastic, showed loss of expression of either MLH1 or MSH2. The three adenomas from the two patients who had not undergone genetic testing all showed loss of MLH1 staining, corresponding to the known mutation in their kindreds. Eight adenomas with loss of MLH1 expression were located in the proximal colon and two were situated in the rectum. Loss of MSH2 expression was observed in three distal, one proximal, and one rectal polyp. The 10 adenomas, all from proven carriers, that had normal expression of MMR proteins were small and low grade dysplastic. Three of these adenomas were located in the proximal colon, four in the distal colon, and the remaining three were located in the rectum.

DISCUSSION

Our results differ from previous studies concerning morphological descriptions of adenomas in patients at risk of HNPCC. The results support the accelerated adenoma-carcinoma sequence of carcinogenesis in HNPCC but at the same time strongly suggest that lesions at different locations, proximal or distal in the colon or in the rectum, behave differently in this respect. More specifically, similar to the high frequency of hereditary non-polyposis colorectal carcinomas proximal to the splenic flexure, HNPCC adenomas have a right preponderance, although less so than carcinomas, and these right sided adenomas are more prone to malignant conversion compared with left sided adenomas.

The large group of HNPCC adenomas included in this study is representative of the Groningen HNPCC population as they comprise all the available benign neoplastic lesions consecutively removed over a 12 year period. The majority of HNPCC adenomas were obtained during surveillance examinations while sporadic adenomas were acquired during colonoscopy to diagnose clinical symptoms such as rectal blood loss, diarrhoea, and abdominal pain. The groups are thus not readily comparable and this probably explains why the HNPCC adenomas were smaller, more often tubular, and not more dysplastic than sporadic adenomas, findings in contrast with reports in the literature.^{9, 11, 20, 26, 27} It is noticeable however that the small and tubular HNPCC adenomas were not less dysplastic than the larger and more often villous sporadic cases, as would be expected.²⁷ To reduce the discrepancy arising from comparing adenomas obtained through surveillance and diagnostic colonoscopies, we also evaluated only the index HNPCC adenomas, those adenomas found during the first colonoscopy (data not shown). The characteristics of these index neoplastic lesions did not differ from those of all HNPCC adenomas reported in the present study. In accordance with Green's observation, they were mostly small and tubular.²⁸ The same team of gastroenterologists performed all colonoscopies. The number of small (<5 mm) HNPCC and sporadic adenomas in all sections of the colon illustrates the thorough endoscopic search in both groups throughout the entire colon. Nevertheless, we cannot exclude the fact that greater care was taken in the examination of the colons of HNPCC subjects than of those of other patients.

The prevalence and grade of dysplasia of adenomas in the general population differ with regard to sex, age, and geographic region.^{27, 29, 30–35} Any bias was minimised by including the same male to female ratio of adenomas in both groups. A sufficient number of age matched subjects however was not available. The histological characteristics of our control group were similar to those of Griffioen et al in another Dutch clinical investigation.³⁶ The results from the control group also correspond to findings of several other studies.^{37, 38} However, other reports, mainly autopsy and surveillance studies, favour a substantially higher frequency of low grade dysplastic sporadic polyps and a lower frequency of villous lesions.^{17, 27, 30, 33} The proportion of proximal adenomas varies in the literature between 11% and 40% (Griffioen et al 17%³⁶).

The essential role of the proximal colon in the pathogenesis of HNPCC is evident from numerous reports of the high incidence of right sided (interval) carcinomas in HNPCC patients.^{1, 6–8, 10, 12, 14, 39–42} In accordance with this, we found that HNPCC adenomas were located more often in the proximal colon than sporadic ones. The proximal propensity suggests alteration in the initiation of neoplastic growth in HNPCC in comparison with the general population. A recent report on the presence of microsatellite instability in both hyperplastic and dysplastic aberrant crypt foci in the colons of HNPCC patients also suggests a possible role for MMR dysfunction in the initiation of neoplastic lesions in HNPCC.⁴³ However, Leach and others proposed that the adenomas in HNPCC develop on a sporadic basis only to provide a substrate for defective DNA MMR genes.⁴⁴

We therefore attempted to substantiate Leach's theory or the “initiation” proposal by performing immunohistochemistry for gene products of MLH1 and MSH2 on adenomas from subjects with a known mutation. In accordance with most reports, absence of immunohistochemical staining was seen in two thirds of HNPCC adenomas—that is, in 44% of low grade and in all high grade dysplastic adenomas.^45–50 Thus our data strongly suggest that DNA repair deficiency is not responsible for the initiation of an adenoma but determines the subsequent progression of the lesion. Although MMR dysfunction is not the first event, it is surely a very early occurrence in the tumorigenesis of HNPCC lesions and it heralds development to high grade dysplasia.

Our results illustrate that the proposed accelerated adenoma-carcinoma sequence in HNPCC is probably site specific—that is, limited to the proximal colon. While the group of HNPCC adenomas as a whole did not exhibit features of increased susceptibility to malignant conversion in comparison with sporadic adenomas, a difference was observed within the group of adenomas proximal to the splenic flexure. The large (≥5 mm) HNPCC adenomas in the proximal bowel were highly dysplastic suggesting more advancement in the malignant transformation than sporadic polyps of a similar size at this location. This and the fact that nine of the 15 HNPCC adenomas that showed loss of protein expression were located in the proximal colon emphasise the predilection of the proximal colon for tumorigenesis in HNPCC. Characteristics of HNPCC adenomas at specific sites in the large bowel have had limited attention to date as authors have only reported results on HNPCC adenomas in general. In contrast, sporadic adenomas have been described with regard to location. The National Polyp Study observed an increased frequency of high grade dysplasia in adenomas located distal to the splenic flexure but attributed this mainly to increased size and a villous component rather than to location per se.²⁷ Nusko et al reported that sporadic right sided adenomas have a lower risk of becoming malignant compared with sporadic left sided adenomas.⁵¹ The reason for the reverse situation in HNPCC is still unclear.^{25, 52–54}

In conclusion, half of the HNPCC adenomas were located in the proximal colon but this does not fully explain the proximal propensity of cancers. Our data showed that progression to high grade dysplasia was more common in proximal than in distal HNPCC adenomas, indicating a faster transformation rate from early adenoma to cancer in the proximal colon. MMR gene malfunction probably does not initiate adenoma development but is present at a very early stage of tumorigenesis and seems to herald the development of high grade dysplasia. At this time there is no explanation for this site specific behaviour of HNPCC lesions or for the susceptibility of epithelial cells in the proximal colon to somatic MMR gene mutations.

Acknowledgments

Abbreviations

REFERENCES