JPS is caused by a germline defect in SMAD4
or BMPR1A. 16,17
Transgenic mice develop distinct JPS-like phenotypes depending on which of the JPS causing genes is targeted. Smad4
mutant mice show hyperplastic or serrated epithelium and minor stromal overgrowth whereas Bmpr1a
mutant mice or mice with inhibited BMP signalling through transgenic expression of noggin show polyps with reactive changes of the epithelium, crypt dilatation and a prominent stromal compartment.13–15,27
We investigated and compared the histological phenotype of human juvenile polyps from patients with a SMAD4
Consistent with earlier reports, histological evaluation revealed a subset of JPS polyps featuring an epithelial phenotype (40%), deviating from classic juvenile polyps characterised by a prominent stromal compartment (60%).10,19
The epithelial phenotype was more prevalent in cases with a SMAD4
germline mutation, whereas, juvenile polyps with a BMPR1A
germline mutation predominantly had the classic juvenile polyp phenotype (p<0.001).
Interestingly, quantitative evaluation of the crypt-stroma ratio confirmed our initial histological findings. Juvenile polyps with a SMAD4 germline mutation had a significantly higher crypt-stroma ratio compared to those with a BMPR1A germline mutation indicating a higher crypt density in the former and confirming the epithelial phenotype. These results underscore the relevance of crypt-density as discriminatory feature between the classic and epithelial type juvenile polyp.
Nevertheless, 8 JPS polyps showed a discrepancy between histological and stereological classification and may thus be considered to display intermediate features (). Re-evaluation of these polyps revealed that massive crypt dilatation may result in a juvenile polyp initially labelled as classic phenotype to be considered a B type polyp by stereologic means. On the other hand, surface erosion and subsequent inflammation in juvenile polyps with a proliferative core may cause a polyp to be classified histologically as an epithelial phenotype yet stereologically be scored as type A.
Similar frequencies of indefinite, low grade and high grade dysplasia were found in juvenile polyps from patients with either a SMAD4
germline defect, contradicting earlier reports of a more dysplasia prone intestinal phenotype in polyps with a SMAD4
Interestingly, 50% of all foci of low or high grade dysplasia in juvenile polyps with a BMPR1A
germline defect were found in type A polyps, whereas, none of the type A polyps with a SMAD4
germline defect contained dysplasia.
To investigate whether the neoplastic change in the juvenile polyps could be attributed to mutations in the conventional adenoma-carcinoma sequence, the dysplastic areas were investigated for APC
mutations. Our results revealed only 2 polyps with K-ras
mutations, consequently, prior reports of APC
mutations in dysplastic polyps 31
could not be confirmed. These data suggest that the conventional adenoma-carcinoma sequence may not play a distinct role in JPS tumour formation, as has been concluded by other investigators.32
Moreover, the different phenotypes could not be attributed to either an APC
ras mutation. Evaluation of the Ki67 proliferation marker demonstrated that focal loss of compartmentalisation of Ki67 i.e. expanded cell cycle activity, could not be linked to an A or B phenotype when stratified by dysplasia. This finding is consistent with the concept that loss of compartmentalisation of the proliferative zone is a general feature of dysplasia regardless of the underlying genetic defect.
Few prior studies have been dedicated to the evaluation of a relation in genetic make-up and histological phenotype in juvenile polyps. Handra-Luca et al analyzed a series of juvenile polyps for percentage and morphology of epithelial and stromal components, blood vessels, level of inflammation, hyperplasia and dysplasia.12
They characterized several distinctive features of polyps with a SMAD4
mutation: association with various grades of dysplasia, upper digestive tract location and malformative vessels in the stroma. BMPR1A
polyps were exclusively of the lower digestive tract and were not associated with dysplasia or malformative vessels. As mentioned, our results showed no distinction with regard to presence and/or grade of dysplasia between juvenile polyps in the setting of a SMAD4
germline mutation. Although presence of vascular malformation was evaluated in the initial screening it was not a discriminatory feature in our series of juvenile polyps (data not shown). No polyps of the upper digestive tract were present in our cohort.
Also, Aretz et al describe various histological phenotypes ranging from juvenile polyps to hyperplastic polyps and pseudopolyps albeit with adenomatous components or even adenomas in juvenile polyposis patients with an established germline mutation in one of the associated genes but no correlation between genotype and histological phenotype is provided.1
Nevertheless, our results confirm the wide array of histological phenotype variations encountered in the setting of juvenile polyposis.
are both key components of the TGF-β/BMP signalling pathway maintaining homeostasis of the intestinal lining through processes of cellular proliferation (TGF-β) and differentiation and apoptosis (BMP). Signal transduction takes place through phosphorylation of the type 1 transmembrane receptor kinase (i.e. BMPR1A) by the type 2 receptor. The activated type 1 receptor phosphorylates the pathway restricted SMAD2 and 3 (TGF-β) or SMAD1,5 and 8 (BMP) which, in complex with the common mediator SMAD4, is translocated to the nucleus where target gene transcription is regulated.23
Individuals with germline defects in SMAD4
and consequent disrupted TGF-β/BMP signalling develop multiple hamartomatous malformations in the gastrointestinal tract. These hamartomas are often characterized by an abnormal stromal component suggesting a prominent role for the stroma in polyp formation. The polyp epithelium initially shows normal maturation, although inflammation is common and may cause reactive changes. Subsequent dysplastic progression of the epithelium has been proposed to be the result of the altered microenvironment.21
Recent studies provide evidence that conditional inactivation of Bmpr2
in the intestinal mesenchyme leads to mice developing hamartoma-like polyps, whereas, conditional deletion of Bmpr1a
in the epithelium showed elongation of the villi, but no de-novo crypt or polyp formation.2,3
Consistent loss of heterozygosity (LOH) of the BMPR1A
locus has thus far not been detected in the epithelium or stroma of JPS polyps from patients with a BMPR1A
germline mutation 16
; although one study reports somatic loss of the 10q22 region exclusively in the lamina propria and not in the epithelium suggesting inactivation of BMPR1A
might be a stromal event.18
Selective loss of Smad4-dependent signalling in T cells leads to a JPS-like phenotype reminiscent of what we described as a type A polyp with cystic spaces lined by columnar epithelium surrounded by abundant stroma.20 Smad4
heterozygous mice on the other hand develop polyps with an epithelial phenotype (type B) and show LOH specifically in the epithelium of larger polyps.27,32
Likewise, LOH of the SMAD4
locus occurs in the epithelium of juvenile polyps from patients with a SMAD4
The exact role of SMAD4 and timing of SMAD4
inactivation in polyp initiation and progression remains poorly understood but it seems that these polyps develop mainly through an epithelial defect.22,30
In addition to phenotype classification, SMAD4 immunohistochemistry may provide a specific marker for the detection of a SMAD4
Although the number of polyps in this study is limited, we propose that juvenile polyps with a SMAD4 germline defect have a higher crypt density regardless of the dysplastic status. On the contrary, juvenile polyps with a BMPR1A defect are more often classic juvenile polyps with a prominent stromal compartment. Crypt density in these polyps is initially low but may increase due to neoplastic change of the epithelium. Investigation of Ki67 immunohistochemistry reveals that the difference in crypt density in juvenile polyps with a SMAD4 or BMPR1A germline mutation is not a result of altered proliferative activity.
We conclude that juvenile polyps in the setting of juvenile polyposis syndrome may exhibit distinct phenotypes. Juvenile polyps with a SMAD4 germline mutation more likely express an epithelial phenotype with a relatively high crypt density, whereas, juvenile polyps with a BMPR1A mutation are usually the classic juvenile polyp phenotype with a prominent stromal compartment. Importantly, we find similar rates for all grades of dysplasia in juvenile polyps with either a SMAD4 or BMPR1A background.