DNA sequencing performed by numerous groups around the world shows that the majority of HHT patients have mutations in either ENG
The mutation detection rates in the scans range from 62% to 93%, a range that matches the detection rates found in scans of other genes in mendelian diseases.15
There are several explanations why a patient might appear mutation negative in a mutation screen. For diseases such as HHT that have highly variable clinical presentations, it is possible that some of the referred patients may not actually have the disease in question. In the present study, presuming that all members of the cohort are affected HHT patients, another explanation could be that these patients may harbour non‐coding mutations, such as alterations in poorly characterised regulatory regions of the gene, including the promoter, introns, and 5′ and 3′ untranslated regions. Additionally, large scale genomic rearrangements involving whole exons, partial gene loss or duplication, and intrachromosomal or interchromosomal rearrangements will bypass detection by DNA sequencing. It is also possible that some patients will have mutations in other genes that lead to the same disease phenotype. A recent report of two unrelated HHT families that exhibit linkage to a region on chromosome 5 demonstrates that at least one other gene is involved in HHT.16
HHT patients may harbour a mutation in this as yet unidentified gene.
A final explanation for these ENG/ALK1 mutation negative HHT cases might be that some of these patients harbour mutations in SMAD4. Based on a molecular diagnosis, these patients would have the JP‐HHT syndrome, but they may have been diagnosed with HHT due to occult or unrecognised manifestations of JP. In this study we queried if SMAD4 mutations were found in the general HHT population. To determine the answer, we sequenced the coding exons and adjacent flanking intronic regions of SMAD4 in a cohort of 30 unrelated people with presumptive HHT. These people had been referred for DNA based diagnostic testing for HHT and all were negative for mutations in ENG and ALK1. One of them had colon cancer, which is sometimes the first symptomatic manifestation of JP, but none of the others was known or reported to have JP. Although GI bleeding was reported in five of these subjects, this was interpreted as a symptom of HHT.
All of the SMAD4 mutations identified in JP‐HHT patients to date are found in the COOH terminus of the protein. The three mutations identified in the subjects here are in the COOH terminus of SMAD4 and are nearly identical in location and type of mutation to those seen in JP‐HHT patients (fig 1). This strongly suggests that the three HHT subjects with SMAD4 mutations in this cohort are affected with JP‐HHT syndrome. Two of these three SMAD4‐HHT subjects are known to have colonic polyps, and one of these two has colorectal cancer. The status of the third individual regarding JP symptoms is unknown.
There is growing evidence of distinct phenotypic differences between patients with HHT1 and HHT2.26,27,28
Similarly, other studies have demonstrated phenotypic differences between JP patients with SMAD4
mutations compared with patients with mutations in BMPR1A
the second gene known to be involved in JP. JP is also known to be variably penetrant and is associated with an increased risk of gastrointestinal cancer.22,31
Knowing which gene is responsible for the disease in an individual HHT patient will aid in the proper management and care of the patient and, significantly, of related family members.
We recommend that when genetic testing is advised for HHT patients, SMAD4 should be screened if no mutations are found in either ENG or ALK1. We also recommend that screening for colonic and gastric polyps be considered in people in whom neither ENG nor ALK1 mutations have been found, in whom SMAD4 mutations have been uncovered, or in anyone with anaemia that cannot be completely explained by epistaxis or some other cause. This screening will identify those HHT patients with occult polyposis. Early detection of colonic polyps in any patient, but in particular in JP‐HHT patients, could prevent the development of colorectal cancer by finding and removing precancerous polyps.
There appears to be a high rate of de novo cases of JP‐HHT,20
and all three of the SMAD4
‐HHT patients in this study reported no family history of HHT. Although a positive family history is one of the criteria for HHT diagnosis,21SMAD4
mutation carriers may often lack this key diagnostic feature, potentially making the clinical diagnosis more difficult. In the cases of presumed HHT without any apparent family history, an argument might be made that SMAD4
mutation analysis should precede analysis of ENG
, as de novo mutations in these other HHT genes appear to be rare.8,13,14
An HHT patient found to harbour a SMAD4
mutation would be a prime situation where genetic testing can be used to guide clinical management. Because of the increased risk of gastrointestinal cancer associated with JP, it is critically important for a patient with a SMAD4
mutation to be screened for JP. We suggest that HHT patients harbouring a SMAD4
mutation should be considered at high risk of JP‐HHT, requiring more intensive colorectal cancer screening strategies than those recommended for the average risk population.32
Correspondingly, JP patients with SMAD4
mutations similar to those seen in JP‐HHT patients should be examined for the visceral manifestations of HHT which can present suddenly and catastrophically.
SMAD4, ENG, and ALK1 are all members of the TGF‐β signalling pathway, and mutations in the genes encoding them can cause a broad constellation of phenotypes with both distinct and overlapping clinical features. It has been known for a number of years that mutations in SMAD4
and we have recently shown that certain types of mutations in SMAD4
Here we report that unselected HHT patients have SMAD4
mutations that are strikingly similar to those seen in JP‐HHT patients. In JP cases with the same types of SMAD4
mutations, it remains to be seen if these JP patients exhibit symptoms of HHT due to the paucity of clinical descriptions in the literature. Mutations in ENG
have recently been reported in patients with JP,33
and mutations in ALK1
cause some cases of primary pulmonary hypertension.34,35,36
Molecular dissection of the interconnections between these genes and the effects of aberrant signalling through the TGF‐β and other alternative signalling pathways, will help to further elucidate the pathophysiology of these inherited diseases.