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Serologic
and genetic testing for IBD
By Diana Swift
Serologic testing for irritable bowel disease will soon play a role
in diagnosis, prognosis and patient management, said William Sandborn,
MD, Professor of Medicine, Division of Gastroenteroloy, Hepatology
and Internal Medicine, Mayo Clinic, Rochester, MN.
Although there have been no definitive trials in screening adults,
some work has been done in the pediatric setting (Ruemmele et al,
Gastroenterology 1998;115:822-829), and there has been some modeling
and decision analysis done in adult patients, he said.
The candidate tests for a predictive role are; pANCA (anti-neutrophil
cytoplasm autoantibody with perinuclear staining); ASCA (anti-Saccharomyces
cerevisiae antibody); and newer assays such as I-2 antibody (a bacterial
transcription factor) and ompC (outer membrane protein core).
Work done about a decade ago at Cedars-Sinai Medical Center, Los
Angeles, found that while normal controls were negative for pANCA,
it was present in patients with ulcerative colitis, even after colectomy
(Duerr et al, Gastroenterology 1991;100:1385-1391). “There
was a frequency for the perinuclear pattern of about 60 per cent.
You see a little bit of this in Crohn’s and collagenous colitis,
but the majority of patients are negative. So this test is fairly
specific but not that sensitive.”
With ASCA, a U.K. study done in the late 1980s found that while
only a minority of controls and UC patients show elevated IgG and
IgA antibodies to S. cerevisiae, about one-half to two-thirds of
Crohn’s patients have elevated titres of IgG and/or IgA-ASCA.
“So again, this is fairly specific and moderately sensitive,”
Dr. Sandborn said.
Studied in combination by French investigators at the University
of Lille, pANCA/ASCA improved specificity and positive predictive
value. A Crohn’s patient was likely to be ASCA-positive/ANCA-negative.
With an ANCA-positive/ASCA-negative result, the chances of having
UC were 97 per cent. “So you now have a very high specificity
and positive predictive value and a sensitivity of about 60 per
cent,” Dr. Sandborn said. Similarly, if a patient is pANCA-negative/ASCA-positive,
the sensitivity is about 50 per cent, and the specificity is once
again high at 97 per cent.
“If you’re both ANCA-negative and ASCA-negative, or if
you’re double-positive, then it’s not entirely clear what
you have. But this doesn’t mean you don’t have IBD, just
that we can’t differentiate between the two diseases. With
only one of these two specific patterns, we pretty well know what’s
going on.”
For sensitivity, the Prometheus pANCA assay outperforms other labs
(commercial or academic), has good PPV and is equal to other labs
for specificity, he said.
Looking at indeterminate colitis, a multicentre European study followed
100 patients for several years to see if they would remain indeterminate
or progress to a clinical diagnosis of CD or UC, and to ascertain
if ASCA-positives were more likely to progress to CD, and ANCA-positives
to UC. “Investigators found that if you’re ASCA-positive
and ANCA-negative, there’s an 89 per cent positive predictive
value that you will move on to Crohn’s,” he said.
ANCA-positives/ASCA-negatives have a 64 per cent chance of moving
on to UC, whereas double-negatives have an 85 per cent chance of
remaining indeterminate.
“But is all this good enough to base a treatment decision on,
or to decide if a patient should move on to Remicade or methotrexate
or get an ileoanal pouch?” he asked. “This is really just
another piece of the puzzle that goes into the mix of clinical history
and endoscopic and radiologic findings to help decide the best course
of treatment.”
As for the newer assays, he noted that I-2 antibody, which will
soon be available for clinical use, has been associated with CD
patients but not with normal controls, UC patients or those with
other inflammatory conditions.
Recent profiling by Prometheus found that some Crohn’s patients
are ompC-positive and ASCA-negative. “As we keep adding markers
to the serologic panel, we will improve the sensitivity and clinical
characteristics of these tests,” he said.
Diagnosis to prognosis
A Cedars-Sinai study found that UC patients who were pANCA-positive
were more likely to develop pouchitis after surgery. “At the
Mayo, we had already seen that the frequency of acute and chronic
pouchitis was double in patients who were pANCA-positive versus
pANCA-negative,” he said (Sandborn et al Gastroenterology 2000;118:A106).
The Cedars-Sinai researchers also found that not just positivity,
but also titres, affected risk: patients with high pANCA titres
had a 50 per cent increased risk of pouchitis. “This study
needs confirmation in practice. So don’t withhold an ileoanal
pouch based on this alone. But you might want to go the extra mile
to avoid surgery in such patients.”
As for ASCA, it is associated with small bowel Crohn’s, for
which it has a sensitivity of 70 to 80 per cent. “We need additional
markers to pick up Crohn’s colitis, but for small bowel disease,
this looks pretty good,” he said.
Interestingly, the Cedars-Sinai study observed that a small subset
of 10 to 15 per cent of patients with CD colitis and ANCA positivity
have clinical features that resemble the UC phenotype. “I don’t
know what to do clinically with this information at present, but
likely these observations will pull together in a serology-based
treatment algorithm—at least in an adjunctive way,” Dr.
Sandborn said.
He noted that the NOD2 gene puts Crohn’s patients at risk for
small bowel disease and progression to fibrostenosis and fistulas.
If a patient is ASCA-positive, he is more likely to develop fibrostenosis
and internal penetrating disease. “These are probably linked,”
he said. ANCA-positives are more apt to have left-sided colitis.
“It’s a bit early to hang your hat on this in terms of
managing patients, but you can see where we’re going—which
ones you want to operate on and which ones you want to move more
into additional medical therapy.”
Dr. Sandborn said ASCA-positive patients had a relatively high rate
of response to infliximab, while in their ASCA-negative/ANCA-positive
counterparts response was somewhat lower. “But in my view,
you should not use this lab test to withhold infliximab from a deserving
patient,” he said.
TPMT activity
Dr. Sandborn addressed the utility of testing AZA/6-MP patients
for TPMT activity to avoid leukopenia, and also for blood concentrations
of these prodrugs’ active metabolites such as 6-TGN and 6-MMP
to determine if patients are receiving therapeutic doses.
TPMT activity is genetically mediated, he said, with about 90 per
cent of individuals having normal enzyme activity, 10 per cent having
high activity, and about one in 300 having low activity. This last
group has homozygous low-activity alleles and hence is unable to
metabolize AZA/6-MP.
A French study genotyped 40 CD patients with severe myelosuppression
on AZA. In those with two low-activity alleles, leukopenia occurred
within a month or a month and a half. In intermediates it occurred
within six months, and in normals it was spread over an extended
period, in some cases as long as 87 months.
“Doing a TPMT test at baseline does not let you off the hook
for measuring CBCs, but it does protect some patients from severe
leuokopenia early on,” Dr. Sandborn said. One of his intermediate-metabolizing
patients on AZA developed early myelosuppression and subsequent
Pneumocystis carnii pneumonia. “You can go for a long time
without trouble, but then you can get a severe case with opportunistic
infections. So I think it’s worth a one-time genetic test at
the start of treatment to see which patients you should dose down.”
As for therapeutic doses, he said, a study by Ernest Seidman found
that patients with 6-TGN blood levels of over 235 had a 65 per cent
response rate to AZA. Only 27 per cent of those with lower concentrations
did so. “This study suggested that it is possible to target
dosing to blood levels and so improve the efficacy of AZA and 6-MP,”
he said. On the other hand, we performed a study at Mayo that did
not show a correlation between disease activity as measured by IBDQ
and levels of 6-TGN (Lowery et al, Gut, 2001)
At his institution, TPMT is measured at baseline, and patients are
given high doses of more than 2 g/kg/d at the initiation of AZA
treatment. In the community setting, however, patients are more
likely to start on 50mg to 100mg. “But physicians are becoming
more comfortable with getting patients up to therapeutic doses now
that we can measure blood concentrations of 6-TGN,” Dr. Sandborn
said.
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