Commentary: Mendelian randomization—an update on its use to evaluate allogeneic stem cell transplantation in leukaemia

Keith Wheatley and Richard Gray

University of Birmingham Clinical Trials Unit, 1 Somerset Road, Birmingham B15 2RR, UK. E-mail: k.wheatley{at}bham.ac.uk

In this issue of the International Journal of Epidemiology a Lancet letter by Martijn Katan is reprinted.1 Katan's letter introduced the notion that genotype–disease associations could be studied as a way of imputing the causal nature of the association between an environmentally influenceable intermediate phenotype and disease. This approach has come to be referred to as ‘Mendelian randomization’.2,3 This term had previously been introduced in a somewhat different context with reference to the evaluation of the efficacy of allogeneic stem cell transplantation (SCT) in leukaemia. In this commentary we elucidate this method and provide an update regarding its implementation.

Randomized controlled trials (RCT) are the gold standard for the evaluation treatment efficacy.4 Randomization to one treatment or another is necessary in order to avoid the selection biases that might be introduced if some patients were electively chosen to receive one treatment and different types of patient another treatment (e.g. young and fit patients might be given the more intensive therapy, older and frailer patients the milder treatment). RCT have provided much valuable evidence on which treatments work, and often on those that are ineffective too, in many medical specialties. One disease area in which RCT have not proved feasible is the assessment of the role of SCT in haematological malignancy. SCT is a procedure in which stem cells, either from bone marrow or peripheral blood, are harvested from a suitable donor and infused into the patient after their own bone marrow has been wiped out by high-dose chemotherapy and/or radiotherapy. For a donor to be suitable, he/she must have a tissue type that is compatible with the patient's own tissue type, otherwise the risk of rejection is great. The commonest source of a suitable donor is a sibling of the patient, although unrelated donor transplant (UDSCT) is becoming more frequent as greater numbers of healthy volunteers contribute to registries of potential donors.

We present here an update on a method of overcoming this lack of randomized evidence, using acute myeloid leukaemia (AML) as an example. Many haematologists have strong prior beliefs on the role of SCT in AML and there is a widely held view that SCT is the treatment of choice for younger patients who have a human leucocyte antigen (HLA)-matched sibling donor available (though this view is based largely on non-randomized evidence, often from transplant registries,5 that is intrinsically unreliable because of the possible selection biases mentioned above6). The UK Medical Research Council (MRC) AML12 trial attempted to evaluate the role of SCT compared with conventional intensive consolidation chemotherapy (CCT) with a randomization between the two treatment modalities in patients who had a HLA-matched sibling donor available.7 This approach did not prove successful, with only 40 patients being randomized. We are not aware of any other trials in which patients with AML have been randomized between SCT versus CCT.

Thus, we need an alternative, unbiased method for evaluating SCT. The answer is provided by Mendelian, or genetic, randomization.8 This is named after Gregor Mendel (1822–1884), a Moravian and an Augustinian monk. In 1865 Mendel published a book titled Treatises on Plant Hybrids in which he reported the results of his experiments on heredity, including the inheritance of traits by offspring randomly half from the mother and half from the father. In the haematological context, for a patient's sibling to be a suitable donor, he/she must have inherited the same tissue type as the patient from their mother and father. Since the chances of there being a match depend on the random assortment of genes at fertilization, only one in four siblings will be expected to have the same tissue type as the patient. Thus, whether or not a patient has a matched sibling donor available is essentially a random process and the presence or absence of a donor can be used as a surrogate for randomization. The efficacy of SCT compared with CCT can then be assessed, in an essentially unbiased fashion, by the comparison of all the patients in a study who have a donor available, and who can potentially receive an SCT, with those who do not have a donor, and who cannot undergo SCT. Importantly, the analysis must follow the same intention-to-treat principle that is applied in RCT, i.e. all patients must be analysed in the group to which they are allocated irrespective of whether or not they receive that treatment.

Over the last few years, a number of reports have used donor versus no donor analysis appropriately to evaluate SCT. The BGMT,9 EORTC,10 and MRC11,12 groups have performed analyses that included all patients in the donor and no donor groups. These studies clearly show that SCT reduces the risk of relapse substantially (e.g. 36% versus 52% at 7 years in the MRC AML10 trial, hazard ratio = 0.63, P < 0.0001). There are also clear risks with SCT and procedure-related mortality following transplant can be greater than 20%. For example, in the donor versus no donor comparison in MRC AML10, 19% of donor patients, with those who did not actually receive a transplant included in the denominator, died in first complete remis- sion (CR)—the stage of treatment/disease, following induc-tion chemotherapy, at which SCT is preferentially delivered—compared with 9% of the no donor group (hazard ratio = 2.08, P < 0.001). These competing factors meant that in none of these studies was there a significant benefit for SCT on overall survival when taken individually. However, in all three SCT was somewhat better, so a meta-analysis might indicate a significant survival benefit.

Unfortunately, other groups that have claimed to have performed donor versus no donor analysis have not actually done so. The CCG13 and US Intergroup,14 while appropriately including all patients with a donor in the donor group, failed to analyse all patients without donors in the no donor group. Instead, they included only those patients who were randomized between CCT and autologous transplant (ASCT) in the no donor group. Thus, in the CCG trial 24% of children without a donor were excluded, while the US Intergroup study excluded 33% of the patients who had achieved remission. Similar inadequately analysed studies continue to be reported.15 If these studies are then included in a meta-analysis,16 the meta-analysis is also flawed. This failure to comply with the intention-to-treat principle could introduce serious bias into the analysis, since patients who do not undergo the randomization to chemotherapy versus ASCT may differ systematically, not necessarily in an identifiable way, from those who do. The reason for this invalid analysis is probably the wish to make direct comparisons between SCT and CCT, and SCT and ASCT. One drawback of the donor versus no donor approach is that such comparisons are not possible and the no donor group, irrespective of treatment, has to be treated as a whole. This is because it is not possible to identify those patients with a donor who would have had CCT had they not had a donor and compare them with the no donor CCT patients; similarly, those who would have received ASCT had they not had a donor cannot be identified for comparison with the no donor group who had ASCT. This means that it is only possible to evaluate SCT against all other consolidation therapies combined.

A problem with donor versus no donor analysis is that compliance of the donor group is often poor, with a substantial proportion not receiving SCT. While this will lead to a dilution of any treatment effects and hence to underestimation of the benefits (or indeed harms) of SCT, it does not introduce any qualitative bias.

For donor versus no donor comparisons to be effectively a randomized comparison, and hence be truly reliable, it is necessary to ensure that the tissue typing data are as accurate as possible. In a standard randomization, the patient population is readily definable, but this may be less clear cut in the case of a Mendelian randomization. Ideally, tissue typing data would be obtained direct from the typing laboratory and would include the number of siblings typed, with dates and results for each sibling. This was done in the MRC AML9 trial, but is logistically demanding and has not been feasible in subsequent, much larger, MRC trials.

Another issue to consider is whether to include patients who have no siblings in the no donor group, since to do so may introduce bias in favour of the donor group. The usual reference time point for donor versus no donor analysis is the date that first complete remission was achieved. A patient with no siblings can be immediately assigned to the no donor group on this date and becomes at risk of suffering an event straightaway. However, since tissue typing may not have been completed, or even initiated, for patients with siblings, these patients start off in an unknown donor status category and only after some time, possibly several months, are assigned to either the donor or the no donor group. Thus there is a time lag in patients with siblings, some of who will be found to have a donor. If a patient with a sibling were to relapse early or die in CR before tissue typing was complete, they would be excluded from the analysis on the basis that the presence or absence of a donor had not been established, leading to differential exclusion of early events in patients who would have been found to have a donor. Empirically, the MRC found in the AML10 trial that there was no difference in outcome between the no sibling and no donor groups so the former were included with the latter in the analysis, thereby giving larger numbers, but this should not be considered automatic policy and should be reviewed in each case.

The above argument could be extended to the number of siblings that a patient has. The more siblings there are, the greater the chance of finding a donor, but also the longer time it might take to complete tissue typing. This means that a patient in a small family may, on average, be found to have no donor sooner than a patient in a larger family is found to have a donor, so becomes at risk of having an event earlier, again introducing a possible slight bias in favour of the donor group. The ideal method to take account of the time needed to identify a donor would be to perform a Mantel-Byar analysis,17 with all patients starting in the no donor group. Those who are found to have a matched sibling would then be right censored from the no donor group and left censored into the donor group on the date that a match is identified. Such an analysis does require detailed information on the tissue typing process to be available and, as discussed above, this may not be logistically feasible.

Furthermore, if there are important prognostic factors that correlate with the number of siblings that a patient has, then account will need to be taken of these in the analysis. For example, children (especially young ones) are likely to have fewer siblings than adults because their family may not yet be complete—and because there is a trend towards smaller families nowadays—but they also have a better prognosis than older patients with AML. Thus, the analysis should be stratified by age in order to adjust for this confounding factor.

Even with the use of Mendelian randomization, the role of SCT in AML for the majority of patients remains unclear (though it has been shown that the risks of transplant outweigh the benefits in the small group of patients with favourable risk disease as defined by cytogenetic abnormalities and these patients are no longer transplanted). Despite the large size of the MRC studies (nearly 2000 patients aged <45 years have been analysed in the AML10 and AML12 trials), confidence intervals are still relatively wide and, excluding good risk patients, are compatible both with no survival benefit for SCT and with a moderate, and possibly worthwhile (patient quality of life also needs to be considered, as does cost), improvement in outcome. Moreover, continuing advances in both chemotherapy regimens and in transplant technology and strategies mean that the balance between the two approaches may be in a state of flux. After 20 years of investigating this issue, the current MRC trial, AML15, is still evaluating the role of SCT, using a donor versus no donor comparison based on the principle of Mendelian randomization. In the absence of direct randomized comparisons, this will remain the best way to evaluate the role of SCT.


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