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In their electronic letter Smith et al. claim to have found evidence that adult Prader-Willi syndrome (PWS) patients with maternal uniparental disomy (UPD) have an increased mortality compared to PWS patients with deletion (Del). The main results can be summarised as in Table 1.
Table 1 Prader-Willi syndrome patients: Number of UPD and Del divided into dead and alive.
Obviously, 50% (4/8) of UPD patients and 23% (5/22) of Del patients died during follow-up. The events are few, and the difference is far from significant (P=0.20; Fisher's exact test). However, Smith et al. claim that 44% (4/9) with UPD died while 18% (4/22) with UPD were still alive. Similar calculations were made for the Del patients, and a P-value of 0.05 is mentioned. These calculations make no sense. We are unable to guess the source of the significant P-value.
A reasonable mortality comparison should include the time at risk; it was 44 years in the UPD group and 95 years in the Del group. A Cox regression gave a hazard ratio of 1.31 (95% CI 0.32-5.33; P=0.37) when comparing UPD with Del, adjusted for age and sex. We are, however, not certain about this result due to inconsistent information in Table 2 concerning age on admission, age at death, and follow-up time for three patients (e.g. patient F13 was 17 years on admission and 22 years at death, but had a follow-up time of 0 years). For these patients we recalculated the follow-up time, using the time span from admission to death.
Thus, the results cannot be considered to give any evidence of an increased mortality among UPD compared to Del patients or vice versa. PWS patients, especially those with UPD, and their families should not have added any further unjustified reason to worry.
(1) Smith A, Loughnan G, Steinbeck K. Death in adults with Prader-Willi syndrome may be correlated with maternal uniparental disomy. J Med Genet 2003;40:e63.