Department of Reproductive Medicine, The General Infirmary, Leeds LS2 9NS, UK. E-mail: adam.balen{at}leedsth.nhs.uk
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Abstract |
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Key words: BSE/prion disease/recombinant gonadotrophins/urinary gonadotrophins/variant CJD
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Introduction |
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Prion diseasesbackground |
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Prions are pathogenic forms of proteins that are naturally produced by nerve cells and other cells. The normal protein isoform is referred to as prion protein cellular (PrPc). All forms of prion disease are associated with the accumulation of an abnormal form of prion protein (scrapie, PrPSc) within the central nervous system (CNS). PrPSc is relatively protease resistant and so accumulates in plaques in the CNS. There is increasing evidence to indicate that the transmissible agent may be composed entirely of the abnormal form of prion protein (Department of Health, 2001a). Prions are not micro-organisms.
Between 1970 and December 2000, the National CJD Surveillance Unit identified 970 cases of classical CJD in the UK of which 888 were sporadic, 40 were familial and 42 were acquired as a result of treatment with human pituitary extract or brain membranes (National CJD Surveillance Unit, 2000). New variant CJD (vCJD) was first recognized as a distinct clinical entity in 1996 (Will et al., 1996
). This new disease is believed to be caused by the same abnormal prion protein (PrPSc) that causes BSE and is thought to result from eating contaminated beef products. To 6th August 2001, there have been a total of 106 confirmed or probable cases of vCJD in the UK (Department of Health, 2001b
). It is not known how many people have been infected but have not yet developed symptoms. Although there have been no documented cases of transmission of vCJD through medical interventions to date, it must be assumed that vCJD has the potential for transmission between patients as has been shown for classical CJD (Department of Health, 2001a
). The incubation time for the emergence of the disease may be many decades (Bruce et al., 1997
).
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Could human urine contain prion infectivity? |
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The issue to tax us here is whether pooled urine from post-menopausal women might contain infective agents that could cause prion disease in women treated with gonadotrophins. There is particular sensitivity because of the development of CJD in patients treated both by gonadotrophins and growth hormone purified from cadaveric human pituitary glands. More than 80 cases of growth hormone related CJD have so far been reported in the UK, USA and France, with an incubation time of around 10 years (Committee On Safety Of Medicines, 1998). A genetic susceptibility to infection has been demonstrated. Gonadotrophins were prepared from human pituitaries for a shorter period of time than growth hormone and only four cases of CJD have been reported to date (all in Australia) (Committee On Safety Of Medicines, 1998).
The report that has stirred up the current debate is the finding of a protease resistant isoform of PrP in the urine of scrapie-infected hamsters, BSE-infected cattle and humans suffering from CJD (Shaked et al., 2001). Most of the CJD patients were actually patients carrying the E200K mutation (Gabizon et al., 1996
). Urine from humans infected with vCJD was not examined. The conclusions drawn by Shaked and colleagues were that urine could be used to provide a simple, non-invasive test for prion diseases. They also demonstrated the appearance of PrPSc in urine before its accumulation in brain, thus inferring that the PrP urine test could be used during subclinical stages of infection.
There is no suggestion, however, that urine might contain an infective agent and furthermore, inoculation experiments have been performed (Shaked et al., 2001) which failed to confirm infectivity. In their experiments hamsters were inoculated with either samples containing urine PrP from normal or hamsters with symptomatic scrapie or with brain samples from symptomatic-scrapie hamsters that had been diluted to contain similar concentrations of PrPSc. Whilst the animals inoculated with scrapie-infected brain samples suffered from fatal symptoms after about 80 days, none of the animals inoculated with urine samples from hamsters with symptomatic scrapie developed clinical symptoms of prion disease up to 270 daysalthough they did test positive for the urinary PrPSc and one of three hamsters that was killed after 120 days showed low concentrations of PrPSc in the brain. This suggests that inoculation with urinary PrPSc may result in a subclinical or carrier state prion infection. The implications of this are unclear and there is certainly a need to repeat and confirm these observations (Professor R.Will, personal communication) before drawing conclusions about the putative infectivity of urine from prion-infected humans.
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Transmission of prion disease |
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Urinary gonadotrophins |
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Recombinant FSH |
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Both urinary derived and recombinantly engineered gonadotrophin preparations require purification. Procedures for ensuring that gonadotrophin preparations are free from contamination with infective agents include a number of filtration, anionic exchange chromatography and precipitation stepsdetailed description of which is beyond the scope of this paper (Recombinant Human FSH Product Development Group, 1998; British Pharmacopoeia, 2001
). Thus bacteria, viruses and prions may be physically removed from the final gonadotrophin preparation leaving FSH (and LH) as the only active ingredients. It is also reassuring to note that similar fractionation procedures that are used for the manufacture of human plasma products (e.g. albumin, immunoglobulins, factor VIII etc) contain steps that are capable of removing prion proteins (Foster, 2000
).
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Should the use of urinary-derived gonadotrophins be discontinued? |
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In conclusion, hundreds of thousands of women have been treated with gonadotrophin preparations, since the introduction of the urinary-derived HMG products in the early 1960s and the more recent introduction of recombinant FSH (and now LH) in the 1990s. All of the gonadotrophin preparations appear to have a good safety record without evidence of contamination with infectious agents, in particular, at the present time, with prions. The current procedures for sourcing of products from countries free of vCJD combined with a purification process that appears to minimise the risk of infectivity provides further reassurance. Extensive research is underway to identify reliable, non-invasive screening tests for prion disease together with methods for ensuring their elimination from biological products. In the meantime, we can be reassured that both urinary-derived and recombinant gonadotrophins appear to carry a minimal risk of prion infectivity and there is no evidence to change current prescribing habits.
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Acknowledgements |
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References |
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