minor amount of precursor protein is allowed to undergo further processing through RCE1 cleav- age. However, this finding needs further deepening since activation of the endoprotease RCE1, which is suggested to carry-out the first cleavage step of prelamin A processing in the absence of ZMP- STE24 activity, could depend on several factors. For instance, it could be triggered by a threshold level of preLA-farnesyl-CSIM or by other unknown stimuli involving cell signalling events. In this con- text, however, it is noteworthy that HIV-protease inhibitors, though administered to human fibrob- lasts in long-term treatments, failed to induce high levels of carboxymethylated farnesylated prelamin A. We suggest that a relevant amount of preLA-far- nesyl-C-CH3 is only accumulated if prelamin A mutations affect the availability of the second ZMPSTE24 cleavage site, as it occurs in HGPS cells (Eriksson et al., 2003) and possibly in other LMNA-linked laminopathies. Prelamin A processing in laminopathies The antibodies we have obtained are a useful tool for the study of prelamin A in progeroid laminopathies, metabolic laminopathies and lipody- strophies (Navarro et al., 2004; Capanni et al., 2005; Filesi et al., 2005; Glynn and Glover, 2005; Caron et al., 2007). Data so far reported show involvement of farnesylated prelamin A in these dis- orders (Maraldi and Lattanzi, 2007). It is notewor- thy, however, that the presence of carboxymethylat- ed farnesylated prelamin A could not be investigat- ed so far in laminopathic cells, except than in HGPS as progerin, since antibodies specifically directed to preLA-farnesyl-C-CH3 were not avail- able. Therefore, testing of laminopathic cells with antibody 1188-2 could give important insights. Moreover, the use of 1188-1 or 1188-2 antibody in the analysis of prelamin A processing in patholog- ical and experimental models may give new insights into the function of the lamin A precursor relative to the post-translational modification harboured by the protein (Barton and Worman, 1999; Capanni et al., 2005; Taylor et al., 2005; Crisp et al., 2006; Lattanzi et al., 2007; Mattioli et al., 2008). In fact, while prelamin A toxicity has been so far attributed to the carboxymethyl-farnesyl residue of prelamin A (Glynn and Glover, 2005), the effect of full-length farnesylated prelamin A accumulation is still unknown. However, we recently published that AFCMe treatment leads to formation of highly dys- Original Paper morphic nuclei in human fibroblasts and to severe heterochromatin loss and LAP2α mislocalization (Mattioli et al., 2008). Based on the data obtained in the present study, those pathogenetic effects can be ascribed to farnesylated prelamin A in its full- length form. Another unsolved question in the study of prelamin A in laminopathies concerns the possibili- ty that inhibition of one processing step may acti- vate feedback mechanisms leading to accumulation of other prelamin A forms. For instance, we cannot exclude that blockade of ZMPSTE24 activity may also affect protein farnesylation due to a feedback mechanism. In the context of laminopathy studies, this issue appears particularly relevant. In fact, although the farnesyl residue has been shown to confer toxicity to prelamin A and to cause nuclear dysmorphism (Glynn and Glover, 2005; Caron et al., 2007), we cannot rule out that more than one prelamin A form might be accumulated in lamino- pathic cells and that the rate between different prelamin A forms might affect the severity of the disease. Finally, detection of prelamin A forms in lamino- pathic cells may help setting therapeutic strategies, mostly those based on the use of inhibitors of prelamin A farnesylation (Columbaro et al., 2005; Yang et al., 2005). Acknowledgements The technical support of P. Sabatelli, A. Valmori, S. Grasso and D. Zini is gratefully acknowledged. This work was supported by EU grant FP6 “Euro- laminopathies” no. 018690, by Italian ISS “Rare diseases Italy-USA program” grant no. 526/D30, by a grant from “Fondazione Carisbo”, Italy, by funds from INSERM (Institut National de la Santé et de la Recherche Médicale), Agence Nationale pour la Recherche sur le SIDA et les hépatites virales (ANRS) and the Programme National de Recherche sur le Diabète (PNRD-ARD). References Adjei AA, Erlichman C, Davis JN, Cutler DL, Sloan JA, Marks RS, et al. A Phase I trial of the farnesyl transferase inhibitor SCH66336: evidence for biological and clinical activity. Cancer Res 2000;60:1871-7. Arora P, Muralikrishna B, Parnaik VK. Cell-type-specific interactions at regulatory motifs in the first intron of the lamin A gene. FEBS Lett 2004;568:122-8. Barton RM,Worman HJ. Prenylated prelamin A interacts with Narf, a 51