Investigating Lamin A Mutations in Progeroid Syndromes and Partial Lipodystrophy

Lamin A/C is a component of the nuclear lamina that contributes to nuclear integrity, chromatin organisation, gene transcription and DNA replication. Mutation of the LMNA gene, encoding lamin A/C, causes a number of diseases affecting different tissues, but the mechanism(s) by which this widely expressed protein causes tissue-specific disease remains unclear. Hutchinson-Gilford progeria syndrome (HGPS) is an early-onset premature aging disorder. The most common LMNA mutation (G608G) prevents complete posttranslational processing of lamin A, resulting in aberrant retention of a farnesyl group. In this study, a cohort of children with progeroid phenotypes were screened for genetic defects. The G608G mutation was identified in one patient with a classical phenotype. A second patient with mild progeria carried a rare T623S mutation, which also results in aberrant farnesylation of lamin A. In contrast, a severe progeroid phenotype resulted from homozygous mutation of ZMPSTE24, the key enzyme in lamin A processing. Studies of skin fibroblasts showed a correlation between farnesylated lamin A level and disease severity. FRAP studies revealed that retention of the farnsesyl group causes a 50% decrease in mobility of lamin A, irrespective of the exact mutation. Interestingly, one non-farnesylated mutant also had a 50% reduction in mobility, whilst other non-farnesylated mutants had normal mobility. The results of these studies indicate that incomplete processing of lamin A is an important contributor to severity of progeroid disorders but, in agreement with other reports, is not the only disease mechanism involved. Familial partial lipodystrophy (FPLD) is a fat wasting disorder also resulting from LMNA mutations. Preliminary analysis of the adipogenic potential of mesenchymal stem cells isolated from FPLD patients do not produce detectable levels of adipogenesis. Preliminary immunofluorescence and binding studies in FPLD and progeria tend to support existing evidence that mislocalisation of the adipogenic factor SREBP1 may underlie the lipodystrophy phenotype.