Dunnigan's FamilialPartial Lipodystrophy is a rare automal syndrome attributed to specic mutations, primarily R483Q, in the Lamin A/C gene. Afflicted subjects, mostly females, experienc a post pubral loss of subcutaneous fat primarily in the limbs and increased fat in the head and neck region. We have introduced the mutant lamin gene, as a transgene driven by the adipoycte-specific aP2 promoter into mice. After a prolonged delay, depending on diet, the mice experience a progressive loss of adipose tissue, hepatic steatosis, and insulin resistance as determined by hyperinsulinic, euglycemic clamp studies. Mice carrying the wt lamin as a transgene experience non of these problems. The loss of adipose tissue may be attributed to a failure of mutant preadipocytes to differentiate into mature adipocyes in culture, which is attributed to their failur to express the adipogenic factor, PPARgamma. Thus, the mutant lamin mouse represents an animal model of Dunnigan's Familial Partial Lipodystrophy.? ? Dunnigan-type familial partial lipodystrophy (FPLD) is a rare autosomal dominant genetic disorder with post-puberty onset. Affected patients develop progressive loss of subcutaneous adipose tissue from the trunk and extremities, while accumulating excess adipose tissue on the face and neck. The disease is also characterized by a host of metabolic complications including insulin resistance, dyslipidemia, and hepatic steatosis. While both genders are equally affected by FPLD, the clinical manifestation such as insulin resistance and cardiovascular complications tend to be exacerbated in females compared to males. FPLD is caused by certain mutations in the LMNA gene with approximately 90% localized to exon 8. LMNA encodes two isoforms via alternate splicing: lamin A and lamin C. Lamin A shares 566 amino-terminal amino acids with lamin C, but has 98 unique carboxy-terminal amino acids. Lamin C is produced via alternative splicing of exon10 and contains 6 unique amino acids at its carboxy-terminus. Lamins are intermediate filament proteins localized at the inner aspect of the nuclear membrane that form a meshwork called the nuclear lamina. They are also found at sites of DNA replication and RNA processing, suggesting they may influence gene duplication and expression. Other LMNA-linked diseases, collectively called laminopathies, include Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy, madibuloacral dysplasia and Hutchison-Gilford progeria syndrome. The paticipation of lamins in many cell processes and the broad spectrum of diseases that arise from mutations in LMNA, suggest that lamin proteins have different roles in different somatic cells. ? ? So, how can genetic variants in a widely expressed nuclear lamin protein result in an adipose tissue specific phenotype such as FPLD? The underlying mechanisms are unclear, and no mouse model has been developed. An understanding of the pathogenesis of FPLD will provide insights not only into lipodystrophies, but also into the mechanisms of adipose tissue maintenance and insulin resistance as it occurs in obesity. ? We have constructed a transgenic mouse in which the human LMNA gene has been introduced as a transgene driven by the aP2, adipocyte-specific promoter. Transgenic mice express either mutant lamin A or lamin C containing the common R482Q FPLD mutation or wild-type human lamin A/C. Subsequent analysis of the transgenic mouse revealed a phenotype similar to the human disease. After a delay of many weeks, especially on a high-fat diet, animals with the mutant lamin gene cease to accrue adipose tissue and develop insulin resistance and fatty livers. These data support our transgenic mouse, herein called FPLD mouse, as a suitable model in which to study FPLD and lipodystrophic mechanisms.? Two major mechanisms may contribute to the inability of FPLD mice to accumulate fat; abnormal increases in the breakdown of adipose fat stores or an inability of adipocyte precursors to differentiate into mature adipocytes and accumulate lipid. We found that increased lipolysis did not contribute to fat loss in FPLD mice. However, preadipocytes from FPLD mice showed clear defects in differentiation. Microscopic analysis revealed that stromal vascular cells isolated from adipose tissue of FPLD mice retained their fibroblastic appearance and failed to differentiate into mature adipocytes and accumulate lipid even after 7 days of differentiation. These results were confirmed with real-time PCR, which showed reduced expression of prodifferentiation factors, such as aP2, PPAR and CEBP. Our results are similar to recent studies showing that expression of FPLD mutated lamin A inhibits differentiation in 3T3-L1 preadipocytes. ? When challenged with excess energy load, adipose tissue must recruit a new pool of adipocytes, once existing adipocytes reach their maximum capacity for storing lipid. Because FPLD animals cannot recruit a mature pool of adipocytes, they eventually lose their capacity to accumulate fat. The mechanisms underlying these differentiation defects are not clear. Nonetheless, it is clear that the defects in these mice can be both induced and amplified by challenging with high-fat diet. This is evidenced by the differences we saw in response to chow versus high-fat diet. One hypothesis is that recruitment of new adipocytes, in spite of the fact that they are unable to fully mature and store much lipid, is sufficient for some time to accommodate excess energy stores. A threshold is reached, at which time adipocytes cannot keep up with the excess energy demands and cannot accumulate more fat. A second hypothesis is that the pool of preadipocytes is finite. In a normal animal, a new pool of adipocytes is recruited in response to excess fat load. Because these cells can differentiate normally, they are able to fully mature and accumulate significant amounts of lipid. Conversely, preadipocytes from FPLD animals are not able to fully mature, causing a more rapid recruitment and turnover of precursor cells. If this pool is finite, then eventually, adipocyte precursors will run out, thus preventing fat accumulation in adipose tissue. This hypothesis is supported by the recent development of an inducible fatless mouse model, called FAT-ATTAC mouse. These studies showed that functional adipocytes could be recovered after cessation of treatment that causes their ablation. However, recent data has shown that if treatment lasted more than 12-14 weeks, the effects were not fully reversible, thus suggesting preadipocyte pools can be depleted. We are currently addressing potential experimental approaches that may address these hypotheses.? So, how do mutations in lamin A lead to defective differentiation? One clue comes from studies by Capanni et al. They have shown that the immature form of lamin A, called pre-lamin A, is processed at a reduced rate and accumulates in FPLD fibroblasts. In addition, they demonstrate that sterol regulatory element binding protein (SREBP1), a transcription factor that mediates adipocyte differentiation, interacts with pre-lamin A. Overexpression of pre-lamin A sequesters SREBP1 at the adipocyte nuclear envelope, thus preventing its translocation to the nuclear interior. These events were concomitant with impaired adipocyte differentiation. Perhaps FPLD transgenic mice have accumulation of pre-lamin A, which binds SREBP1 and prevents it from entering the nucleus. This would subsequently preclude activation of other transcription factors that mediate adipocyte differentiation such as PPAR and CEBP. Studies are ongoing to determine if this is a plausible mechanism in our FPLD-transgenic mouse. ? ? A ms. describing the mutant lamin FPLD mouse has been submitted to Nature Medicine.
