**2.2 Type 2 CGL (CGL2) and SEIPIN**

In CGL2 (OMIM #269700), mutations happen in the *BSCL2* gene located on chromosome 11q13. The *BSCL2* gene encodes Seipin. Null mutations found in CGL2 patients occupy approximately 75% of all mutations in the *BSCL2* gene, and the rest are missense mutations [39]. Phenotypically, there are no significant differences between patients with null and missense mutations. However, mutagenic truncated forms of Seipin fail to bind lipin-1, while missense mutants still preserve this type of interaction. Although CGL2 affects a diverse range of people from Europe, Mediterranean and Middle Eastern Arabs, and Japanese, patients that have Lebanese origin share a common homozygous *BSCL2* mutation of c.315\_319delGTATC (p. Tyr106Cysfs\*6) [20, 40].

Unlike patients with CGL1, patients with CGL2 suffer a near-total loss of both metabolically active and mechanical adipose tissues [41]. In addition, these patients have significantly low median serum levels of leptin (0.01 ng/ml) and adiponectin (3.3 μg/ml) as compared to normal healthy individuals who had median serum levels of 4.6 ng/ml and 7.8 μg/ml for leptin and adiponectin, respectively [42]. Also, they suffer cardiomyopathy and mild mental retardation in a more prevalent way [7, 8, 43]. One single patient with CGL2 has been reported to have teratozoospermia, a condition characterized by sperm defects including abnormalities in sperm morphology and bundled sperm with two or more sperms joined together by large ectopic LDs [44]. Additionally, three patients in a family from Pakistan with *BSCL2* mutations have been reported to suffer spastic gait, a movement-related complication due to upper motor neuron [45]. While null mutations in *BSCL2* can also result in early-onset, fatal neurodegenerative syndrome as reported in four patients, gainof-function heterozygous *BSCL2* mutations located in the N-glycosylation motif causes distal hereditary motor neuropathy [46].

SEIPIN is a 398 amino acid transmembrane protein in the endoplasmic reticulum (ER), which regulates the transport of macromolecules including proteins and lipids between the ER and the LD [47]. Therefore, it plays a role as a docking protein to regulate LD biogenesis and adipogenesis [48, 49]. Seipin, an integral ER membrane, participates in lipid homeostasis via various complex mechanisms. One of them is to assist LD assembly and fusion as well as adipocyte differentiation [50]. In fact, SEIPIN deficiency in mammals or its yeast ortholog Fld1p/Sei1p can lead to changes in LD morphology, manifested as clustering of multiple small LDs or supersized LDs [5]. Recently, SEIPIN/Fld1p has been found to be stabilized to

*Lipodystrophy - A Rare Condition with Serious Metabolic Abnormalities DOI: http://dx.doi.org/10.5772/intechopen.88667*

ER-LD contacts to assist the protein and lipid trafficking into growing LDs. SEIPIN strengthens the contact site between ER and LD to regulate the growth of immature LDs [51]. The protein might also be involved in PL and TAG synthesis by its binding and interaction with phosphatidic acid phosphatase lipin-1 and AGPAT2 [52, 53].

*Bscl2*<sup>−</sup>/<sup>−</sup> mice manifest complete loss of WAT and have most metabolic complications including hyperinsulinemia, insulin resistance, and hepatic steatosis [54–56]. These animals have low plasma levels of glucose and TAG compared to their wildtype (WT) counterparts but have postprandial hypertriglyceridemia [54, 57]. Insulin signaling in the liver of *Bscl2*<sup>−</sup>/<sup>−</sup> mice was diminished after 4 h of fasting but improved after 16 h [57]. Knocking out *Bscl2* specifically in adipose tissue causes the mice to have an adipocyte hypertrophy with enlarged LDs, reduced lipolysis, adipose tissue inflammation, progressive loss of both WAT and BAT, insulin resistance, and hepatic steatosis [40].

*Bscl2*<sup>−</sup>/<sup>−</sup> MEFs and stromal vascular cells failed to differentiate due to uncontrolled lipolysis activated by cyclic AMP (cAMP)-dependent protein kinase A (PKA) [55, 58]. It causes loss of LDs and subsequently impaired adipogenesis by silencing the expression of adipose-specific transcription factors. This loss of function can be rescued by inhibitors of lipolysis but not by a PPARγ agonist. Thiazolidinediones, antidiabetic drugs, are able to restore adipogenesis but unable to intervene the unbridled lipolysis happening in *Bscl2*<sup>−</sup>/<sup>−</sup> MEFs [56]. Interestingly, LDs in lymphoblastoid cell lines have been reported to have an increase in numbers but shrinkage in size from 12 patients with CGL2 with null *BSCL2* mutations [59]. In addition, it has been observed that there is a shift in the proportion of monounsaturated fatty acids to saturated fatty acids in hepatic triglycerides and phosphatidylethanolamine, implicating a defect in acyl-CoA desaturase activity in these cell lines [59].

#### **2.3 Type 3 CGL (CGL3) and CAV1**

CGL3 (OMIM #612526) is induced by mutations in the *CAV1* gene located on chromosome 7q31. The *CAV1* gene encodes caveolin-1. Kim et al. first described a CGL3 patient from Brazil who has a homozygous "loss of function" mutation at p.E38X in the *CAV1* gene [60]. This patient possesses generalized lipodystrophy with near-total loss of metabolically active adipose tissue but preserves mechanical adipose tissue and fat in the bone marrow. She also manifests acanthosis nigricans, severe hypertriglyceridemia, hepatic steatosis and splenomegaly, short stature, functional megaesophagus, hypocalcaemia, primary amenorrhea, and chronic diarrhea. She also developed diabetes mellitus at age 13 [60]. Almost at the same time, Cao et al. reported a case of a typical partial lipodystrophy caused by heterozygous frame-shift mutation in *CAV1*, designated I134 fsdelA-X137 and −88deltaC [61]. This female patient has subcutaneous fat loss in the upper part of her body and face but spared her legs, gluteal region, and visceral fat stores [61]. Since 2015, two novel variants of *CAV1* have been identified: de novo heterozygous null mutations, c.424C>T (p.Q142X) and c.479\_480delTT (p.F160X) in a 7-year-old male and a 3-year-old female of European origin, respectively [62]. Both of the patients have generalized fat loss, thin mottled skin, and progeroid features at birth [62]. The male patient has cataracts requiring extraction at 30 months of age, and the female patient has pulmonary arterial hypertension [62–64].

Caveolin-1 is the scaffolding protein primarily constituting specialized vesicular invaginations of 50–100 nm called caveolae [1, 36]. It was discovered by Anderson Lab in 1992 [65, 66]. Its potential function in vesicle transport was reported by Glenney using cDNA encoding caveolin-1 from lungs [67]. There are three caveolin isoforms: CAV1, CAV2, and CAV3. While CAV3 is muscle specific, CAV1 and CAV2

are predominantly expressed in adipose tissue, endothelial cells, and fibroblasts [68, 69]. As the main component of caveolae, caveolin-1 plays an essential role in the caveolae assembly alongside with other caveolar proteins, such as cavin1–cavin4, Pacsin2, and EH domain-containing 2 (EHD2) [70]. In addition to its essential role in caveolae formation, caveolin-1 is also a key determinant of normal lipid homeostasis, vesicular trafficking, and signaling transduction [71]. Caveolae plays a regulatory role in maintaining the integrity and function of the LDs as well as binding and transporting fatty acids and cholesterol by budding off the plasma membrane [70]. Moreover, they serve as a platform for augmenting insulin and PKA signaling [72]. In adipocyte cells, the expression of caveolin-1 is increased 10-fold during adipogenesis [73, 74]. The abundance level of caveolin-1 determines the PL and surface protein composition in LDs and the LD growth. In fact, the expression of *Cav1* in adipose cell lines and mice leads to an increase in the density of caveolae and enhances adipocytes' capacity to have larger LDs and promote cell growth through increased glucose utilization [75].

Similar to caveolin-1 deficiency, lacking another caveolar protein cavin-1 in humans also causes another type of congenital generalized lipodystrophy (type 4 CGL) [76–78]. Interestingly, subcutaneous injection of caveolin-1 overexpressed preadipocytes could form fat pads and larger adipocytes [75]. Both caveolin-1 and cavin-1 are the strong indicators of adipogenic differentiation in human tumors and liposarcoma [79]. Taken together, these data indicate the critical role of adipocyte caveolae in adipose tissue development.

*Cav1*<sup>−</sup>/<sup>−</sup> mice show complete loss of morphologically identifiable caveolae in endothelial and adipose tissue [74]. Likewise, primary cells derived from *Cav1*<sup>−</sup>/<sup>−</sup> MEFs fall short of functional caveolae [74]. *Cav1*<sup>−</sup>/<sup>−</sup> mice have dramatically smaller fat pads, reduced adipocyte size, and poorly differentiated white adipose parenchyma than their WT littermates [80]. While the animals are hyperphagic, they are resistant to dietinduced obesity [20, 81]. Their BAT undergoes hypertrophy [80, 82].
