Sequencing of the reannotated LMNB2 gene reveals novel mutations in patients with acquired partial lipodystrophy

Abstract

The etiology of acquired partial lipodystrophy (APL, also called "Barraquer-Simons syndrome") is unknown. Genomic DNA mutations affecting the nuclear lamina protein lamin A cause inherited partial lipodystrophy but are not found in patients with APL. Because it also encodes a nuclear lamina protein (lamin B2) and its genomic structure was recently reannotated, we sequenced LMNB2 as a candidate gene in nine white patients with APL. In four patients, we found three new rare mutations in LMNB2: intron 1 -6G-->T, exon 5 c.643G-->A (p.R215Q; in two patients), and exon 8 c.1218G-->A (p.A407T). The combined frequency of these mutations was 0.222 in the patients with APL, compared with 0.0018 in a multiethnic control sample of 1,100 subjects (P = 2.1 x 10-7) and 0.0045 in a sample of 330 white controls (P = 1.2 x 10-5). These novel heterozygous mutations are the first reported for LMNB2, are the first reported among patients with APL, and indicate how sequencing of a reannotated candidate gene can reveal new disease-associated mutations.

Figure 1.

A, Genomic structure of LMNB2 as it was known in 2001 and 2005 (with respective reference sequence numbers indicated). Boxes indicate exons, with the exon number shown above. B, Electropherogram tracings of genomic DNA sequences of the LMNB2 gene from healthy control subjects (top row of sequences) and subjects with APL (lower row of sequences). Normal genomic sequence for each region is shown above the tracings, in addition to 5′ and 3′ orientation. Mutation names are given below the tracings, with arrows pointing to mutant heterozygous sequences. The sequence designated c.82G→A in exon 1 was later determined to be a polymorphism that was seen at relatively high frequency in control subjects of several ethnic backgrounds and was not significantly associated with APL in this sample. C, Schematic diagram of head, rod, and tail domains of lamins B2 and A. Subdomains of the rod domain are shown as numbered coil domains and numbered “linker” domains (L). The numbers of amino acid residues for each region of each lamin type are shown in parentheses above the domain maps. The positions of the two LMNB2 missense mutations are shown (arrows). For the p.R215Q mutation within Coil 1B of the highly conserved rod domain, the homologous regions from lamins B2, A, and B1 are shown below the map, with the respective amino acid numbers on each side of this conserved region. R215 in lamin B2 and the corresponding residue in the other lamins are boxed.