Characterization of two human monoclonal IgM antibodies that recognize nuclear lamins

PNUTS enhances in vitro chromosome decondensation in a PP1-dependent manner

PP1 (protein phosphatase-1) is a serine/threonine phosphatase involved in mitosis exit and chromosome decondensation. In the present study, we characterize the subcellular and subnuclear localization of PNUTS (PP1 nuclear targeting subunit), a nuclear regulatory subunit of PP1, and report a stimulatory role of PNUTS in the decondensation of prometaphase chromosomes in two in vitro systems. In interphase, PNUTS co-fractionates, together with a fraction of nuclear PP1, primarily with micrococcal nuclease-soluble chromatin. Immunofluorescence analysis shows that PNUTS is targeted to the reforming nuclei in telophase following the assembly of nuclear membranes and concomitantly with chromatin decondensation. In interphase cytosolic extract, ATP-dependent decondensation of prometaphase chromosomes is blocked by PP1-specific inhibitors. In contrast, a recombinant PNUTS(309-691) fragment accelerates chromosome decondensation. This decondensation-promoting activity requires the consensus RVXF PP1-binding motif of PNUTS, whereas a secondary, inhibitory PP1-binding site is dispensable. In a defined buffer system, PNUTS(309-691) also elicits decondensation in an exogenous PP1-dependent manner and, as in the cytosolic extract, a W401A (Thr401-->Ala) mutation that destroys PP1 binding abolishes this activity. The results illustrate an involvement of the PNUTS:PP1 holoenzyme in chromosome decondensation in vitro and argue that PNUTS functions as a PP1-targeting subunit in this process. We hypothesize that targeting of PNUTS to reforming nuclei in telophase may be a part of a signalling event promoting chromatin decondensation as cells re-enter interphase.

Dermal fibroblasts in Hutchinson-Gilford progeria syndrome with the lamin A G608G mutation have dysmorphic nuclei and are hypersensitive to heat stress

Background

Hutchinson-Gilford progeria syndrome (HGPS, OMIM 176670) is a rare sporadic disorder with an incidence of approximately 1 per 8 million live births. The phenotypic appearance consists of short stature, sculptured nose, alopecia, prominent scalp veins, small face, loss of subcutaneous fat, faint mid-facial cyanosis, and dystrophic nails. HGPS is caused by mutations in LMNA, the gene that encodes nuclear lamins A and C. The most common mutation in subjects with HGPS is a de novo single-base pair substitution, G608G (GGC>GGT), within exon 11 of LMNA. This creates an abnormal splice donor site, leading to expression of a truncated protein.

Results

We studied a new case of a 5 year-old girl with HGPS and found a heterozygous point mutation, G608G, in LMNA. Complementary DNA sequencing of RNA showed that this mutation resulted in the deletion of 50 amino acids in the carboxyl-terminal tail domain of prelamin A. We characterized a primary dermal fibroblast cell line derived from the subject's skin. These cells expressed the mutant protein and exhibited a normal growth rate at early passage in primary culture but showed alterations in nuclear morphology. Expression levels and overall distributions of nuclear lamins and emerin, an integral protein of the inner nuclear membrane, were not dramatically altered. Ultrastructural analysis of the nuclear envelope using electron microscopy showed that chromatin is in close association to the nuclear lamina, even in areas with abnormal nuclear envelope morphology. The fibroblasts were hypersensitive to heat shock, and demonstrated a delayed response to heat stress.

Conclusion

Dermal fibroblasts from a subject with HGPS expressing a mutant truncated lamin A have dysmorphic nuclei, hypersensitivity to heat shock, and delayed response to heat stress. This suggests that the mutant protein, even when expressed at low levels, causes defective cell stability, which may be responsible for phenotypic abnormalities in the disease.

Autoimmune cytopenias associated with autoantibodies to nuclear envelope polypeptides

A subset of anti-nuclear autoantibodies (ANA) are directed against nuclear envelope (NE) polypeptides and display by indirect immunofluorescence (IIF) a ring-like fluorescent pattern. We report herein 19 patients with autoimmune cytopenias associated with antibodies (Abs) to NE polypeptides. Anti-NE specificity was determined by immunoblot, using NE preparations and purified lamina fractions. Eleven sera reacted with lamin B(1), and two reacted with both lamin B(1) and an unidentified 150-kDa protein (p150). One serum reacted with only p150. Four sera reacted with lamins A and C, and one reacted with and an unidentified 52-kDa NE polypeptide (p52). Autoimmune cytopenias included hemolytic anemia (7 cases), thrombocytopenia (13 cases), and neutropenia (6 cases). Five patients had 2 (3 cases) or 3 (2 cases) different cytopenias. Antiphospholipid antibodies (APLA) were detected in 14 patients, 2 of whom experienced thromboembolic events. A liver disorder was present in 7 patients. Systemic lupus erythematosus and lupus-like syndrome were diagnosed in 11 and 2 patients, respectively. Cytopenias responded to steroids alone (13 patients), or together with intravenous immunoglobulins (2 patients), or cyclophosphamide (2 patients). Two patients did not require treatment. Our results suggest that anti-NE Abs need to be sought for in patients with peripheral cytopenias, particularly when they are associated with APLA and/or liver disorders. Their detection strongly suggests an autoimmune process. Such cytopenias are often manifestations of a lupus or lupus-like disease and are responsive to steroids.

Nucleotide sequence analysis of the VL and VH domains of five human IgM directed to lamin B. Evidence for an antigen-driven process in the generation of human autoantibodies to lamin B

OBJECTIVE

To gain insight into the genetic origin of human antilamin autoantibodies, we determined the nucleotide sequence of the light and heavy chain variable region (VL and VH) domains of 5 IgM antibodies directed to lamin B. These antibodies represent a distinct subset of antinuclear antibodies, and their presence is associated with a particular lupus-like syndrome.

METHODS

We derived and cloned lymphoblastoid cell lines from peripheral blood B cells of 3 patients, selected anti-lamin B-producing subclones, and sequenced the messenger RNA coding for Ig heavy and light chains.

RESULTS

We isolated 2 subclones (1 IgM kappa, 1 IgM lambda) from one patient (FUR) and 2 subclones (both IgM lambda) from another (HER). In contrast, all 8 lines derived from B cells isolated from the third patient (BEN) synthesized identical anti-lamin B IgM kappa antibodies: All VL and VH domains from these 5 IgM were encoded by different VL or VH genes. DH regions were all different, and there was no restriction in the use of JL or JH segments. Analysis of the nucleotide sequence of the VL domains allowed the identification of the putative germinal gene in 3 instances (V kappa IV, Humkv325, and V lambda III.1); the overall ratios of replacement:silent mutations (R:S) were 6.5 and 1.2 in the complementarity-determining regions (CDRs) and framework regions (FRs), respectively. The 2 other lambda sequences belonged to the V lambda III family. With regard to VH domains, 3 of 5 derived from previously identified germline genes (VHIV 4.19, VHIV 4.22, and VHIII 9.1); the overall R:S ratio for these genes was 8 and 1.5 in CDRs and FRs, respectively.

CONCLUSION

Taken together, these data provide evidence that the repertoire of human antilamin autoantibodies is not restricted and that the antigen (or another kind of selective pressure) plays a role in the generation of autoantibodies to lamin B. This hypothesis is in accordance with the reactivity of these antibodies to discrete epitopes of lamin B.