Epidemiology

The reported prevalence of AICTDs is variable, depending on methodology, nature of referral bias, and ethnicity.³ It is generally accepted that Sjögren’s syndrome has the highest prevalence (0.5 to 3.6%), with SLE being much lower at about 15 to 50 per 100,000. Scleroderma, polymyositis, and dermatomyositis are relatively rare AICTDs, occurring in fewer than 10 per 100,000. Experts are increasingly realizing that overlap syndromes of scleroderma and myositis are more common than the “pure” forms of the disease.⁴ There are no epidemiology studies of overlap syndromes, apart from Japan, where the reported prevalence of mixed connective tissue disease (MCTD) was 2.7 per 100,000. The syndrome of MCTD usually occurs as an isolated finding, but there are reports of a familial occurrence. Unlike SLE, precipitation by sun exposure has not been described in patients with MCTD. Likewise, drug exposure has not been related to the onset of MCTD, although a transient appearance of anti-RNP antibodies has been seen at the initiation of procainamide therapy.⁵ Vinyl chloride and silica are the only environmental agents that have been associated with MCTD so far.

Autoimmunity in Overlap Syndromes

Compelling evidence indicates that autoimmunity is often antigen driven by components of subcellular particles, in particular spliceosomes, nucleosomes, and proteasomes.⁶

Autoimmunity to Spliceosomal Components

Certain components of the spliceosome are common targets of autoimmunity in the AICTDs.⁷ Furthermore, it appears that post-translational modifications of these molecules, as occurs during apoptosis, are often associated with increased immunogenicity.⁸ Spliceosomes are complex nuclear particles made up of some 300 distinct proteins and 5 RNAs, which are involved in the processing of premessenger RNA (pre-mRNA) into mature “spliced RNA.”⁹ Two major spliceosomal subunits are antigenic targets in autoimmunity: (1) small nuclear ribonucleoprotein protein particles (snRNPs) and (2) heterogeneous nuclear RNP particles (hnRNPs).

The snRNPs contain small RNA species ranging in size from 80 to 350 nucleotides that are complexed with proteins.⁶ These RNAs contain a high content of uridine and are therefore called U-RNAs; 5 different U-RNAs were defined on the basis of immunoprecipitation (U1, U2, U4, U5, and U6). Autoantibodies to these complexes are mainly directed to the protein components. Anti-Sm antibodies precipitate five proteins with molecular weights of 28,000 (B′B), 16,000 (D), 13,000 (E), 12,000 (F), and 11,000 (G); five of these polypeptides are common to the U1, U2, U4, U5, and U6 RNAs. Anti-RNP antibodies precipitate three proteins with molecular weights of 68,000 (70K), 33,000 (A′), and 22,000 (C); these polypeptides are uniquely associated with U1 RNA (Figure 86-1).The clinical correlates considered to be distinctive of MCTD are associated with the 70-kD specificity with an immunodominant epitope embracing amino acid residue 125 flanked by important conformational residues at positions 119-126 (see Figure 86-1). On the other hand, SLE is associated with anti-Sm antibodies.

Figure 86-1 The spliceosome is made up of five small nuclear RNAs (snRNAs) complexed with proteins to form a small nuclear ribonucleoprotein particle (snRNP). This subcellular structure is responsible for splicing introns from pre-mRNA to form mRNA via a 59-splice recognition site. Antibodies to various spliceosomal constituents are a common feature of autoimmune rheumatic disorders with a tendency to associate with different clinical profiles (see Table 86-1). The U1 small nuclear RNP (U1-snRNP) particle of the spliceosome is composed of U1-RNA, RNP proteins (70 kD, A, and C), and common Sm proteins (B′B, D, E, F, and G). The structure of U1-RNA consists of single-stranded RNA and double-stranded RNA called stem loops (SL) 1, 2, 3, and 4. An electron density map of the functional core of U1-snRNP at 5.5 resolution has enabled the spatial visualization of the RNA and placement of the seven Sm proteins, U1-C, and U1-70K into the map. A striking feature is the amino (N)-terminal polypeptide of U1-70K, which extends over a distance of 180 from its RNA binding domain, wraps around the core domain consisting of the seven Sm proteins, and finally contacts U1-C, which is crucial for 59-splice-site recognition.

(Modified from Newman J: Structural studies of the spliceosome, Curr Opin Struct Biol 20:82–89, 2010; and Pomeranz AD: Crystal structure of human spliceosomal U1 snRNP at 5.5 A resolution, Nature 458:457–480, 2009.)

The hnRNPs are among the most abundant proteins in the eukaryotic cell nucleus. They contain pre-mRNA associated with 30 small proteins that are all structurally related and have molecular weights of 33 to 43 kD. Nine hnRNP core proteins have been designated A1, A2, B1a, B1b, B1c, B2, C1, C2, and C3. An antibody termed anti-RA33, which targets the 33-kD hnRNP-A2, is particularly interesting because it is found in about one-third of sera from patient with RA, SLE, and MCTD.¹⁰ It also has associations with patient subsets of erosive arthritis in SLE, scleroderma, and MCTD and predicts the eventual development of RA in patients with early polyarthritis.¹¹ Importantly, this association with anti-RA33 is not seen in scleroderma (sine erosions), PM, or overlaps of PM/Scl or PM/DM. The antigenic epitopes of hnRNP-A2 contain two RNA binding regions at the N-terminal end and a glycine-rich C-terminal region. Certain disease subsets target these two RNA binding regions differently. For instance, RA and SLE sera preferentially react with the complete second RNA binding domain, whereas MCTD sera target an epitope that spans both RNA binding domains.

Autoimmunity to Nucleosomal Components

Nucleosomes are the compact building blocks of chromatin and consist of an octamer of two copies of histones H2A, H2B, H3, and H4, wrapped around approximately 146 base pairs of DNA (Figure 86-2). During apoptosis endonucleases cleave chromatin with the liberation of nucleosomal particles. Following the release into the cytoplasm, nucleosomes migrate to the surface of the dying cell¹² and thus become accessible to B cell receptors. The development of autoimmunity has been linked to defective phagocytosis of apoptotically released constituents.¹² Nucleosomal antibodies are directed to antigenic determinants on the intact nucleosome rather than its individual components, DNA and histones. In a study of 496 patients with 13 different AICTDs and 100 patients with hepatitis C, antinucleosome antibodies were found in the sera of patients with SLE (71.7%), Scl (45.9%), and MCTD (45.0%).¹³

Figure 86-2 The nucleosome is the fundamental repeating unit of chromatin. The central part of the nucleosome is composed of a tetramer composed of two molecules of histones H3 and H4, flanked by two dimers of histones H2A and H2B. This central core is surrounded by two super-helical turns, consisting of 146 base pairs of histone-free DNA. Histone H1 is located at the point where DNA enters and exits the nucleosome. Antibodies to the nucleosome arise early in the evolution of systemic lupus erythematosus—before anti-DNA and antihistone antibodies. Thus the nucleosome is thought to be an important early autoantigen in the development of epitope spreading. Nucleosome antibodies are also found in scleroderma and mixed connective tissue disease.

(From Amoura Z, Koutouzov S, Piette C, et al: The role of nucleosomes in lupus, Curr Opin Rheumatol 12:369–373, 2000.)

Autoimmunity to Proteasomal Components

The 26S proteasome is a large subcellular particle involved in the degradation of proteins that have been tagged with ubiquitin, resulting in the generation of peptides for presentation by the MHC class I molecules (Figure 86-3). There is good evidence that it is the target of an autoimmune response in AICTD. Antibodies to proteasomal subunits have been reported in patients with autoimmune myositis, systemic lupus erythematosus, and primary Sjögren’s syndrome. Circulating 20S proteasomes (c20S) subunits appear to have an association with disease activity in MCTD and SLE.¹⁴

Figure 86-3 Most proteins in the cytosol and nucleus are degraded via the proteasome-ubiquitin pathway. The 26S proteasome is a huge complex of 2.5 mega-daltons, made up of approximately 35 different subunits. It contains a proteolytic core complex, the 20S proteasome, and one or two 19S regulatory complexes that associate with the termini of the barrel-shaped 20S core. The function of proteasomes is twofold: (1) to degrade intracellular proteins that have been tagged with ubiquitin and (2) to generate antigenic peptides for presentation by the major histocompatibility complex class I molecules. Antibodies to proteasomal subunits have been reported in several autoimmune diseases (especially systemic lupus erythematosus and polymyositis/dermatomyositis), and elevated levels of proteasomes have been correlated with disease activity.

Undifferentiated Connective Tissue Disease

Rheumatologists frequently see patients who present with a weakly positive ANA and nonspecific symptoms such as arthralgias, fatigue, and cold sensitivity. The critical question in such patients is “will they develop a connective tissue disease?” or “do they have fibromyalgia?”

The answer to this question is not always straightforward because fibromyalgia is not a diagnosis of exclusion²⁷; it is a common comorbidity with the well-defined CTDs and is often associated with cold-induced vasospasm.²⁸ An algorithm for diagnosing UCTDs is given in Figure 86-4. In the early stages of a CTD, there may be just one or two suspicious clinical and laboratory features, but a definitive diagnosis cannot always be made. In such cases a working diagnosis of undifferentiated connective tissue disease (UCTD) may be appropriate.²⁹ Most patients with this UCTD have Raynaud’s phenomenon with or without an unexplained polyarthralgia and a positive ANA with usually just a single autoantibody specificity, often anti-Ro and anti-RNP.³⁰ A 5-year follow-up study of 665 patients with UCTD reported that only 34% developed a well-defined CTD (RA—13.1%, Sjögren’s—6.8%, SLE—4.2%, MCTD—4%, Scl—2.8%, systemic vasculitis—3.3%, and PM/DM—0.5%).³¹ Certain combinations of features are predictive for the development of an established CTD: Polyarthritis plus U1-RNP antibodies predict MCTD, sicca symptoms plus anti-SSA/SSB antibodies predict Sjögren’s syndrome, Raynaud’s phenomenon plus a nucleolar ANA pattern predict Scl, polyarthritis plus high levels of rheumatoid factor (RF) predict RA, and fever/serositis plus a homogeneous ANA pattern or anti-dsDNA antibodies predict progression into SLE (Table 86-2). The identification of a pathologic nail-fold capillary pattern can provide some early indication that the UCTD may progress to systemic sclerosis (SSc) or MCTD (Figure 86-5).³² Low levels of vitamin D are also reported to be a risk factor for the development of UCTDs and should be evaluated and corrected in all such patients.³³

Figure 86-4 Algorithm for evaluating patients with undifferentiated connective tissue disease (UCTD). CREST, calcinosis, Raynaud’s phenomenon, esophageal dysmotility, sclerodactyly, and telangiectasia; MCTD, mixed connective tissue disease; SLE, systemic lupus erythematosus.

Table 86-2 Disorders Associated with Increased Fibrosis

Figure 86-5 Nail-fold capillaroscopy in patients with systemic sclerosis and mixed connective tissue disease (MCTD). A, Normal capillaries. B, MCTD patient showing dilated and thickened capillary loops. C, Early scleroderma with irregular capillaries and mild dropout. D, Advanced scleroderma with capillary dropout and neoangiogenesis.

(Modified from Cutolo M, Sulli A, Pizzorni C, Smith V: Capillaroscopy as an outcome measure for clinical trials on the peripheral vasculopathy in SSc—is it useful? Int J Rheumatol pii:784947, 2010.)

Scleroderma Overlaps

Several fibrotic conditions may mimic scleroderma (see Table 86-2). Scleroderma itself has a widespread heterogeneity of disease expression ranging from a diffuse cutaneous disease, with a poor prognosis, to a limited cutaneous involvement, with generally a good prognosis. Furthermore, some patients with Scl have a prominent overlap with other connective tissue diseases.³⁴ In many cases, these overlaps occur in patients who do not have prominent skin involvement (sine scleroderma) or with the limited form of the disease—CREST. Approximately 90% of patients with Scl have a positive ANA. Scleroderma-related antibodies include topoisomerase I (Scl-70), anticentromere (ACA), hnPNP-I, RA33, p23, p25, RNA polymerase I (RNAP-1), RNA polymerase III, U1-RNP, PM/Scl, fibrillarin, histone, Ku, endothelial cell, and Th/To³⁵ (see Table 86-1).

A German registry for scleroderma has reported on patterns of organ involvement in two subsets of 1483 SSc patients.³⁶ Limited distal skin involvement (distal to the knee and elbows) was seen in 46% (the lcSSc group), and 33% had progressive widespread scleroderma (rapid involvement of trunk, face, and extremities—the dcSSc group). An overlap syndrome was seen in 11%, and 8% were undifferentiated. The extent of organ involvement varied between subgroups. For instance, musculoskeletal involvement was seen in 68%of the overlap group compared with 57% of the dcSSc group. Pulmonary fibrosis (56%) and pulmonary hypertension (19%) were most common in the dcSSc group, but pulmonary hypertension was seen in 15% of the dcSSc group.

Specific antibody profiles tend to be associated with distinctive patterns of morbidity and mortality.³⁷ Patients possessing anticentromere, anti-U3 snRNP, and anti-Th/To antibodies tend to have the limited form of Scl, whereas anti-Scl-70, ACA, and anti-RNAP are associated with diffuse skin involvement and systemic disease. Anti-PM/Scl antibodies are associated with a myositis/Scl overlap and a tendency to develop pulmonary interstitial disease.³⁸ About 60% of patients with scleroderma have obvious synovitis, and 35% are positive for RF. Erosive arthritis in Scl has an association with anti-RA33; the Scl component in such overlap patients is often an incomplete form of CREST.³⁹ The limited form of scleroderma has a well-documented overlap with primary biliary cirrhosis (PBC). The distinctive antibody association of scleroderma with PBC is antimitochondrial antibodies.⁴⁰ Conversely, anticentromere antibodies have been found in 10% to 29% of patients with PBC; approximately half developed some features of the CREST syndrome. Hence a serologic overlap between the two syndromes is more prevalent in the clinical overlap. Low-grade muscle involvement is not uncommon in scleroderma, being described in between 50% and 80% of patients. A European review of 114 scleroderma overlap patients reported a 95% PM/Scl antibody positivity⁴¹ with 80% having an inflammatory myositis. This “scleromyositis” differed from MCTD by coexistent features of dermatomyositis (myalgia, myositis, Gottron sign, heliotrope rash, calcinosis), but no overlap SLE features, as is characteristic of classic MCTD. Many of these patients had a deforming arthritis of the hands. In general they had a chronic benign course, and most were steroid responsive. Scleroderma lupus overlaps are less common. However, Scl patients often have antinuclear antibodies other than ACA and Scl-70.

Nonscleroderma fibrotic disorder may be mistaken for a scleroderma overlap at initial presentation (see Table 86-2); although these disorders may have some systemic involvement, they seldom exhibit overlap features with other AICTDs.

Nephrogenic systemic fibrosis (NSF) is a fibrotic disorder that develops in some patients following exposure to gadolinium-containing contrast agents; most patients have pre-existing renal disease.⁴² Histologically there is fibroblast proliferation, thickened collagen bundles, and deposits of mucin, similar to those observed in scleromyxedema. The clinical presentation is a rapid progression with confluence of initially focal areas of indurated skin (Figure 86-6). The face is usually spared, but joint contractures may occur at the elbows and knees, and systemic involvement with pulmonary and neurologic symptoms can develop in refractory cases. NSF is usually nonresponsive to corticosteroids and immunosuppressive therapy.

Figure 86-6 Four examples of fibrosing disorders. A, Scleromyxedema (mucinous skin lesions appear as waxy papules on the face). B, Eosinophilic fasciitis (induration of skin and subcutaneous tissues). C, Scleredema (diffuse subcutaneous edema with swelling over right trapezius). D, Nephrogenic systemic fibrosis (coalescence of indurated nodules with joint contracture).

(Modified from Boin F, Hummers LK: Scleroderma-like fibrosing disorders, Rheum Dis Clin North Am 34:199–220, 2008.)

Eosinophilic fasciitis presents with limited scleroderma-like skin changes involving the extremities (see Figure 86-6). The correct diagnosis is suggested by finding a peripheral eosinophilia and a hypergammaglobulinemia.⁴³ The definitive diagnosis is established by a full-thickness skin biopsy that shows a diffuse inflammation of the fascia. Initial treatment is with corticosteroids (prednisone 0.5 to 1 mg/kg) tapering according to the clinical response; some patients need to continue moderate-dose corticosteroids for up to 2 years. Methotrexate and mycophenalate may be used in refractory cases.

Scleromyxedema is characterized by cutaneous mucinosis and is often associated with a gammopathy, usually IgM and light chains.⁴⁴ The mucinous skin lesions appear as waxy papules on the face, neck, and limbs. If the papules coalesce, it may be mistaken for scleroderma (see Figure 86-6). Systemic involvement may occur with dysphagia, proximal muscle weakness, pulmonary, cardiac, and renal complications. It is difficult to manage; corticosteroids are usually tried initially, in refractory cases some benefit has been reported for intravenous immunoglobulin and thalidomide.

Scleredema is a cutaneous mucinosis that often starts with a febrile episode and resolves spontaneously.⁴⁵ More chronic scleredema has been associated with paraproteinemias including multiple myeloma and diabetes mellitus. The dermis is thickened with increased collagen glycosylation, as in diabetic stiff skin syndrome. The face and neck are commonly involved, and there is relative sparing of the hands and feet (see Figure 86-6). Systemic organ involvement is rare, but a monoclonal gammopathy is sometimes seen. Such cases need to be worked up for lymphoma. Refractory cases have been helped by local radiotherapy.