U1-snRNP antibodies, also referred to as U1-RNP and Smith (Sm)/RNP, target 3 components of the U1 small nuclear ribonucleoprotein complex: U1-snRNP RNP A, U1-snRNP RNP C, and U1-snRNP RNP-70kd.
These antibodies, found in 2% to 14% of SSc patients, are more frequent in lcSSc than in dcSSc.
The 2 main types of SSc are defined according to the pattern of skin involvement: limited cutaneous SSc (lcSSc) and diffuse cutaneous SSc (dcSSc).
In lcSSc, skin thickening is present distal to the elbows and knees, and facial skin thickening may or may not be present. In contrast, dcSSc is characterized by thickening of the skin of the whole extremity, as well as that of the anterior chest, abdomen, and back, with or without facial skin involvement. Multiple organs, including the heart, lungs, gastrointestinal tract, and kidneys, can be affected in both forms, though organ involvement is generally less severe in lcSSc.
CREST syndrome (calcinosis, Raynaud phenomenon, esophageal dysmotility, sclerodactyly, and telangiectasia) occurs frequently in lcSSC but can also occur in longstanding dcSSc.
The antibodies are also found in patients with other autoimmune diseases, including approximately 90% of patients with mixed connective tissue disease.
Patients with U1-snRNP antibodies tend to have less prominent skin thickness and less renal involvement, but have increased risk of PAH, arthritis, and esophageal dysfunction.
Overall, the presence of U1-snRNP antibodies is associated with a good response to corticosteroids and a favorable prognosis.
Clinical Characteristics Associated With Autoantibodies Used in the Diagnosis and Classification of Systemic Sclerosisa:
|
Antibody |
Prevalence in SSc, % |
Presence in other autoimmune diseases |
SSc type most likely |
Clinical associations |
Relative SSc prognosis |
|
ANA |
85-95 |
Common |
Does not help differentiate type |
Most common in rheumatic diseases |
Varies |
|
Centromere A or B |
20-40 b |
Uncommon |
lcSSc |
PAH, calcinosis, digital ischemia, intestinal involvement |
Better |
|
Ku |
1.5-5 |
Common |
Overlap syndromes or lcSSc |
Muscle and joint involvement |
Unclear |
|
PM/Scl-75 or PM/Scl-100 |
2-11 |
Common |
SSc-myositis overlap syndrome or lcSSc |
Muscle and joint involvement, calcinosis; GI and lung involvement for those with PM/Scl-75 |
Better |
|
RNA Polymerase III |
7-41 |
Rare |
dcSSc |
Renal crisis, progressive skin thickening, GAVE, malignancy |
Poorer |
|
Scl-70 |
9.4-71 b |
Rare |
dcSSc |
Severe pulmonary fibrosis, cardiac involvement, renal crisis, digital ulcers |
Poorer |
|
Th/To |
2-5 |
Rare |
lcSSc |
Pulmonary involvement |
Poorer |
|
U1-snRNP |
2-14 |
Common (90% in patients with MCTD) |
lcSSc |
PAH, arthritis, esophageal dysfunction |
Better |
|
U3-RNP (Fibrillarin) |
4-10 b |
Rare |
dcSSc |
Multiorgan involvement, PAH |
Poorer |
| ANA, antinuclear antibodies; dcSSc, diffuse cutaneous systemic sclerosis; GAVE, gastric antral vascular ectasia; GI, gastrointestinal; lcSSc, limited cutaneous systemic sclerosis; MCTD, mixed connective tissue disease; PAH, pulmonary arterial hypertension; SSc, systemic sclerosis. | |
| a | Data presented in this table have been compiled from the cited references. It is important to note that autoantibody frequency in SSc as well as other autoimmune diseases varies with sex, ethnicity, and population studied. |
| b | In the United States, prevalence has been shown to vary across ethnicities. |
References:
Stochmal A, Czuwara J, Trojanowska M, et al. Antinuclear antibodies in systemic sclerosis: an update. Clin Rev Allergy Immunol. 2020;58(1):40-51. doi:10.1007/s12016-018-8718-8
Mehra S, Walker J, Patterson K, et al. Autoantibodies in systemic sclerosis. Autoimmun Rev. 2013;12(3):340-354. doi:10.1016/j.autrev.2012.05.011
Hamaguchi Y, Takehara K. Anti-nuclear autoantibodies in systemic sclerosis: news and perspectives. J Scleroderma Relat Disord. 2018;3(3):201-213. doi:10.1177/2397198318783930
Young A, Khanna D. Systemic sclerosis: commonly asked questions by rheumatologists. J Clin Rheumatol. 2015;21(3):149-155. doi:10.1097/RHU.0000000000000232
Denton CP, Khanna D. Systemic sclerosis. Lancet. 2017;390(10103):1685-1699. doi:10.1016/s0140-6736(17)30933-9
U1-snRNP RNP A
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