Some patients have features of more than one
rheumatic disease and thus do not fit into traditional classification. Patients with combination of clinical finding similar to those of
systemic lupus erythematosus (SLE),
progressive systemic sclerosis (PSS),
polymyositis,
rheumatoid arthritis (RA) and with unusually high titers of circulating
antinuclear antibody with specificity for nuclear
ribonucleoprotein (RNP) are considered to have
mixed connective tissue disease (
MCTD). The overlap was described by Sharp and colleagues in 1972. During the post 20 years many studies exposed the clinical correlates of this antibody system (now called anti U1RNP). Controversy arose about whether
MCTD was a distinct entity or would be better defined as subset of SLE. Anti RNP
antibodies precipitate three
proteins uniquely associated with U1RNP. Clinical correlates considered to be distinctive of
MCTD are associated with 68 kD
antigen specificity. Its to be expected that T cells receptors and HLA molecules are involved in the generation of these
antibodies. Several observations have indicated, that 68 kD anti U1RNP antibody response in associated with
HLA DR 4 and DR2 phenotype. Several studies have pointed a role of viruses initiating an antibody response against URNPs. Initial observations of
MCTD suggested infrequent renal disease, a good response to
corticosteroids and favourable prognosis. Future study has shown that some patients may require aggressive and prolonged pharmacologic
therapy and that pulmonary involvement is common.
Pulmonary hypertension associated proliferative vascular lesions may be serious complication with not always favourable prognosis.