PSMB8 encoding the β5i proteasome subunit is mutated in joint contractures, muscle atrophy, microcytic anemia, and panniculitis-induced lipodystrophy syndrome.
We performed homozygosity mapping in two recently reported pedigrees from Portugal and Mexico with an autosomal-recessive autoinflammatory syndrome characterized by joint contractures, muscle atrophy, microcytic anemia, and panniculitis-induced lipodystrophy (JMP). This revealed only one homozygous region spanning 2.4 Mb (5818 SNPs) on chromosome 6p21 shared by all three affected individuals from both families. We directly sequenced genes involved in immune response located in this critical region, excluding the HLA complex genes. We found a homozygous missense mutation c.224C>T (p.Thr75Met) in the proteasome subunit, beta-type, 8 (PSMB8) gene in affected patients from both pedigrees. The mutation segregated in an autosomal-recessive fashion and was not detected in 275 unrelated ethnically matched healthy subjects. PSMB8 encodes a catalytic subunit of the 20S immunoproteasomes called β5i. Immunoproteasome-mediated proteolysis generates immunogenic epitopes presented by major histocompatibility complex (MHC) class I molecules. Threonine at position 75 is highly conserved and its substitution with methionine disrupts the tertiary structure of PSMB8. As compared to normal lymphoblasts, those from an affected patient showed significantly reduced chymotrypsin-like proteolytic activity mediated by immunoproteasomes. We conclude that mutations in PSMB8 cause JMP syndrome, most probably by affecting MHC class I antigen processing.
Department of Internal Medicine, Center for Human Nutrition, University of Texas Southwestern Medical Center, Dallas, 75390, USA.
Anil K
AK
Agarwal
Chao
C
Xing
George N
GN
DeMartino
Dario
D
Mizrachi
Maria Dolores
MD
Hernandez
Ana Berta
AB
Sousa
Laura
L
Martínez de Villarreal
Heloísa G
HG
dos Santos
Abhimanyu
A
Garg
D000740:Anemia / Q000235:genetics*; D020134:Catalytic Domain; D003286:Contracture / Q000235:genetics*; D006801:Humans; D008060:Lipodystrophy / Q000209:etiology / Q000235:genetics*; D009133:Muscular Atrophy / Q000235:genetics*; D020125:Mutation, Missense; D015434:Panniculitis / Q000150:complications*; D020641:Polymorphism, Single Nucleotide; D046988:Proteasome Endopeptidase Complex / Q000235:genetics*
C077436:LMP7 protein; D046988:Proteasome Endopeptidase Complex