Novel exon 3B proteolipid protein gene mutation causing late-onset spastic paraplegia type 2 with variable penetrance in female family members.

Spastic paraplegia type 2 (SPG2) is allelic to Pelizaeus-Merzbacher disease (PMD), with both conditions resulting from mutations in the proteolipid protein gene (PLP). We report an SPG2 family in which 3 male members and a heterozygous female member were affected with spastic paraplegia characterized by relatively late onset and mild clinical manifestations. A unique H147Y mutation in exon 3B of the PLP altering the proteolipid protein (PLP) but not the alternatively spliced DM20 isoform was identified as the cause of this distinct disease phenotype. Cellular pathology studies of SPG2 mutations offer an explanation for the paradoxical finding that mutations associated with the mildest phenotype in male family members also affect female carriers.