porphyria, acute intermittent, nonerythroid variant
A HMBS-related hepatic porphyria subtype caused by variants in HMBS exon 1, which is not expressed in erythrocytes, causing HMBS activity in erythrocytes to be at a normal level. The housekeeping promoter produces the HMBS transcript containing exons 1 and 3-15. In contrast, erythrocytes use an erythroid-specific promoter downstream of the housekeeping HMBS promoter to produce a transcript that contains only exons 2-15. Therefore deleterious variants occurring within exon 1 or affecting the splicing of exon 1 to exon 3 do not impact the erythrocyte isozyme, but do impact the more broadly expressed housekeeping isozyme.