LMX1B is a LIM-homeodomain transcription factor required for the normal development of dorsal limb structures, the glomerular basement membrane, the anterior segment of the eye, and dopaminergic and serotonergic neurons. Heterozygous loss-of-function mutations in LMX1B cause nail patella syndrome (NPS). To further understand LMX1B gene regulation and to identify pathogenic mutations within the coding region, a detailed analysis of LMX1B gene structure was undertaken. 5' -RACE and primer extension identified a long 5' -untranslated region of 1.3 kb that contains two upstream open-reading frames (uORFs). Transient transfection assays showed that sequences required for basal promoter activity extend no further than 112 bp upstream. An additional 47 mutations have been identified in the coding region, as well as nine deletions of large portions of the gene, but not in the promoter or highly conserved intronic sequences. The range of mutations and the identification of uORFs suggest further complexity in the regulation of LMX1B expression.

We report twenty-two novel mutations in the gene encoding the transcription factor LMX1B, previously shown to be mutated in persons with Nail Patella Syndrome (NPS). The mutations comprised eight missense, one splice-site, three insertion/deletion and ten nonsense or frameshift mutations. A sub-set of five recurrent mutations within the homeodomain represents over one-quarter of the described NPS mutations. The type and distribution of the mutations is consistent with the hypothesis that NPS is the result of haploinsufficiency for LMX1B.

Nail patella syndrome (NPS) is a pleiotropic condition characterized by dysplasia of the nails, hypoplasia of the patellae, elbow dysplasia, and progressive kidney disease. The syndrome is inherited in an autosomal dominant manner and has been shown to result from mutations in the LIM-homeodomain encoding LMX1B gene. The LMX1B transcription factor plays a role in defining the development of dorsal-specific structures during limb development. To date, a total of 64 point mutations and small deletions or insertions have been reported, concentrated within either the LIM or homeodomains. No NPS mutations have been observed within the carboxy-terminal third of the coding sequence, suggesting that mutations in this region are not inactivating. These findings support the hypothesis that NPS results from a 50% reduction in LMX1B function via a reduction in synthesis, disruption of secondary structure, or failure to bind DNA. Hum Mutat 14:459-465, 1999. (C) 1999 Wiley Liss, Inc.