Transforming growth factor alpha: a modifying locus for nonsyndromic cleft lip with or without cleft palate?

Most (but not all) studies have found weak but significant association between restriction fragment length polymorphisms at the transforming growth factor alpha (TGFA) locus on chromosome 2p13 and nonsyndromic cleft lip with or without cleft palate (CL +/- P). However, all attempts to demonstrate genetic linkage between TGFA and CL +/- P in families have produced consistently negative lod scores which provide evidence against linkage. We typed a 3-allele single-strand conformation polymorphism at TGFA in 14 extended families with multiple CL +/- P members from West Bengal, India. No significant TGFA differences were observed between the entire sample of 34 affected people and a sample of 38 unaffected people unrelated to each other (p = 0.39). However, affected individuals with CL only showed significant differences from unaffected individuals (p = 0.008). More interestingly, the CL only and CL+P groups of individuals differed strongly from each other in their TGFA frequencies (p = 0.0002). Using an autosomal dominant model with reduced penetrance for the inheritance of a major CL +/- P locus (suggested by our prior segregation analyses), a non-significant maximum lod score of 0.13 at a recombination frequency of 20% was obtained. We suggest that the TGFA locus only modifies expression (severity) of the CL +/- P trait, which is controlled by a major (necessary) locus elsewhere; this could explain the difficulty in obtaining positive linkage results.