Genetic heterogeneity and minor CYP1B1 involvement in the molecular basis of primary congenital glaucoma in Gypsies

Abstract

Primary congenital glaucoma (PCG) is a genetically heterogeneous disorder of autosomal recessive inheritance, with mutations in the cytochrome P450 1B1 (CYP1B1) gene detected in an average of approximately 50% of cases worldwide. The Roma/Gypsies are considered to be a rare example of a single founder CYP1B1 mutation, E387K (identified in the Slovak Roma), accounting for 100% of disease alleles. Contrary to this concept, unusual genetic heterogeneity was revealed in this study of 21 Gypsy PCG patients from Bulgaria and 715 controls from the general Gypsy population. In our small sample of affected subjects, we identified five different CYP1B1 mutations - four known (E229K, R368H, E387K and R390C) and one novel and potentially pathogenic (F445I), which together accounted for approximately 30% of disease alleles. E387K was rare in both the patient and the control group, indicating that its high frequency in the Slovak Roma is the product of local founder effect not representative of the overall molecular pattern of PCG in the Gypsy population. Data on other Mendelian disorders and on the population genetics of the Gypsies suggest that a true founder mutation is likely to exist and has remained undetected. Our analysis of another candidate gene, MYOC, and the GLC3B and GLC3C loci did not provide support for their involvement. The molecular basis of PCG in the Gypsies is thus unresolved, and diagnostic analyses should be extended beyond the E387K mutation.

LTBP2 and CYP1B1 mutations and associated ocular phenotypes in the Roma/Gypsy founder population

“…17 The sample included 34 families with 42 affected subjects. In families with affected firstdegree relatives, only the proband was included in mutation frequency estimates; cousins were considered independent, due to observed intra-familial heterogeneity (example in Figure 1).…”

“…Carrier testing for the CYP1B1 p.E387K mutation was performed on 827 population controls, of whom 715 have been reported previously 17 and 112 were newly added Gypsy samples from Bulgaria and Spain. LTBP2 p.R299X was analysed in 782 samples: 552 control Gypsy individuals from diverse sub-isolates 18 and 230 Indians (180 from the northwestern state of Gujarat and 50 members of the Jat Sikh caste from the Northern state of Punjab).…”

“…25 The allele frequency of LTBP2 p.R299X was 0.013 (Supplementary Table 3) and the proportion sampled was based on 600 000 population size. For p.E387K, whose distribution is limited to a specific geographic region (Figure 3), the allele frequency was set at 0.008 (based on the sub-isolate data in Sivadorai et al 17 ) and the estimates were carried out using a population size of 60K or 100K.…”

“…Conditions were as described. 13,17 Where these analyses failed to identify the mutations on both chromosomes, we sequenced all coding regions, parts of the flanking introns and untranslated regions of LTBP2 (NM_000428). 13,14 Novel changes were analysed for potential functional effects using Human Splicing Finder 19 and PolyPhen.…”

“…17 The discovery of LTBP2 allowed the identification of an ancestral Gypsy mutation, p.R299X, which was shared with an affected Pakistani family on an identical haplotype background and explained B40% of Gypsy PCG alleles. 13 A substantial proportion of alleles remained unknown, an unusual situation in the Gypsy population, characterised by strong founder effect and restricted mutation diversity in Mendelian disorders.…”