A. Mukhopadhyay

Myocilin gene implicated in primary congenital glaucoma

Primary congenital glaucoma (PCG) has been associated with CYP1B1 gene (2p21), with a predominantly autosomal recessive mode of inheritance. Our earlier studies attributed CYP1B1 mutations to only 40% of Indian PCG cases. In this study, we included 72 such PCG cases where CYP1B1 mutations were detected in only 12 patients in heterozygous condition, implying involvement of other gene(s). On screening these patients for mutations in myocilin (MYOC), another glaucoma‐associated gene, using denaturing high‐performance liquid chromatography followed by sequencing, we identified a patient who was double heterozygous at CYP1B1 (c.1103G>A; Arg368His) and MYOC (c.144G>T; Gln48His) loci, suggesting a digenic mode of inheritance of PCG. In addition, we identified the same MYOC mutation, implicated for primary open angle glaucoma, in three additional PCG patients who did not harbor any mutation in CYP1B1. These observations suggest a possible role of MYOC in PCG, which might be mediated via digenic interaction with CYP1B1 and/or an yet unidentified locus associated with the disease.

Recent advances in molecular genetics of glaucoma.

Glaucoma represents a heterogeneous group of optic neuropathies, with different genetic bases. It can affect all ages generally with a rise in intra-ocular pressure. Three major types of glaucoma have been reported: primary open angle glaucoma (POAG), primary acute closed angle glaucoma (PACG) and primary congenital glaucoma (PCG), as well as a few others associated with developmental abnormalities. In recent years impressive progress has been made in the molecular genetic studies of POAG and PCG. These include the discovery of three genes – Myocilin, Optineurin and CYP1B1 – defects in which results in Mendelian transmission of glaucoma. Identification of single nucleotide polymorphisms in multiple other genes that are associated with glaucoma and alteration of drug sensitivity are enriching our knowledge regarding the complex nature of the disease. This review attempts to present the recent progress made in the molecular genetics of glaucoma.

Molecular pathology of haemophilia B: identification of five novel mutations including a LINE 1 insertion in Indian patients.

Summary.u2002 Heterogeneous mutations in factor IX (FIX) gene cause haemophilia B and a large number of mutations have been characterized. However, reports on gene defects among Indian haemophilia B patients are rare despite a high estimate of such patients in the country. We report identification of 22 independent mutations including five novel mutations in 24 unrelated patients. The novel gene defects include two point mutations, two deletions and one insertion of a LINE 1 element. Majority of the mutations (14 of 24) occurred on the same haplotype background, but do not suggest any founder effect. Direct identification of mutations can be utilized to perform the carrier detection and prenatal diagnosis, especially in families with isolated patients.

Analysis of haemophilia B database and strategies for identification of common point mutations in the factor IX gene

Summary.u2002 Haemophilia B is an X‐linked recessively inherited bleeding disorder caused by heterogeneous mutations spanning the entire factor IX gene. As spontaneous germ‐line mutations are known to occur mostly at CpG dinucleotides in the FIX gene, control of the disease would require continuous carrier detection and mutation screening. Identification of point mutations, the most common type of mutation in FIX gene, is more challenging compared with deletion and insertion mutations. We examined the haemophilia B database to identify specific nucleotides in the FIX gene that are mutated in relatively large number of patients and the variability (if any) in the mutational hotspots at CpG dinucleotides. It was found that while mutations responsible to account for all 2348 haemophilia B patients covered 20% of the FIX cDNA, only 1% of the cDNA involving mostly CpG dinucleotides accounted for mutation in 42.41% of the patient pool. Thus, only 27 nucleotides need to be investigated to identify the common point mutations, among which 15 are predicted to undergo change in restriction sites on mutation. It is interesting to note that seven nucleotides occurring in CpG dinucleotides do not have any reported mutation despite each of those being predicted to harbour mutation as a result of transition and having mutations recorded in the database for the neighbouring nucleotides. Strikingly large number of mutation in codon 296 causing T to M change in catalytic domain originally proposed to be the result of the founder effect also contains largest number of haplotype suggesting recurrence of deu2003novo mutation.

OCA1 in Different Ethnic Groups of India is Primarily Due to Founder Mutations in the Tyrosinase Gene

Oculocutaneous albinism (OCA) is a heterogeneous group of autosomal recessive disorders characterized by an abnormally low amount of melanin in the eyes, skin and hair, and associated with common developmental abnormalities of the eye. Defects in the tyrosinase gene (TYR) cause a common type of OCA, known as oculocutaneous albinism type 1 (OCA1). The molecular basis of OCA has been studied extensively in different population groups, but very little information is available on Indian patients. Our investigation covering thirteen ethnic groups of India, some representing >20 million people, revealed that among 25 OCA families 12 were affected with OCA1, and that these cases were primarily due to founder mutations in TYR. We detected nine mutations and eight SNPs in TYR, of which six mutations (five point mutations & one gross deletion) were novel. In contrast to most reports describing compound heterozygotes, the presence of homozygotes in 10 out of the 12 pedigrees underscores the lack of intermixing between these ethnic groups in India. Haplotype analysis suggested a few founder chromosomes causing the disease in the majority of the patients. Direct detection of the mutations prevalent in specific ethnic groups could be used for carrier detection and genetic counselling.