Frecuencia de Celo

Assigning correct allele frequencies for marker loci is important for accurate statistical assessment of linkage. If a disease was to segregate with a common allele in the population the calculated lodscores will be low or obliterated if the family size is small due to the assumption that the appearance o f this allele in the family maybe from an unrelated spouse. However, a relatively common allele in one population maybe extremely rare in another, therefore in these situations calculation based on the use of the common frequency may mask a true linkage. Similarly, if a relatively frequent allele is erroneously assumed to be rare, evidence o f false linkage will be obtained. Most present day linkage studies rely on the CEPH/Généthon microsatellite marker panel where allele frequencies are based upon the values obtained for the CEPH group o f families. Often, equal frequencies for all alleles of a marker is presumed for ease of calculation, but this can lead to erroneous estimations o f linkage with detection o f false linkages or failure to detect true linkages. Estimation o f linkage for DHRD pedigrees I and II enrolled in this study founded allele frequencies on the published frequencies for the CEPH set of families* However as it was soon observed that these allele frequencies did not match those seen in the study population as extra alleles were frequently observed, new unrelated spouses were recruited into the study to provide more realistic allele frequencies in concordance with the gene pool o f that region.

3.17 Is D H R D allelic w ith M alattia leventinese ?

The clinical similarities between these two diseases coupled with their mapping to the same region o f chromosome 2p strongly suggests the two conditions to be allelic, implying mutations within a single gene being responsible for the two different clinical phenotypes. This is not an unusual phenomenon and is encountered in a number o f retinal dystrophies, the classic example being mutations in the outer segment disc membrane protein peripherin/RDS

which can manifest as a gamut of phenotypes ranging from typical retinitis pigmentosa to typical macular dystrophy (Wells et al, 1993). This is possible where a protein has several cellular functions. It is well documented that alterations in structural proteins typically result in dominant disease phenotypes while those in functional proteins usually enzymes lead to recessive diseases. Many exceptions to this view do exist, but in retinal disease, none exemplifies this more than the gene encoding rhodopsin. Mutations in rhodopsin can cause either dominant or recessive RP (Rosenfeld et al, 1992: Al-Maghtheh et al, 1993). This can be envisaged as certain mutations may alter its structural attributes resulting in altered protein folding which can lead to structural instability of the cell and cell death, but rhodopsin also plays a catalytical role bringing about its own hydrolysis with absorption o f light. M utations that affect this enzymatic role may lead to the recessive disease phenotypes o f RP that have been observed. These findings strengthen our view on the possible allelic nature of DHRD and Malattia leventinese.

A radial pattern o f drusen is a clinical finding in both these diseases, however it is more the exception in DHRD while being the norm in Malattia leventinese. Although the drusen deposits in Malattia leventinese are believed to be continuous with, and internal to the basement membrane of the RPE while those in DHRD lie external to the basement membrane, the recent evaluation o f the phenotypic diversity in our DECRD family by Evans and coworkers (In Press) states that the differentiation of the Malattia leventinese phenotype from that of DHRD can only be based upon the constancy and quantity o f the sub-retinally observed radial drusen. If DHRD and Malattia leventinese are allelic conditions, it is possible that the clinical variation may result from the presence o f a double mutant allele o f a single gene in the more severe Malattia leventinese phenotype resulting in prominent radial drusen while a single mutant allele results in the milder DHRD phenotype in which mild radial drusen is observed. Such speculation is not inconceivable as double mutant alleles have previously been reported in diseases such as cystic fibrosis (Savov et al, 1995).

The possibility o f two genes with related functions mapping closely to the same region of the genome cannot however be excluded as such findings have been observed for the red and green opsin genes which lie in tandem array on the X chromosome (Nathens et al, 1986), genes encoding the major histocompatibilty proteins on chromosome 6 (Auffrey and Strominger, 1986), and the interleukin genes, IL-4 and IL5 on chromosome 5 (Stock et al, 1992). If two different genes responsible for DHRD and Malattia leventinese exist in close proximity on 2p21-16, they may individually be mutated or more likely, cytogenetically invisible microdeletions encompassing both genes (contiguous gene syndrome) may be occurring. In this situation, the similarities in the two disease phenotypes can be explained by the causative genes encoding two different components o f a common pathway. The varying properties observed in DHRD alone suggest that the mutant gene maybe affecting a number o f biochemical pathways resulting in the accumulation of debris of a varied nature. Identification of the causative gene/genes for both these clinical entities and their mutations await the clarification of these thoughts.

3.18 Relevance for the future

One o f the difficulties of counselling families afflicted with late onset diseases such as DHRD is the inability to detect the clinical manifestations at an early age. In DHRD this is further complicated by the phenotypic diversity observed which is not an age-related feature. W ith the mapping of DHRD to chromosome 2p l6 and subsequent haplotype analysis, molecular genetics can now form the basis for identification o f disease in younger generations o f this family and other linked families enabling a better understanding o f the disease and its prognosis to be provided. This is of specific importance in guiding the careers of affected individuals away from areas centred on acute vision. The present work in our laboratory is centred upon

Genetic analysis o f S orsby’s fundus dystrophy

CHAPTER IV

RESULTS II