The Retina


Retina is one of the most important parts of the eye which play a major role in vision. It is consists of two major structures which rods and cones. Rods are larger in number in compare to cones and are more concentrated in the peripheral retina. They are responsible for scotopic or night vision. Rods have high sensitivity and convergence in retinal pathway. However, they have low spatial and temporal resolution in compare to cones. Rods dysfunction affects night vision and loss of peripheral field. Cones are fewer in number and more concentrated in fovea. It is responsible for photopic or day vision. Cones dysfunction can lead to poor color vision, central flied and day vision.15

Retinitis pegmentosa (RP) is an inherited disease cause by mutation in many genes (genetically heterogeneous) characterized by destruction of photoreceptors and vision loss. The main symptoms of RP are night blindness (due to destruction of rods involved with night vision), and reduction of the visual field. Patients with RP do not usually present with systemic manifestations, but there are other systemic conditions that can present as RP such as Laurence-Moon syndrome and Bardet-biedl syndrome. (1)

Bardett-biedl syndrome (BBS) is an autosomal recessive disease that results in cone-rod dystrophy (atypical RP). BBS is distinguished from Laurence-Moon syndrome, in which retinal pigmentary degeneration, hypogonadism and mental retardation occur in conjunction with sever, progressive distal muscle weakness and spastic paralysis, but With no polydactyly. Moreover, BBS is more common than LMS, with a prevalence of 1 in125,000'160,000 in Europe and 1 in 65,000 in an Arab population.

In BBS, both copies of BBS gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition. The systemic features of this condition include postaxial polydactyly, mental retardation, central obesity, hypogonadism, and renal failure. There are other rare symptoms which not present in all patients such as short stature, hepatic fibrosis, reproductive abnormalities, diabetes mellitus, endocrinological disturbances, developmental delay, and speech deficits. BBS is a genetically heterogeneous condition characterized by involvement of many chromosomal loci. The most common one are BBS1 (11q13), BBS3 (3p13), BBS2 (16q22) and BBS4 (15q21). (8)

Mutations in these genes lead to problems with the structure and function of cilia. Defects in these cell structures probably disrupt important chemical signaling pathways during development and lead to abnormalities of sensory perception. Researchers believe that defective cilia are responsible for most of the features of Bardet-Biedl syndrome.

A recent study has shown that BBS1 (11q13) is the most common type especially among Caucasian families (45%). However, BBS among Middle Eastern countries and Asia don't show relation to any of the above loci. (8) A comparative study was done in Canada and showed that BBS3 is the most common type among Canadian people. All the samples were homozygous reflecting a common heritage among their patients (6).

In most of North America and Europe, Bardet-Biedl syndrome has a prevalence of 1 in 140,000 to 1 in 160,000 newborns. The condition is more common on the island of Newfoundland (off the east coast of Canada), where it affects an estimated 1 in 17,000 newborns. It also occurs more frequently in the Bedouin population of Kuwait, affecting about 1 in 13,500 newborns.(2).

A cohort study was conducted among Newfoundland population including 46 patients (26 males, 20 females) showed that 91% of the patients have blindness, 63% with postaxial polydactyl, 95% with Syndactyly, and Brachydactyly was present in all patients` feet and in the hands of 86% of the patients. Moreover, about 96% of the patients were obese, 48% with diabetes mellitus, 86% with Neurological, speech, and psychiatric abnormalities and renal abnormalities in all patients. 33

A phenotypic and molecular study was conducted on 18 families of different origin (14 from North Europe and 4 from Middle East) showed that 72% of the patients present with polydactyl. More ever, all the patients have renal and genital abnormalities. Twenty eight of the children were in special need schools while the remaining 22% went to normal schools. For the genotype of the eighteen families, the study showed that 11q13 locus is the most common BBS gene mutation among the families follow by 16q21, where 15q22 and 3q12 loci are less common. 8

Comparative study of three relative Arabian families suggests that the severity and the number of symptoms in BBS, specially polydactyly is associated with specific chromosomal subtypes of BBS.6

Material & method:

Retrospective, qualitative, Case series study was conducted among 13 patients from 5 unrelated families. The sample was selected according to specific criteria in which the patients should be Omanis, at any age or gender, referred from any health center to ophthalmology clinic at Sulltan Qaboos University Hospital (SQUH).the study was planned to enrolled 6 unrelated families but one family was not able to come for the systemic and genetic examination, so we did not include it in the systemic and genetic results. Patients with pseudoretinitis pigmentosa and other systemic conditions associated with RP were not included. After the selection of the sample from SQUH Ocular Genetics Database, the ocular, genetic and the systemic data of the patients were collected from the old medical files as well as the electronic medical records system at the medical record department. Then, all the families were called to be informed about the research and to obtain an inform consent. After that, each family was scheduled for an appointment at the genetic and ophthalmic clinics at SQUH.

During the visits, the history of the patients was taken from their parents for any abnormal or delay in their developmental milestones and their behavior at home. The psychological adaptation of the patient was evaluated by questioner. All the patients underwent a full physical and ocular examination. Cardiovascular examination, respiratory examination and inspection of the hands and feet (for polydactyl, sundactaly and bradlydactyly) and the head (for craniofacial deformities) were also performed. The orodental deformities were also detected as a part of the examination. The blood pressure, height, weight and head circumference were measured. A urine sample was collected from all patients for urine cultured and urine dipstick. Females' patients underwent an ultrasonography (US) of the reproductive system to asses any gentile abnormalities were the gentile abnormalities in males were assisted by general examination of the external gonads. Liver and renal abnormalities were recorded in the system. Then, patient's behavior, developmental and speech delay were assisted by the psychosocial team at the genetic clinic.

The families' pedigrees were drawn by the help of past medical history taken from the families and depending on the data available in the system.

The genotype mutations were already recorded in the system for all families except for one family in which a genetic molecular test was ordered. Venus blood sample was taken from the patients for the genetic and molecular test. DNA was extracted from leucocytes and each family member was genotyped using the polymerase chain reaction (PCR) incorporating fluorescently labeled microsatellite marker flanking known loci. We amplified 40 ng of genomic DNA in 10 gl reaction volumes containing 1 ,ul PCR buffer (10 x) (670 mmo Tris-HCl, pH 8.0, 166 mmol/I NH4S04, mmol/l MgCl,, 1.7 mg/ml BSA), 5 mmol/l each dNTP (dATP, dCTP, dGTP, dTTP), pmol of each fluorescent primer, and 0.6 uni Taq polymerase. Typical reaction condition were as follows: initial denaturation at 94??C f 10 minutes then 24 cycles of 94??C for 45 sei onds, annealing temperatures 54-60??C for 4 seconds, extension at 72??C for two minute and then extension at 72??C for 10 minutes. The PCR products were separated on a 6' polyacrylamide gel and their fluorescent sign detected by a DNA fragment analyser (Al 373). Alleles were assigned using the incorporated GENOTYPER software and then taste for two point linkage using MLINK (part ( the LINKAGE software package). A model autosomal recessive inheritance was used.8

The data was analyzed in order to meet the objectives of the study by using statistical package for social sciences (SPSS). Then, the data was summarized as number and percentage and classified and represented in graphs and charts.

Finally, the families were counseled about the inherited pattern of the disease and how to prevent farther affected members at the family.

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