Email: michelle. The grass genetic model Brachypodium Brachypodium distachyon L. Brachypodium and wheat shoot and root development and anatomy were highly similar. Main stem leaves and tillers side shoots emerged at the same time in both grasses in four temperature and light environments.
Both developed primary and nodal axile roots at similar leaf stages with the same number and arrangement of vascular xylem tracheary elements XTEs. Brachypodium, unlike wheat, had an elongated a mesocotyl above the seed and developed only one fine primary axile root from the base of the embryo, while wheat generally has three to five.
Roots of both grasses could develop first, second and third order branches that emerged from phloem poles. Brachypodium has the overwhelming advantage of a small size, fast life cycle and small genome, and is an excellent model to study cereal root system genetics and function, as well as genes for resource partitioning in whole plants. Whereas the main mode of inheritance for primary congenital glaucoma is autosomal recessive 12 , 13 , 22 , for juvenile and adult-onset glaucoma the more frequently reported mode of inheritance is autosomal dominant with reduced penetrance 23— A significant proportion of other ocular conditions associated with glaucoma are also inherited as autosomal dominant traits 27— Table 1 provides a summary of the chromosomal location for different types of primary glaucoma and other associated conditions.
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In this article, the molecular genetics of different types of glaucoma will be reviewed in order to provide some insight into the molecular etiology of two well studied types of glaucoma, namely, the juvenile-onset primary open angle glaucoma and the pediatric form of primary congenital glaucoma. A brief summary on the current status of other types of ocular conditions associated with glaucoma will also be presented. This is the most common form of this group of eye conditions, usually accompanied with variable severity and phenotypic expressivity 1 , 9.
POAG have been arbitrarily divided into two groups of juvenile and adult with an overlapping clinical presentation and a sliding scale age of onset and IOP values 1. Although there are some differences between the rare form of juvenile-onset open angle glaucoma JOAG as compared with the more frequent form of adult-onset chronic open angle glaucoma COAG , the clinical diagnosis of both groups are based on the presentation of visual field loss, glaucomatous changes of the optic nerve and optic nerve damage that is usually accompanied with an increased IOP 9.
Apart from the clear differences in the age of diagnosis in these two groups of POAG, the condition in juvenile subjects is more severe, presenting with significantly higher IOPs i. In contrast, the late-onset form has a quite different phenotype, usually with a milder presentation, progressive development, moderate elevation of IOP and medical treatments often yield a satisfactory outcome 6 , 7 , 32— The painless progression often leads to a late diagnosis, when irreversible damage to the optic nerve has already occurred, thus complicating the prognosis of this type of POAG.
Although there is some controversy about the exact mode of inheritance in these two groups of eye condition, pedigree structure of the majority of families used in genetic linkage analysis clearly suggest that inheritance is autosomal dominant with an incomplete penetrance. A new page in the molecular genetic study of JOAG was opened in early , when using a single large American family, Sheffield et al. After this initial report, the efforts of other investigators from the US and Europe were focused on testing additional glaucoma families from different genetic backgrounds. Confirmation of this initial linkage was soon followed in another American JOAG family 36 , families with Irish, British and German backgrounds 37 , two French families 38 and one large Danish family Morissette et al.
In this study, 36 subjects were diagnosed between the ages of 25 and 35 and, therefore, classified as having JOAG, while four subjects were diagnosed after the age of 40 and considered as COAG. Six other members were diagnosed with ocular hypertension and several other asymptomatic obligate carriers were also identified. Twenty family members developed glaucoma between 11 and 51 years of age median A 35 year old healthy female had a severely affected daughter and therefore classified as a case of incomplete penetrance.
Nine more normal family members aged from 14 to 66 had also inherited the affected haplotype.
Clinical features in the affected individuals from all of the families reported so far were very similar and conformed to the typical form of juvenile-onset primary open angle glaucoma. Despite the existing variability in the exact age of detection, the majority of the patients were diagnosed with glaucoma in childhood to early adulthood average 18 years. The IOP was typically very high, often in the range of 40—50 mm Hg. Medical treatment was initially effective, but surgery was required to control the progress of glaucoma.
Chromosomal location for different types of primary glaucoma and other associated ocular conditions. The translated regions in both genes are lightly shaded. Frameshift mutations that are predicted to truncate the open reading frames of these two genes are shown with dark up-arrows below the exons.
Missense mutations are indicated with contour down-arrows above the exons. The region of the CYP1B1 gene affected by a large deletion is shown by a bi-directional arrow below the exon line. Note that all the missense mutations identified in our study are in these regions.
Evidence for genetic heterogeneity of this condition has also been reported 39 , 42 , Graff et al. The affected members of this family developed glaucoma before the age of 25 and maintained IOP pressure without treatment in the 28—50 mm Hg range. This may possibly prove to be linked to the 4q25 or 6p25 regions, where other families with both glaucoma and iris hypoplasia have been mapped. In two JOAG families unlinked to the 1q21—q31 region, some differences in the clinical presentation of glaucoma were noted For the first family the age of onset and severe optic nerve damage was before the age of 10, but the IOP was mildly elevated 25—30 mm Hg range.
The second family was of African American descent. Further evidence for genetic heterogeneity of this condition has also been reported for French JOAG families A similar contig map of the region has also been reported by another group A more comprehensive physical map of GLC1A has recently been published These investigators also completed a 4.
Of the 10 familial cases with mutations, at least one has been indicated to be of the adult-onset type. A predominantly reported mutation is the GlnSTOP observed in six familial, three sporadic and one normal healthy subject. The mutation truncates the TIGR protein by amino acids prematurely note that due to an earlier error in the cDNA sequence of this gene, the amino acid numeration has now changed. The responding protein was named TIGR and subsequently shown to be present not only in the TM but also in the ciliary body, the two major tissues that are involved in the production and regulation of aqueous humor which normally nourishes ocular tissues and controls the intraocular pressures AF , cloned named CBS and characterized from a human ciliary body library These authors showed that, in the ocular tissues, the TIGR transcript is abundantly present in iris and ciliary body, but in low or undetectable levels in cornea, lens and retinal pigment epithelium.
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In the non-ocular tissues, the transcript preferentially hybridizes to heart and skeletal muscle RNA, but is undetectable in brain, placenta, lung, liver, kidney and pancreas. Therefore, it was suggested that TIGR is restricted preferentially to muscle in ocular and non-ocular tissues At the top is the helical configuration of the leucine zipper and arginine motifs between residues and of the coding region of CBS Only leucine L and arginine R residues are highlighted.
The position of amino acid residues indicated are approximately positioned relative to the main features indicated. SP, signal peptide; dark box, leucine zipper-like region. Figure kindly provided by Dr Miguel Coca-Prados. The TIGR gene encodes for a polypeptide that is amino acids long. The gene consists of three exons , and bp, respectively that are separated by two large introns Table 2 and Fig.
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The leucine-rich region contain an a-helical conformation and on its opposite sites, five arginine residues form a positively charged domain Fig. Interestingly, this region contains all the known TIGR mutations 48 and has been suggested as possibly being involved in the cell binding activity of this protein Therefore, mutations in this region may interfere with uptake or metabolism of the protein leading to its accumulation, obstruction of aqueous outflow and increased IOPs. This abnormal IOP regulation and control would eventually result in damage of the vulnerable optic nerve tissues, thus leading to glaucoma and possible blindness.
D that maps to the 1q23—q24 region has been described It has been recently confirmed that these two genes are identical N. Shimizu, personal communication. The Myocilin gene was cloned from the human retina cDNA library and shown to encode an acidic protein isoelectric point 5.
These authors suggest that this is a new cytoskeletal protein involved in the morphogenesis of ciliated neuroepithelium such as photo-receptor cells.
Northern blot analysis with a cDNA clone corresponding to nt — i. However, no signals were detected in other tissues including heart, brain, placenta, lung, liver, kidney and pancreas. These data indicate that this gene is predominantly expressed in only limited types of highly differentiated tissues including retina and perhaps muscle. These authors also showed that Myocilin is actively produced in the ellipsoid of the rod inner segment where most protein synthesis takes place, and it is apparently transported to the basal apparatus of the connecting cilium.
They further speculated that the hydrophobic regions at the N-terminus may act as glue connecting the microtubules or, alternatively function as a connector between the nucleus and the basal body. Based on the characteristic nature of this gene, the authors also suggested that myocilin may be a candidate gene for an inherited retinal disease such as Usher syndrome However, the precise localization of the MYOC in the rootlet and basal body of connecting cilium and specific tissue expression of this protein should provide further understanding of the role and function of this gene or TIGR in patients affected with POAG.
In summary, the gene that is mutated in patients with POAG has been independently cloned and isolated from trabecular mesh-work 49 , ciliary body 50 and retina We have also identified a mutation in one family from Edinburgh, Scotland The only deletion of 3 bp in exon III was reported in four families from a small southern Italian village where all individuals carry a common haplotype. This mutation most likely resulted from a founder effect with a common ancestral mutation One other interesting patient has been described who is hemizygous for the GLC1A region This patient is 24 years of age and has no clinical evidence of glaucoma.
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Therefore, it was indicated that the loss of function of one of the copies of the GLC1A gene i. However, it is still likely that this patient will develop glaucoma at a later stage of life and that the haploinsuffi-ciency detected in this subject may still be adequate to cause the phenotype, a phenomena that has recently been reported for the chromosome 4q-linked Rieger syndrome Another interesting piece of evidence is presented in a large French-Canadian pedigree that has previously been shown to be segregating for the GLC1A locus in which four subjects born to two affected parents inherited two copies of the affected GLC1A haplotype from their parents, but all were clinically normal for glaucoma These patients were aged between 41 and 49 and they have already produced two clinically affected offspring who have inherited only a single affected haplotype.
This observation indicated that homoallelic complementation of the affected haplotypes is a likely mechanism to account for these phenotypically normal homozygous affected carrying haplotype subjects. If these patients do not share the same TIGR mutation with other branches of the pedigree, a separate mutation in another part of the genome or mutation in one of the closely located genes within the GLC1A critical region could be responsible for this observation.
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