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Lattice corneal dystrophy (LCD), an inherited form of amyloidosis, is characterised by the development of lattice lines and opacity in the cornea. LCD is classified clinically into four subtypes: I, II, III, and IIIA. Several distinct mutations of TGFBI have been associated with LCDIIIA: P501T,1 L527R,2 N544S,3 A546T,4 N622K (T1913G and T1913A), and V627S.5 All cases of LCD characterised at the molecular genetic level to date have been attributed to heterozygous point mutations of TGFBI. We now present the first example of a homozygous point mutation of TGFBI in an individual with LCD, a diagnosis supported by clinical, histological, and molecular genetic findings.
A 52 year old Japanese man visited our corneal clinic in July 1997 with a main complaint of gradual impairment of vision. His parents, who were related, were no longer alive and he had no children. He had two brothers and four sisters. His reporting suggested that his father had had LCD and that his younger sister had also developed this condition.
Slit lamp examination revealed a large region of opacity and thick lattice lines in the middle to deep portion of the corneal stroma in both eyes (fig 1A and B). Given that his visual acuity in both eyes decreased to 20/40, we performed penetrating keratoplasty on his left eye in December 2000 and on his right eye in May 2002. After the surgeries, his visual acuity improved to 20/25 in each eye and no recurrence has been observed to date.
Histological analysis revealed that the amorphous component of the middle to deep region of the stroma of both corneas stained with eosin (fig 1C) and with Congo red (fig 1D). The detection of apple green dichroism by polarised light microscopy was also consistent with amyloid deposition in the middle to deep region of the corneal stroma (fig 1E). Although most of the epithelial cell layer, basement membrane, and hemidesmosomes of the right cornea appeared normal by transmission electron microscopy (TEM) (fig 1F), a region was detected that seemed to be devoid of basal cells, basement membrane, and hemidesmosomes. TEM also revealed amyloid deposits in the middle to deep corneal stroma (fig 1G), although Descemet’s membrane and endothelial cells appeared normal.
After obtaining informed consent, we purified DNA from the white blood cells isolated from 10 ml of the patient’s blood. With appropriate primers,6 we amplified exons 4 and 12 of TGFBI by the polymerase chain reaction (PCR) and directly sequenced the products. We detected a homozygous point mutation, CTG→CGG (L527R), in codon 527 of TGFBI in the proband (fig 2). Point mutations were not detected in codons 124, 518, 544, 546, or 555, mutations in which have been associated with corneal dystrophies.
Fujiki et al2 reported that LCDIIIA in L527R heterozygotes is characterised by a late onset and mild clinical findings. Hirano et al7 reported that the condition caused by L527R heterozygosity was associated with amyloid deposition in the deep corneal stroma but not with corneal erosion. In contrast, LCD in our patient with a homozygous L527R mutation was characterised by onset in middle age, recurrent corneal erosion, and amyloid deposition in the middle to deep region of the stroma. The deposits in the proband showed right eye-left eye asymmetry in size and shape, as previously described for some L527R heterozygotes.2,7
In general, corneal dystrophies caused by homozygous point mutations in TGFBI are characterised by an earlier onset, more severe symptoms, and a higher frequency of recurrence after keratoplasty compared with those attributable to the corresponding heterozygous mutations.8–12 Our study and the two previous studies2,7 of this mutation suggest that this is also the case for L527R. However, the difference in the findings of slit lamp examination between L527R heterozygotes and homozygotes with LCD appears to be less marked than that observed between R124H heterozygotes and homozygotes with Avellino corneal dystrophy. The reason for this discrepancy remains unclear at present.
This research was supported by grants from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science, and Technology of Japan (to MI and TN).