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Published in final edited form as:
Br J Ophthalmol. 2010 August ; 94(8): 1094–1099. doi:10.1136/bjo.2009.171892.
Nonsense mutation in MERTK causes autosomal recessive
retinitis pigmentosa in a consanguineous Pakistani family
Amber Shahzadi1, S Amer Riazuddin1, Shahbaz Ali1, David Li2, Shaheen N Khan1, Tayyab
Husnain1, Javed Akram3, Paul A Sieving2, J Fielding Hejtmancik2, and Sheikh Riazuddin1,3
1National Centre of Excellence in Molecular Biology, University of the Punjab, Lahore, Pakistan
2Ophthalmic
Genetics and Visual Function Branch, National Eye Institute, National Institutes of
Health, Bethesda, Maryland, USA
3Allama
Iqbal Medical College, University of Health Sciences, Lahore, Pakistan
Abstract
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Background—Retinitis pigmentosa (RP) is one of the most common ophthalmic disorders
affecting one in approximately 5000 people worldwide. A nuclear family was recruited from the
Punjab province of Pakistan to study the genetic basis of autosomal recessive RP.
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Conclusion—Our results strongly suggest that the nonsense mutation in MERTK, leading to
premature termination of the protein, is responsible for RP phenotype in the affected individuals of
the Pakistani family.
Methods—All affected individuals underwent a thorough ophthalmic examination and the
disease was characterised based upon results for fundus photographs and electroretinogram
recordings. Genomic DNA was extracted from peripheral leucocytes. Exclusion studies were
performed with short tandem repeat (STR) markers flanking reported autosomal recessive RP loci.
Haplotypes were constructed and results were statistically evaluated.
Results—The results of exclusion analyses suggested that family PKRP173 was linked to
chromosome 2q harbouring mer tyrosine kinase protooncogene (MERTK), a gene previously
associated with autosomal recessive RP. Additional STR markers refined the critical interval and
placed it in a 13.4 cM (17 Mb) region flanked by D2S293 proximally and D2S347 distally.
Significant logarithm of odds (LOD) scores of 3.2, 3.25 and 3.18 at θ=0 were obtained with
markers D2S1896, D2S2269 and D2S160. Sequencing of the coding exons of MERTK identified
a mutation, c.718G→T in exon 4, which results in a premature termination of p.E240X that
segregates with the disease phenotype in the family.
INTRODUCTION
Retinitis pigmentosa (RP) is a group of retinopathies involving progressive photoreceptor
degeneration.1 Patients experience night blindness, constriction of peripheral visual field in
Correspondence to Dr Sheikh Riazuddin, National Centre of Excellence in Molecular Biology, 87 West Canal Bank Road, Lahore
53700, Pakistan; riaz@aimrc.org.
AS, SAR, JFH and SR contributed equally to this study.
Competing interests None.
Patient consent Obtained.
Ethics approval Institutional Review Board approval was obtained for this study from the Centre of Excellence in Molecular Biology
(CEMB) (Lahore, Pakistan) and the National Eye Institute (Bethesda, Maryland, USA).
Provenance and peer review Not commissioned; externally peer reviewed.
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the early stages and complete vision loss at later stage due to melanin-containing bone
spicule formation on the fundus. Vascular attenuation and waxy pallor of the optic disc are
other characteristic features of RP.2 Epidemiological studies estimate that 50–60% cases of
non-syndromic familial RP are inherited in an autosomal recessive fashion.3 To date 29
genes have been identified with mutations known to cause non syndromic autosomal
recessive RP (http://www.sph.uth.tmc.edu/Retnet). Most of these genes are expressed in
photoreceptors or retinal pigment epithelium and thus are directly involved in phototransduction cascade or in photoreceptor and retinoid metabolism.4
Mer tyrosine kinase protooncogene (MERTK) belongs to receptor tyrosine kinase family of
cell surface receptors. The gene consist of 19 exons encoding a 999 amino acid protein
expressed in retinal pigment epithelium (RPE).5MERTK was identified due to the
widespread retinal degeneration resembling RP in a large family harbouring a partial
deletion of the gene.6 After its first association with arRP, three mutations, IVS10-2A→G,
R651X and 2070delAGGAC, were identified in patients affected with arRP.7 Subsequently
three additional novel sequence variations, R722X, R844C and R865W, causing severe
retinal dystrophy in a single patient, were reported.5 Recently, a splice donor site mutation
in intron 16 has been implicated in rod–cone dystrophy and arRP.89 Altogether seven
mutations in MERTK have been associated with arRP.
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Here, we report a consanguineous Pakistani family diagnosed with arRP. Exclusion analyses
localised the critical interval to chromosome 2q and bi-directional sequencing identified a
nonsense mutation in MERTK that segregates with disease phenotype in the family. These
results strongly suggest that a nonsense mutation in MERTK is responsible for the disease
phenotype in this family.
MATERIALS AND METHODS
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Seventy-five consanguineous Pakistani families with non-syndromic RP were recruited to
participate in a collaborative study for identification of retinal disease causing loci between
the National Centre of Excellence in Molecular Biology (NCEMB), Lahore, Pakistan and
the National Eye Institute (NEI) Bethesda MD, USA. The family described in this study is
from the Punjab province of Pakistan. A detailed medical history was obtained by
interviewing family members. All of the ophthalmological examinations were completed at
either the Layton Rahmatulla Benevolent Trust (LRBT) hospital or at NCEMB, Lahore,
Pakistan. Fundus photographs were acquired by a camera manufactured by Fuji (Tokyo,
Japan). Electroretinogram (ERG) responses were recorded using ERG equipment
manufactured by LKC (Gaithersburg, Maryland, USA). Scotopic responses were recorded
under dark-adapted conditions using a single bright flash stimulus; photopic responses were
recorded under light-adapted conditions using a 30 Hz flicker stimulus. Blood samples were
collected from affected and unaffected family members. DNA was extracted by a nonorganic method as described previously.10
Genotype analysis
Multiplex PCR were completed in GeneAmp 9700 PCR System (Applied Biosystems,
Foster City, California, USA). Briefly, each reaction was carried out in a 5 μl mixture
containing 40 ng genomic DNA, various combinations of 10 μM fluorescently labelled
primer pairs, 0.5 μl 10× PCR Buffer (Applied Biosystems), 1 mM dNTP mix, 2.5 mM
MgCl2 and 0.2 U of Taq DNA polymerase (Applied Biosystems). Initial denaturation was
carried out for 5 min at 95°C, followed by 10 cycles of 15 s at 94°C, 15 s at 55°C and 30 s at
72°C and then 20 cycles of 15 s at 89°C, 15 s at 55°C and 30 s at 72°C. The final extension
was performed for 10 min at 72°C and followed by a final hold at 4°C. PCR products from
each DNA sample were pooled and mixed with a loading cocktail containing HD-400 size
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standards (Applied Biosystems). The resulting PCR products were separated in an ABI3100
DNA sequencer and analysed by using GENESCAN 4.0 software package (Applied
Biosystems).
Linkage analysis
Two point linkage analyses were performed using the FASTLINK version of MLINK from
the LINKAGE Program Package.1112 Maximum logarithm of odds (LOD) scores were
calculated. using ILINK. arRP was analysed as a fully penetrant trait with an affected allele
frequency of 0.001. The marker order and distances between the markers were obtained
from the Marshfield database (http://research.marshfieldclinic.org/) and the National Center
for Biotechnology Information chromosome 2 sequence maps
(http://www.ncbi.nlm.nih.gov). For the initial genome scan equal allele frequencies were
assumed, while for fine mapping allele frequencies were estimated from 96 unrelated and
unaffected individuals from the Punjab province of Pakistan.
Mutation screening
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Primer pairs for individual exons were designed using the primer3 program
(http://primer3.sourceforge.net/). The primer sequences and annealing temperatures are
available upon request. Amplifications were performed in 25 μl reaction containing 50 ng of
genomic DNA, 400 nM, of each primer, 250 μM dNTPs, 2.5 mM MgCl2 and 0.2 U Taq
DNA polymerase in the standard PCR buffer provided by the manufacturer (Applied
Biosystems). PCR amplification consisted of a denaturation step at 96°C for 5 min, followed
by 40 cycles, each consisting of 96°C for 45 s followed by 57°C (or primer set specific
annealing temperature) for 45 s and at 72°C for 1 min. PCR products were analysed on 2%
agarose gel, precipitated and purified by ethanol precipitation. The PCR primers for each
exon were used for bidirectional sequencing using Big Dye Terminator Ready reaction mix
according to manufacturer instructions (Applied Biosystems). Sequencing products were resuspended in 10 μl of formamide (Applied Biosystems) and denatured at 95°C for 5 min.
Sequencing was performed on an ABI PRISM 3100 Automated sequencer (Applied
Biosystems). Sequencing results were assembled with ABI PRISM sequencing analysis
software version 3.7 and analysed using SeqScape software (Applied Biosystems).
RESULTS
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PKRP173 was recruited from the Punjab province of Pakistan. A detailed medical and
family history was obtained by interviewing family members. Three affected and one
unaffected individual underwent detailed ophthalmic examination. According to medical
records available to us, all affected individuals started experiencing visual difficulties during
the first decade of their life; at the time of examination vision was reduced only to
perception of light (table 1). Fundus examination revealed marked vascular attenuation, and
bony spicules in peripheral regions of the retina combined with atrophic maculopathy with
pigment dispersion (figure 1). ERG recordings at 0 dB (scotopic) showed no rod and cone
response. Isolated cone responses measured at 30 Hz flicker (photopic) were absent (figure
2), illustrating an advanced stage of retinopathy encompassing both rod and cone cells of the
retina. Retinal attributes and ERG recording in the unaffected individual 12 were completely
normal (figures 1 and 2)
Exclusion studies were performed with closely linked short tandem repeat (STR) markers
for 23 reported arRP loci (data not shown). Haplotypes were constructed to investigate
homozygous regions shared by all the affected individuals of the family. PKRP173 was
found linked to markers D2S1896, D2S2269 and D2S160 at chromosome 2q14.1 flanking
MERTK (figure 3). All the affected individuals were homozygous for D2S1896, D2S2269
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and D2S160. Additional STR markers D2S2264, D2S293 and D2S347 were designed to
define the proximal and distal boundaries. As shown in figure 3, there is proximal
recombination in affected individual 18 at D2S293. Similarly, there is a distal recombination
in affected individuals 10, 11 and 18 at D2S347. Taken together, this places the disease
interval in the 13.4 cM (17 Mb) region flanked by markers D2S293 proximally and D2S347
distally. These results were statistically evaluated by investigating the probabilities of
linkage to markers on chromosome 2q; two point LOD scores of 3.2, 3.25 and 3.18 at θ=0
were obtained with markers D2S1896, D2S2269 and D2S160 (table 2).
The critical interval harbours MERTK, a gene previously associated with arRP. All coding
exons and exon–intron boundaries were sequenced to identify any and all sequence
variations present in the affected individuals. We identified six sequence variations
including two missense variations (Arg466Lys and Ile587Val), a non-synonymous variation
(Ser627Ser), two non-coding intronic variations (IVS6-46G→A, IVS15-11C→A), and most
importantly a homozygous c.718G→T substitution in exon 4. All affecteds were
homozygous, whereas unaffected individuals were heterozygous carriers for this change
(figure 4a). This mutation introduces a premature termination at amino acid 240 and was not
present in 96 ethnically matched control samples.
DISCUSSION
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Here, we report association of arRP phenotype in a consanguineous Pakistani family with
MERTK, a gene previously associated with arRP. Ophthalmological examination of affected
individuals revealed classical features of RP, including arteriolar attenuation, pigmentation
on peripheral and mid peripheral regions of retina and optic disc pallor. Exclusion analyses
indentified chromosome 2q as the critical interval harbouring MERTK and subsequent
sequential analyses identified a nonsense mutation segregating with disease phenotype in
PKRP173. Linkage to chromosome 2q14.1,presence of a nonsense mutation in gene
previously associated with arRP, segregation of the mutation with the disease phenotype in
the family, along with absence of the variation in ethnically matched controls strongly
suggest that this mutation is responsible for RP phenotype in PKRP173.
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To date seven mutations in MERTK causing arRP have been described for Caucasian, mideastern and Spanish families.578 Of particular interest is a single base deletion, c.2214delT,
causing frame shift and leading to a premature after-addition of 31 novel amino acids.13 The
clinical evaluation includes bull's eye maculopathy and absence of pigmentation for
individuals homozygous for the deletion mutation.13 In our study the pathogenic mutation
would be expected to be degraded by a nonsense-mediated decay (NMD) mechanism,
leading to complete ablation of the mutant mRNA. If even somehow the transcript evades
NMD, the protein thus synthesised would lack functionally important domains, namely
fibronectin and tyrosine kinase domains (figure 4b). The more severe phenotype in affected
individuals of PKRP173 comprising of extensive pigmentation in peripheral and midperipheral region of retina coupled with macular atrophy might be due to the relatively older
age of examination.
In conclusion, a nonsense mutation in MERTK has been identified as a molecular defect
responsible for RP phenotype in a consanguineous Pakistani family. To the best of our
knowledge this is the first report of mutation in immunoglobulin domain of MERTK.This
and other pathogenic mutations thus identified will help us better understand the
pathphysiology of RP at a molecular level. Further, in addition to genetic counselling and
early diagnosis of affected individuals, improved understanding of the mutations that lead to
arRP will help in development of better therapeutic procedures.
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Acknowledgments
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The authors are grateful to all members for their participation in this study. We are also thankful to members of the
staff of Layton Rehmatullah Benevolent Trust (LRBT) hospital for clinical evaluation of affected individuals.
Funding This study was supported, in part by Higher Education Commission and Ministry of Science and
Technology Islamabad, Pakistan.
REFERENCES
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cone dystrophy: loss of mutant protein functions in transfected cells. Invest Ophthalmol Vis Sci.
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7. Gal A, Li Y, Thompson DA, et al. Mutations in MERTK, the human orthologue of the RCS rat
retinal dystrophy gene, cause retinitis pigmentosa. Nat Genet. 2000; 26:270–1. [PubMed:
11062461]
8. Ebermann I, Walger M, Scholl HP, et al. Truncating mutation of the DFNB59 gene causes cochlear
hearing impairment and central vestibular dysfunction. Hum Mutat. 2007; 28:571–7. [PubMed:
17301963]
9. Brea-Fernandez AJ, Pomares E, Brion MJ, et al. Novel splice donor site mutation in MERTK gene
associated with retinitis pigmentosa. Br J Ophthalmol. 2008; 92:1419–23. [PubMed: 18815424]
10. Kaul H, Riazuddin SA, Yasmeen A, et al. A new locus for autosomal recessive congenital cataract
identified in a Pakistani family. Mol Vis. 2010; 16:240–45. [PubMed: 20161816]
11. Lathrop GM, Lalouel JM. Easy calculations of LOD scores and genetic risks on small computers
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12. Schaffer AA, Gupta SK, Shriram K, et al. Avoiding recomputation in genetic linkage analysis
1669. Hum Hered. 1994; 44:225–37. [PubMed: 8056435]
13. Tschernutter M, Jenkins SA, Waseem NH, et al. Clinical characterisation of a family with retinal
dystrophy caused by mutation in the Mertk gene. Br J Ophthalmol. 2006; 90:718–23. [PubMed:
16714263]
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Figure 1.
Fundus photographs of members of family PKRP173. Fundus demonstrating several
features associated with retinitis pigmentosa (RP) including a waxy pallor of the optic disc,
attenuated arterioles, atrophy of retinal pigment epithelium (RPE) and peripheral bone
spicules. Notably, macular atrophy with pigment clumping in peripheral regions is present.
(A) Affected individual 18; (B) affected individual 9; (C) affected individual 10; (D)
unaffected individual 12. OD, right eye; OS, left eye.
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Figure 2.
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Electroretinogram responses of members of family PKRP173. ERG recordings show no rod
and cone response for affected individuals. Isolated cone responses measured at 30 Hz
flicker are absent too, illustrating an advanced stage of retinopathy encompassing both rod
and cone cells of the retina. Individual 18: (A) Right eye (OD) combined rod and cone
response; (B) OD cone response; (C) Left eye (OS) combined rod and cone response; (D)
OS cone response. Individual 9: (E) OD combined rod and cone response; (F) OD cone
response; (G) OS combined rod and cone response; (H) OS cone response. Individual 10: (I)
OD combined rod and cone response; (J) OD cone response; (K) OS combined rod and cone
response; (L) OS cone response. Individual 12: (M) OD combined rod and cone response;
(N) OD cone response; (O) OS combined rod and cone response; (P) OS cone response.
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Figure 3.
Pedigree drawing and haplotype of chromosome 2q markers of family PKRP173. Squares
are males, circles are females, filled symbols are affected individuals, double line between
individuals indicates consanguinity and diagonal line through a symbol is deceased family
member. The haplotypes of six adjacent chromosome 2q microsatellite markers are shown
with alleles forming the risk haplotype are shaded black, and alleles not co-segregating with
retinitis pigmentosa (RP) are shown in white.
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Figure 4.
(A) Sequence chromatograms of MERTK in family PKRP173: unaffected individual 07 (top
row) heterozygous and affected individual 09 (bottom row) homozygous for c.718 G→T
transversion that leads to a premature termination (p.E240X). (B) A schematic
representation of MERTK domains. Structure domains were predicted with simple modular
architecture tools (SMART) algorithms (http://smart.embl-heidelberg.de/). FNB, fibronectin
type 3 domain; IG, immunoglobulin domain; TM, transmembrane domain; Tyrkc, Tyrosine
kinase catalytic domain.
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48
45
43
45
9
10
11
18
10
9
10
9
Age at first diagnosis of RP (years)
PL, perception of light.
Age (years)
ID
PL
PL
PL
PL
Visual acuity
Night blindness
Night blindness
Night blindness
Night blindness
First symptom
Progressive
Progressive
Progressive
Progressive
Disease progression
Macular atrophy, pigment deposition, vascular attenuation
Macular atrophy, pigment deposition, vascular attenuation
Macular atrophy, pigment deposition, vascular attenuation
Macular atrophy, pigment deposition, vascular attenuation
Fundus findings
Clinical characteristics of affected individuals of PKRP173 diagnosed with autosomal recessive retinitis pigmentosa (RP)
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Table 1
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122.9
D2S2269
124.2
112.9
112.9
112.6
107.2
102.4
Mb
–0.06
–∞
3.10
–0.01
–∞
3.18
3.18
3.25
3.13
–0.01
–∞
3.20
0.01
0
0.54
2.84
2.91
2.87
0.47
0.54
0.05
0.64
2.59
2.67
2.61
0.61
0.64
0.09
0.65
2.49
2.62
2.54
0.60
0.65
0.1
0.58
1.85
1.96
1.86
0.53
0.58
0.2
0.4
1.16
1.26
1.18
0.3
0.4
0.3
0.65
3.18
3.25
3.20
0.60
0.65
0.10
0.00
0.00
0.00
0.09
0.10
θ max
LOD scores were calculated at different θ values for each marker with the FASTLINK version of MLINK from the LINKAGE program package Maximum LOD scores for each marker were calculated
using ILINK.
LOD, logarithm of the odds.
131.5
122.9
D2S1896
122.9
118.1
D2S293
D2S347
114.4
D2S2264
D2S160
cM
Marker
Zmax
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Two point LOD scores of chromosome 2q markers for PKRP173
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Table 2
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