Thymidine phosphorylase (TYMP) gene stop mutation, G38X, in a familial case of mitochondrial neurogastrointestinal encephalomyopathy

R. Bhuvaneshwari*

Department of Neurology, Kauvery Hospital, Chennai, Tamilnadu, India

*Correspondence: Tel: +91 95001 67591 Email: bhuvana1675@hotmail.com

Abstract

Background: Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is a rare mitochondrial disease with a spectrum of neurological and gastrointestinal involvement and characterized Mutations in the thymidine phosphorylase (TYMP) gene.

Case presentation: We present this young man with mobility issues and malabsorption who then turned out be positive for MNGIE.

Conclusion: This young man was tested positive for mutation of TYMP gene and the father of the individual was a clear heteroxygote indicating the recessive nature of the disease.

Keywords: Mitochondrial neurogastrointestinal encephalomyopathy, Thymidine phosphorylase, Stop mutation

Background

Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is a chronic and progressive disease that is characterized by ptosis/external ophthalmoplegia, gastrointestinal dysmotility, cachexia, peripheral neuropathy, leukoencephalopathy. The disease was initially described by Okamura et al. [1]. Hirano et al. [2] first established linkage of MNGIE to chromosome 22q13.32. Later, the gene TYMP was observed to be mutated in MNGIE patients [3]. A number of studies have subsequently characterized the clinical features, and identified associated mitochondrial mutations [4–7].

Mutations in the thymidine phosphorylase (TYMP) result in a loss of function of the TYMP gene and eventually cause the disease. The disease was initially associated with deletions in mtDNA. However, the autosomal genomic transmission via chromosome 22 attributed the disease to defects in inter-genomic communication between the mitochondrial and nuclear genome [3,8].

Over 60 mutations in the TYMP gene have been reported thus far [9,10]. Among them, the stop mutation G38X was first reported in a female patient from Iraq [11]. Here, we genetically screened the TYMP gene in a patient (male) from Iraq referred to Global hospital, and identified the same stop mutation. The patient had all clinical features of MNGIE. Besides screening the patient for TYMP mutations, we have collected the family history/pedigree chart. We also investigated the TYMP gene of the father and identified the father as a heterozygous carrier. Genetic analyses confirmed the clinical diagnosis and reiterated the familial nature of the disease. To our knowledge, this is the first report of the stop mutation in a family along with part of the family genetic data.

Case Presentation

A 26-year-old driver from Iraq was referred to our centre with mobility issues and history of profound weight loss for over two years. The problem started with left leg weakness and low back pain for which he underwent spinal surgery at L3 level in 2007. Although he remained stable for sometime, he soon noticed worsening of lower limb weakness. Gradually his right leg was also affected.

Simultaneously, he complained of chronic diarrhea and constant dull pain abdomen and loss of appetite. He had diarrhea 4–5 times in a weak and reported severe weight loss. He lost almost over 15 kg of body weight in the last three years. He weighed 35 kg only. There was no history of bulbar involvement such as breathing or swallowing problems. His upper limbs were not affected.

There was a strong family history with two of his elder brothers and his paternal uncle’s two daughters dying young after having similar muscle wasting disease.

On examination he was cachectic with bilateral mild ptosis but no ophthalmoparesis. Rest of the cranial nerve examination was normal. There was global muscle wasting. Motor examination revealed bilateral severe weakness of the lower limbs most prominent distally. Ankle dorsiflexion was not possible bilaterally (MRC grade 0/5) and plantar flexion was very weak (MRC grade 3/5). In addition, there was mild generalized muscle weakness (MRC grade 4/5).

He was completely being flex with down going plantars. Sensory examination revealed reduced perception of light touch and pin-prick most prominent in the lower limbs up to his knees. Dorsal column signs such as loss of vibration and position sensory perception were elicited up to ankles in the lower limbs but none in the upper limbs. He did not exhibit any cerebellar signs.

Systemic examination was normal including cardiac and respiratory systems. His routine blood tests revealed hypoalbuminemia and slightly raised serum LDH. Rest of the full blood count, ESR were normal. Liver, renal, thyroid function tests were also normal. Serum ferritin, B12, folic acid levels and electrolytes were within normal limits. ANA, ANCA, HIV serology were negative.

Gastrointestinal investigation included stool examination, colonoscopy which were normal but endoscopy showed evidence for esophagitis with candidal infection and generalised reduced luminal pliability and biopsy showed evidence for chronic duodenitis with eosinophilia. Echocardiogram was normal.

MRI brain showed evidence for bilateral T2/flair bright signals in peri-ventricular, subcortical white matter, bilateral cerebellar white matter, thalami, ganglio capsular region and brain stem suggestive of metabolic encephalopathy. MRI spine showed evidence for previous surgery at L3 along with chronic degenerative process of the lumbar spine but rest of the spine was normal (Fig. 1a and b).

thymidine
thymidine2

Fig:1a and b. MRI brain showing evidence for mriephalopathy.

CSF analysis showed evidence for mild increase in protein only. Nerve conduction studies showed evidence for demyelinating neuropathy with un-recordable sensory responses and electromyography showed evidence mild myopathic process (Fig. 2a and b).

thymidine3
thymidine4

Fig. 2a. Nerve conduction studies and showing evidence for demyelinating neuropathy, b. Electromyography showing evidence for myopathic features.

Patient thus had global muscle wasting, diarrhea, loss of weight and distal weakness in the lower limbs. Investigations revealed evidence for leuco-encephalopathy, demyelinating neuropathy and mild myopathic process in the limbs. This spectrum of leuco-encephalopathy, peripheral neuropathy, myopathy and GI symptoms along with strong family history was consistent with MNGIE disease.

Genetic analysis

MNGIE is an autosomal recessive disorder. The TYMP gene has been reported to be mutated in MNGIE patients and could be used as a means to confirm diagnosis. After obtaining informed consent from the patient and his father for the genetic study they were screened for TYMP mutations to confirm the clinical diagnosis. DNA was extracted from peripheral blood samples of the patient and father using the QIAamp Blood DNA kit (Qiagen, Hilden Germany). The TYMP gene, exons 2–10, was amplified using published primers. Primer sequences were obtained from. Nine sets of primers were used to amplify the nine exons. The amplicons were purified and then sequenced by an automatic genetic analyzer ABI 3130xl using BigDye terminator cycle sequencing ready kit (Applied Biosystems). Sequences were analyzed using Chromas software (ver 2.33, Technelysium, South Brisbane, Australia) and aligned with existing sequences using BLAST (http://blast.ncbi.nlm.nih.gov/Blast.cgi) to identify variations. The patient had a homozygous TYMP gene mutation (c.112G>T) which would convert the glutamate residue to stop codon at the 38th amino acid position [p.38E>X]). As a result of the stop mutation, the protein was truncated prematurely and was thereby rendered nonfunctional resulting in the disease phenotype. The same mutation was found to be in the heterozygous form in the father indicating a familial origin of the disease.

Discussion

MNGIE is an autosomal recessive disorder that involves multiple systems. Most common and prominent features include, gastrointestinal dysmotility, abdominal distension, diarrhea and continued weight loss, peripheral neuropathy, diffuse leukoencephalopathy, ptosis and ophthalmoparesis, and severe cachexia [8]. MNGIE was diagnosed via high thymidine and deoxyuridine in plasma and urine, TP enzyme activity and mutations in TYMP [5,10]. Variability in phenotypic expression of the disease including within a single family has been reported [5,9].

Reduced TP activity in MNGIE was an indicator that the TYMP mutations may be loss of function mutations. The enzyme was also known as gliostatin or endothelial growth factor-1 (platelet derived; ECGF1) [11]. Surprisingly, TP was not expressed or was present at low levels in skeletal muscles which were affected in MNGIE. Absence or diminished activity of TP results in increase in its substrates, dThd and dUrd. The toxicity of the increased substrates could be potential means by which the skeletal muscles were affected [11].

Mutations in TYMP was a common cause of MNGIE and also serves as a diagnostic tool. Our patient reported here exhibited classic symptoms of MNGIE and also had the p.38E>X mutation that has been reported previously [11]. The mutation was found to be familial with the father of the individual harboring the mutation in a heterozygous manner. The patient had a strong family history of the disease. Based on the interview with the patient and considering the demographic location it was possible that the patient reported by Baris et al. [11] may be related to our patient reported here. It was also indicated by the patients that his cousin sisters (paternal side) were affected at an early age. The patient reported by Baris et al. [11] had an early age of disease onset.

Conclusion

Although some paper exist that have studied familial MNGIE cases, this is the first report of the familial nature of the p.38G>X mutation. The father of the individual was a clear heterozygote indicating the recessive nature of the disease. It was possible that the mother may have been a heterozygote as well. However, since we were unable to screen the mother for TYMP mutations, we cannot make conclusive statement based on the information from one parent alone. The data suggests that in such clearly defined familial cases with TYMP mutations the mutations screening may serve as an option to diagnose the disease at an early stage before the onset of the symptoms. Early diagnosis may help manage symptoms and save some lives.

The lack of clear therapeutic regimens for this disorder makes it imperative to identify the disorder as early as possible in order to intervene and manage some of the symptoms at least. To be able to do that it is essential to get a clear understanding of the genetics of the disease via screening of individuals and families for existing genes and novel genes. A family history of the mutation in the disease can be used to at the very least to screen other family members and rule out or confirm the disease at an early stage. Further such data along with demographics will add to the data base and could prove beneficial in the future for the better identification, understanding and intervention of the disease.

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