Spinal muscular atrophy (SMA) is the most common genetic cause of infant mortality [1] with an incidence of 1 in 6,000-10,000 live births [2]; and a carrier frequency of 1:50 [3]. It results from homozygous deletions of exon 7 and 8 involving the SMN1 gene located on chromosome 5q13 [4]. Homozygous SMN2 detection, although found in 5-9% of normal control, may be associated with disease phenotype in selected cases [2,4,5]. Most cases of SMA have autosomal recessive inheritance; however, autosomal dominant and X-linked inheritance has also been documented [6].

SMA phenotype varies, depending on the age of onset and motor development milestones [7]. Since in India, SMA remains highly under-diagnosed, present study aimed to analyse the clinical characterization of patients with gross hypotonia and impaired lower motor neuron function and their further molecular confirmation by SMN gene study.

This is a cross-sectional study carried out on patients referred between the period of January 2008 to August 2012 from Gujarat and its vicinity. The main clinical phenotype were marked hypotonia, diffuse proximal muscle weakness, tongue fasciculation along with absent or greatly decreased deep tendon reflexes and electromyographic (EMG) evidence of denervation on systemic examination. After approval from the ethical committee, 105 subjects were included in the study and their name, age, gender, other demographic findings, presenting signs and symptoms, family history were noted. Molecular analysis was carried out after taking informed written consent.

Gene analysis: Genomic DNA was isolated from blood samples using salting out method [8]. Deletion of exon 7 and 8 of SMN1 and SMN2 gene was carried out by polymerase chain reaction (PCR) amplification and restriction endonuclease digestion. PCR products of exon 7 were digested with Dra I, and exon 8 with Dde I and run on 2.5% agarose gel electrophoresis at 100v and observed under UV transilluminator.

Of 105 subjects presenting with suspicion of SMA, 65(62%) were consistent with the clinical diagnosis of SMA. Out of these, 34(52.3%) children were in the acute infantile group with onset within 6 months of age, 12 (18.5%) were in the range of 7-18 months falling in the category of chronic childhood, 18 (27.7%) subjects belonged to the chronic juvenile category while only 1 subject (1.5%) was more than 30 years of age at the time of presentation as shown in Table I. Males had a greater preponderance than females in our study in the ratio of 1.5:1. This skewed ratio in favour of males was most striking in SMA type I subjects.

TABLE I  Summary of Confirmed Cases of SMA

Molecular study demonstrated that, 83.1% of cases showed deletion of both the exons 7 and 8 of SMN1 gene while 6.2% and 4.6% cases showed deletion of only exon 7 and exon 8, respectively. Exon 7 and/or 8 deletion in SMN2 gene was observed in 4.6% of cases. Deletion of both exons of SMN1 and SMN2 gene was detected in one case. Out of 12 cases with SMA type II, none showed deletion in exon 7 and/or 8 of the SMN2 gene while the only case of adult onset SMA (type IV) had deletion in exon 7 of SMN2 gene (Fig. 1).

Fig. 1 Schematic representation of SMA region on chromosome 5 and localization of SMN1 (telomeric copy) and SMN 2 (centromeric copy) on 5q13.

In our study majority of patients presented within the first 6 months and were of SMA type I (53.3%) which is similar to the observations in other studies [6.9]. 　Males had a greater preponderance which is in concordance with earlier study [6] while a female preponderance was reported in an Egyptian cohort [9].

Different genes and microsatellite markers have been identified in the 5q region that can be deleted in SMA patients. Homozygous deletions of the SMN1 gene are detected in more than 90% of the SMA type I to III patients and only exceptionally in SMA type IV [10]. The number of SMN2 copies correlates with the SMA subtype, age of onset, and length of survival [11]. It has been observed that 95% of SMA type I patients have only 1-2 copies of SMN2, whereas almost all patients belonging to type III had 3 or more copies and a less severe disease course [12]. The most common deletion found in our study was of SMN1 gene with deletion of both exons 7 and 8 in majority of cases (83.1%). Similar observation was made in the cohort of Pakistani and Egyptian children [6,9].　　Deletions of either exon 7 or exon 8 of SMN1 gene and SMN2 gene were observed only in around 5% of cases. Other studies have reported exon 7 deletion of SMN1 gene in 18.2% of cases while none of the cases had deletion of only exon 8 [9] and homozygous deletion of SMN2 gene in a childhood onset SMA [13]. A study in Korean population demonstrated an association between sporadic motor neuron disease and SMN2 deletion in adults [14]. With increasing identification of the underlying genetic defects, clinical spectrum and presentation, the awareness of the disease has improved. However, because no cure is yet available, genetic counselling and prognostic considerations are of great importance. With the availability of genetic testing, it is now possible to diagnose these children early so that appropriate counselling can be offered to the family on the risk of future pregnancies. Early identification by genetic testing has also made early antenatal diagnosis possible.　Thus spreading awareness about this lethal but preventive disease becomes imperative.

Acknowledgments: The referring clinicians for their cooperation and support.

Funding: None; Competing interests: None Stated.　

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