Journal of Postgraduate Medicine Medknow Publications and Staff Society of Seth GS Medical College and KEM Hospital, Mumbai, India ISSN: 0022-3859 EISSN: 0972-2823 Vol. 56, Num. 4, 2010, pp. 290-292

Journal of Postgraduate Medicine, Vol. 56, No. 4, October-December, 2010, pp. 290-292

Case Report

Identification of a novel mutation in an Indian patient with CAII deficiency syndrome

C Shivaprasad¹, P Paliwal², R Khadgawat¹, A Sharma²

¹ Department of Endocrinology, All India Institute of Medical Sciences, New Delhi, India
² Laboratory of Cyto-Molecular Genetics, Department of Anatomy, All India Institute of Medical Sciences, New Delhi, India

Correspondence Address: A Sharma, Laboratory of Cyto-Molecular Genetics, Department of Anatomy, All India Institute of Medical Sciences, New Delhi, India, arundhati_sharma@yahoo.com

Date of Submission: 28-Mar-2010
Date of Decision: 11-May-2010
Date of Acceptance: 01-Jul-2010

Code Number: jp10085

PMID: 20935402

DOI: 10.4103/0022-3859.70944

Abstract

Keywords: CA2 gene, molecular analysis, osteopetrosis

Introduction

Carbonic anhydrase II (CAII) deficiency syndrome (MIM 259730) is a rare heterogeneous autosomal recessive (AR) disorder leading to the development of osteopetrosis (OP), renal tubular acidosis (RTA), and cerebral calcifications. ^[1],[2] Other features include growth failure, mental retardation, pathological fractures, dental malocclusion/malalignment, and cranial nerve compression. Diagnosis is usually based on clinical and radiographic evaluation. A study has shown that all patients with CAII deficiency have mutations in the carbonic anhydrase 2 (CA2) gene. ^[3] We present, for the first time, a rare case of the AR CAII deficiency syndrome associated with mental retardation. Molecular analysis revealed a novel homozygous deletion mutation in the CA2 gene resulting in a protein with complete loss of enzymatic activity. Our study provides clinical features of the CAII deficiency syndrome confirmed at the molecular level and reiterates the need to alert clinicians to this diagnosis as clinical features of this syndrome are similar to the other forms of OP in which treatment options are different.

Case Report

A 24-year-old male presented to our clinic for the evaluation of an increased bone density. He was born to normal parents with nonconsanguineous marriage and an uneventful pregnancy. Genetic counseling was provided and absence of any relatedness between the parents was reconfirmed. There was history of delayed attainment of motor and intellectual milestones, poor height gain and scholastic performance, and growth retardation with history of recurrent fractures on minor trauma. Detailed review of the family history was normal. Physical examination [Table - 1] revealed slightly dysmorphic features and overcrowded and malaligned teeth. Musculoskeletal examination, ocular evaluation, and audiogram for speech abnormality were normal. There was no hepatosplenomegaly. Skeletal survey showed hyperdensity of bones and other radiographic features characteristic of OP with calcification seen on the lateral view of the skull. Computerized scan of the head showed calcification of basal ganglia and periventricular regions and a marked increase was seen in the bone mineral density (DXA, Hologic QDR 4500A) [Figure - 1]. Results of routine investigations were within normal range [Table - 1] except for arterial blood gas analysis showing metabolic acidosis with simultaneous urine pH of 6.5. Peripheral blood samples (5.0 ml) were collected in EDTA from the patient and his family members after taking informed consent. DNA was subjected to PCR for all seven CA2 exons as described. ^[4] Amplified products were directly sequenced (Applied Biosystems [ABI], Foster City, CA, USA) and were analyzed on an ABI-3100 Genetic Analyzer. Nucleotide sequences were compared with those of the CA2 gene (ref. ID ENST00000285379). Sequence analysis in the patient revealed a homozygous a deletion, c.251delT in exon 3 [Figure - 2]a resulting in a frameshift and generating a novel amino acid sequence from position 84 to 89 leading to a premature stop codon at position 90 [Figure - 2]b. The sibling and parents were carriers of the same mutation in the heterozygous state [Figure - 2]a.

Discussion

The CAII deficiency syndrome presents with a triad of increased bone density, RTA, and cerebral calcifications. Short stature, developmental delay, recurrent fractures, and dental malocclusion/malalignment are the other prominent features which were also seen in our patient.

Carbonic anhydrase is a zinc metalloenzyme and catalyzes the reversible hydration of carbondioxide (CO ₂ ) to bicarbonate. CAII, one of its 14 isoenzymes, has a wide distribution and is found in the bone, kidney, brain, and erythrocytes with a high concentration in osteoclasts ^[5] and plays a role in osteoclast resorptive activity by acidification. The acid secretion process is initiated by carbonic anhydrase II, which favors CO ₂ hydration, and converts it to bicarbonate. The deficiency of CAII disrupts the balance between osteoclast and osteoblast activity in favor of bone deposition ^[6] causing excessive, but brittle, bone formation leading to an increased fracture risk due to the failure of osteoclast development or function. ^[2] CAII is also involved in proximal renal tubular bicarbonate reclamation and distal renal tubular acidification and many patients with CAII deficiency show characteristics of both proximal and distal RTA. ^[7]

CA2 gene mutations have been reported in families - the common "Arabic mutation" leads to loss of the donor splice junction of the intron 2. ^[3] There is only one CAII deficiency report of an Indian patient ^[8] where a missense mutation in exon 2 was found.

Our index case showed a novel c.251delT mutation leading to frameshift and a truncated protein of 90 instead of a normal 260 amino acid protein. Due to this, the active site of carbonic anhydrase containing a zinc ion coordinated by the imidazole rings of three histidine residues at positions His94, His96, and His119 is completely lost resulting in an inactive CAII enzyme. Mental retardation seen in our patient correlated with a complete loss of the enzyme activity also substantiated by a study showing the effect of CAII deficiency on developing human brain. ^[9]

CAII deficiency syndrome is treated symptomatically with therapy directed toward the correction of metabolic acidosis once the diagnosis is suggested. Acidosis may not always be invariably present in patients with CAII deficiency syndrome. Clinically overt cases may have baseline acidosis which may not be evident. Studies on CAII deficiency syndrome kindreds have documented the presence of phenotypic variability with respect to acidosis in the affected family members. Therefore, it is better to confirm the clinical diagnosis by molecular methods for early and precise diagnosis, counseling, and treatment management purposes.

RTA is the most likely cause for growth retardation, and alkali supplementation before the age of 4 has shown to result in normal height. Hematopoietic stem cell transplantation employed in most severe forms with bone marrow failure offers the best chance for long-term survival. ^[2]

Confirmation of the diagnosis at an early age by molecular techniques leads to an early institution of therapy improving the growth potential and reducing the incidence of other complications like fractures. Our study provides diagnosis of the CAII deficiency syndrome based not only on clinical features but also confirmed at the molecular level. Molecular testing does not provide early diagnosis but only confirms the clinical impression. The confirmation of the clinical features by molecular methods is important as these features are similar to the other forms of OP in which treatment modalities are different.

To conclude, this is the first report of molecular analysis of the CAII deficiency syndrome associated with mental retardation in India. The novel mutation reported here expands the knowledge of the nature of mutations known to cause this syndrome. For a populous country like India, with a high percentage of patients who cannot afford the expensive treatment options, the best alternative is to provide gene testing for carrier detection in families with such disorders based on which genetic counseling regarding recurrence risks and prenatal diagnosis can be offered to parents planning a pregnancy.

Acknowledgments

The authors acknowledge the help extended by Dr. Alok Kumar Mittal from the Department of Radiodiagnosis, AIIMS, New Delhi.

References

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