The Invitae Galactosemia Panel analyzes the GALE, GALK1, and GALT genes, which encode enzymes responsible for galactose metabolism. This test is useful for the diagnosis of patients whose clinical symptoms, abnormal newborn screening results, or biochemical findings indicate galactosemia.
GALE GALK1 GALT
GALE GALK1 GALT
Galactosemia is a group of autosomal-recessive-inherited metabolic disorders that are caused by a deficiency in one of three enzymes involved in the catabolism of the simple sugar galactose. Patients with galactosemia experience toxic elevations of galactose and other galactose metabolites in the blood when they are exposed to milk and other lactose products. Many infants affected with galactosemia are detected through state newborn-screening programs.
Galactose-1-phosphate uridylyltransferase (GALT) deficiency is the most common enzyme deficiency in galactosemia, including classic galactosemia, clinical variant galactosemia, and Duarte variant galactosemia.
In classic galactosemia and clinical variant galactosemia, GALT enzymatic activity is absent or very low. Affected infants appear normal at birth, but once they ingest lactose, they quickly develop life-threatening symptoms, including feeding problems with vomiting and diarrhea, liver damage leading to jaundice, kidney disease, congenital bilateral cataracts, E. coli sepsis, seizures, bleeding diathesis, and failure to thrive. Even with adequate treatment from an early age, children with classic galactosemia remain at increased risk for developmental delays, speech problems, and abnormalities of motor function. Almost all females with classic galactosemia manifest premature ovarian insufficiency. Patients with variant galactosemia who are well-managed typically do not develop the long-term complications that are associated with classic galactosemia.
Duarte variant galactosemia is a common and milder form of GALT deficiency. Individuals with Duarte variant galactosemia is associated with compound heterozygosity for the Duarte mutation, N314D, and a classic mutation. Most patients are asymptomatic, although some have been reported with speech and language difficulties. Treatment for Duarte variant galactosemia remains controversial.
Patients with little or no residual GALT enzyme activity typically have the severe neonatal presentation while patients with higher residual enzyme activity have the less-severe forms. Early diagnosis and elimination of dietary galactose reduce mortality in patients with classic GALT deficiency, but even patients with optimal treatment can still suffer from ovarian insufficiency, mental retardation, speech dyspraxia, ataxia, and learning disabilities.
Galactoepimerase (GALE) deficiency is defined by a spectrum of enzyme deficiencies involving different tissues. Clinical presentations vary from asymptomatic to severe. Patients with severe GALE deficiency accumulate cytotoxic galactose 1-phosphate in the cells and may manifest symptoms resembling classic galactosemia. The severe form of GALE deficiency is extremely rare.
Patients with galactokinase (GALK) deficiency have biochemical findings of galactosemia (increased plasma galactose levels, urinary galactitol excretion), though galactose-1-phosphate levels are not elevated. Patients with GALK deficiency do not develop the life-threatening complications observed in classic galactosemia. Clinically, they are only at high risk of developing cataracts.
For patients with a clinical and biochemical diagnosis of galactosemia, the detection rate of pathogenic variants in one of these three genes is nearly 100%.
|Gene||% of galactosemia cases attributed|
GALT, GALE, and GALK deficiencies are all inherited in an autosomal recessive manner.
Regarding GALT deficiency galactosemia, the prevalence of classic galactosemia is 1 in 10,000–48,000, the prevalence of clinical variant galactosemia is unknown, and the prevalence of Duarte variant galactosemia is approximately 1 in 16,000.
The overall incidence of GALE deficiency is unknown. The generalized form is very rare. The peripheral form is more common in African Americans, compared to other ethnic groups.
The prevalence of GALK is not well-known. The birth incidence for GALK deficiency is approximately 1 in 150,000 or greater in most parts of Europe, USA, and Japan.
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Invitae is a College of American Pathologists (CAP)-accredited and Clinical Laboratory Improvement Amendments (CLIA)-certified clinical diagnostic laboratory performing full-gene sequencing and deletion/duplication analysis using next-generation sequencing technology (NGS).
Our sequence analysis covers clinically important regions of each gene, including coding exons and 10 to 20 base pairs of adjacent intronic sequence on either side of the coding exons in the transcript listed below. In addition, the analysis covers the select non-coding variants specifically defined in the table below. Any variants that fall outside these regions are not analyzed. Any limitations in the analysis of these genes will be listed on the report. Contact client services with any questions.
Based on validation study results, this assay achieves >99% analytical sensitivity and specificity for single nucleotide variants, insertions and deletions <15bp in length, and exon-level deletions and duplications. Invitae's methods also detect insertions and deletions larger than 15bp but smaller than a full exon but sensitivity for these may be marginally reduced. Invitae’s deletion/duplication analysis determines copy number at a single exon resolution at virtually all targeted exons. However, in rare situations, single-exon copy number events may not be analyzed due to inherent sequence properties or isolated reduction in data quality. Certain types of variants, such as structural rearrangements (e.g. inversions, gene conversion events, translocations, etc.) or variants embedded in sequence with complex architecture (e.g. short tandem repeats or segmental duplications), may not be detected. Additionally, it may not be possible to fully resolve certain details about variants, such as mosaicism, phasing, or mapping ambiguity. Unless explicitly guaranteed, sequence changes in the promoter, non-coding exons, and other non-coding regions are not covered by this assay. Please consult the test definition on our website for details regarding regions or types of variants that are covered or excluded for this test. This report reflects the analysis of an extracted genomic DNA sample. In very rare cases, (circulating hematolymphoid neoplasm, bone marrow transplant, recent blood transfusion) the analyzed DNA may not represent the patient's constitutional genome.
|Gene||Transcript reference||Sequencing analysis||Deletion/Duplication analysis|
GALT: Analysis includes the 5 kb deletion NM_000155.3:c.[-1039_753del; 820+50_*789delinsGAATAGACCCCA] as well as the Duarte variant NM_000155.3: c.-119_-116delGTCA.