SUMMARY

 

1. A hemizygous deletion of chromosome 22q11.2 can result in a spectrum of clinical disease including velocardiofacial syndrome (VCFS) and DiGeorge syndrome (DGS). Unlike patients with DGS, VCFS patients do not have thymic hypoplasia or immunodeficiency. This chromosomal deletion syndrome is estimated to occur in approximately 1 in 3000 children.

 

2. Although the most common deletion includes a region containing more than 35 genes, the TBX1 gene has emerged as one of the most likely causes of the DGS phenotype. TBX1 is a transcription factor involved in the development of branchial arch structures. Impaired embryogenesis of the 3rd and 4th pharyngeal arches causes abnormal development of the thymus and parathyroid.

 

3. The classic triad for DGS includes cardiac anomalies, hypocalcemia, and hypoplastic thymus leading to T-cell immune dysfunction. Approximately 90% of patients with DGS have the 22q11.2 deletion. Chromosome 10p deletions and CHD7 mutations can also result in a DGS clinical phenotype. Some patients with DGS lack a defined molecular etiology.

 

4. Low T-cell numbers are present in 80% of patients with 22q11.2 deletion syndrome as a result of thymic hypoplasia. In the vast majority of patients this decrease is mild to moderate. Low initial T-cell numbers can improve over time especially during the first year of life. T-cell function is typically preserved. Humoral immunity is intact in most patients.

 

5.Humoral immunity is intact in most patients. However, antibody defects have been described including IgA deficiency, hypogammaglobulinemia, and specific antibody deficiency.

 

6. Most patients with mild or moderate T-cell decreases do not suffer from opportunistic infections and do not require Pneumocystis prophylaxis. Clinically, patients commonly suffer from frequent upper respiratory infections. Live viral vaccines are generally given to patients who have a CD8 T-cell count > 300 cells/mm3, normal in vitro T-cell proliferative responses (to mitogens and antigens), and normal specific antibody responses to non-live viral vaccine antigens.

 

7. There is an increased incidence of autoimmune disease including juvenile idiopathic arthritis (JIA) and immune thrombocytopenia. This may be due to decreased T-regulatory cells as well as decreased thymic AIRE expression.

 

8. A severe form of this syndrome resulting from complete thymic aplasia (resulting in near total absence of T-cells) occurs in approximately 1% of patients. This is a form of severe combined immune deficiency that is sometimes referred to as complete DGS. This condition is fatal without a thymic transplant or a hematopoietic stem cell transplantation (without T-cell depletion).

 

9. Occasionally patients with complete DGS can have dramatic oligoclonal expansion of a few circulating T-cells leading to normal or elevated total T-cell counts. This is referred to as atypical complete DGS. Patients typically develop erythroderma and rash similar to Omenns syndrome. Helpful laboratory clues include the presence of an increased memory T-cell phenotype (CD4+CD45RO+) with poor T-cell proliferation to mitogens, increased HLA-DR expression on T cells, an oligoclonal T-cell repertoire, and the absence of TRECs.

 

 

 

OVERVIEW

 

           Deletion of chromosome 22q11.2 can result in a spectrum of clinical disease ranging from velocardiofacial syndrome (VCFS) to DiGeorge syndrome (DGS). DGS is the clinical term used for patients who have the classic phenotype of cardiac defect, hypocalcemia and thymic hypoplasia leading to T-cell immunodeficiency. Of the patients who have a DGS phenotype, 90% have a 22q11.2 deletion (10% have other molecular etiologies such as 10p deletion, 17p13 deletion, 18q21 deletion, and CHD7 mutations). While often used interchangeably with DGS, patients with the confirmed chromosomal deletion should be referred to has having 22q11.2 deletion syndrome.

         

           22q11.2 deletion patients have characteristic facial features that include the following:

Low set posteriorly rotated ears

                                 Downward or upward slanting eyes

                                 Ocular hypertelorism

                                 Short philtrum

                                 Hooded eyelids

                                 Broad nasal bridge

                                 Bulbous nasal tip

                                 Micrognathia

                                 High arched palate

         

           Patients with 22q11.2 deletion syndrome have a variety of non-immunologic findings which are listed below:

Cardiac anomalies

Hypocalcemia

Cleft palate or submucous cleft palateVelopharyngeal insufficiency

Renal anomalies (dysplasia, obstruction, reflux)Neurologic (cerebellar hypoplasia, cerebral atrophy)

Skeletal abnormalities (veretebral anomalies)Dental abnormalities (delayed eruption, enamel hypoplasia)

Growth hormone deficiency

Developmental delay

Speech delay

Psychiatric problems (ADHD, Schizophrenia)

 

           Up to 80% of patients with 22q11.2 deletion will have decreased T-cell numbers as a result of thymic hypoplasia. This decrease is mild to moderate in the majority of patients (so-called partial DGS) and can improve significantly during the first year of life. Most patients have some residual thymic tissue (which may exist in an ectopic location) that supports T-cell development. T-cell proliferation to mitogens is typically normal. Immunoglobulin levels and specific antibody levels are normal although IgA deficiency, hypogammaglobulinemia, and specific antibody deficiency have been reported in some patients. Overall, most patients have only minor immune suppression and do not suffer from opportunistic infections or severe infectious complications. There is an increased incidence of sinopulmonary infections in patients which may in part be related to palatal abnormalities.

 

           In about 1% of patients, the complete absence of thymic tissue leads to severely decreased T-cell numbers (comprising only 1-2% of the circulating lymphocytes). This is a form of severe combined immune deficiency and is sometimes referred to as complete DGS. These patients also have very decreased T-cell proliferation to mitogen stimulation. B-cell numbers and NK cell numbers are typically normal, resulting in a T-B+NK+ phenotype. Although B cells are present, immunoglobulin production is greatly impaired due to the absence of T cell help. Thymic transplant or hematopoietic stem cell transplantation (HSCT) is mandatory for these patients. Thymic transplantation is preferred, but HSCT with an HLA-identical donor without T-cell depletion (there is no thymus to educate new T-cell precursors) is a reasonable alternative.

 

          Atypical complete DGS is a variant of complete DGS in which patients still lack thymic tissue but a limited number of circulating T-cells undergo oligoclonal expansion. These expanded cells have poor function and have an activated, predominantly memory phenotype. Patients are still severely immune suppressed despite having high or normal T-cell numbers. A severe dermatitis can develop similar to Omenn syndrome and can be a helpful clue for this diagnosis. These patients also require thymic transplant or HSCT. Atypical complete DGS occurs in approximately one-third of athymic DGS patients.

 

          There is an increased incidence of autoimmune disease in patients with 22q11.2 deletion syndrome. These include autoimmune cytopenias, juvenille idiopathic arthritis, and autoimmune endocrinopathy.

 

 

PATHOGENESIS

 

          The most common chromosome 22q11.2 deletion involves a 3-Mb region and a smaller 1.5-Mb deletion is found in 10% of cases. The deletion results in impaired embryogenesis of the 3rd and 4th pharyngeal pouches. These structures give rise to thymus (which is essential for T-cell development) and the parathyroid (the absence of which causes hypocalcemia).

 

          The most commonly deleted region of chromosome 22q11.2 includes more than 35 genes. TBX1 has emerged as the most likely gene contributing to the cardiac, thymic, and parathyroid abnormalities. TBX1 is a transcription factor that plays a role in the development of branchial arch structures. Mice that are homozygous for TBX1 deletions have a DGS phenotype with thymic hypoplasia, abnormal facies, cleft palate, and cardiac abnormalities. Point mutations in TBX1 have been identified in three patients with a DGS clinical phenotype but without 22q11.2 deletions. These data suggest that TBX1 deletions contribute to the DGS phenotype.

 

          The majority of patients with 22q11.2 deletion do not have a complete absence of T-cells. This is because some residual thymic tissue exists to support T-cell development, often in an ectopic location such as the neck.

 

          Patients have an increased risk for developing autoimmune conditions. A decrease in the percentage of CD4+CD25+ T-regulatory cells has been observed in patients with DGS compared to normal controls. Abnormal thymic development may also result in impaired expression of the autoimmune regulator gene (AIRE) resulting in defective central tolerance.

 

 

DIFFERENTIAL DIAGNOSIS

 

1. Chromosome 10p13-14 deletion

2. Chromosome 17p13 deletion

3. Chromosome 18q21 deletion

4. CHARGE Syndrome

5. In utero ethanol exposure

6. In utero isoretinoin exposure

7. The differential diagnosis for complete DGS includes SCID and other combined immune deficiencies

 

                                                                                 

EVALUATION

 

The presence of dysmorphic facial features, cardiac defects, hypocalcemia during infancy, and recurrent infections should raise suspicion for 22q11.2 deletion syndrome. 

 

Step 1:  Molecular Diagnosis

          FISH for 22q11.2 Deletion

- This test is commonly used to detect the most common deletion

- One disadvantage is that it will not identify other causes of DGS phenotype

          Genome-wide array

- A useful test to study deletions or duplications of genetic material in the entire genome

- This test detects most 22q11.2 deletions as well as other chromosomal abnormalities such as 10p deletions as well as CHD7 deletions.

- Genome-wide array studies are steadily replacing FISH as the diagnostic test of choice to detect 22q11.2 deletions.

 

Step 2:  Immune Evaluation

           CBC with Differential

           Lymphocyte subset enumeration by flow cytometry

           IgG, IgA, IgM levels (if older than 6 months)

           Specific Antibody levels (if older than 6 months)

           

           The absolute lymphocyte count should be calcuated from the CBC

           T-cell enumeration by flow cytometry is particularly important if the absolute lymphocyte count is low (an ALC < 3000 is very low in an                    infant)

           IgA deficiency, low IgG levels, and poor specific antibody response have been described in some patients but these are not typical                          findings.

             Immunoglobulin levels before 6 months of age may reflect transplacentally aquired maternal IgG)

 

STEP 3: Addditional Immune Evaluation -  If the T cell numbers are very low (less than 1-2% of the circulating lymphocytes) complete DGS should be suspected.  Further testing is indicated.

 

           T cell proliferation to Mitogens (PHA, Con A, PWM)
           Naïve (CD45RA) and memory (CD45RO) T cell enumeration by flow cytometry
           TREC Analysis 

           T cell proliferation to Mitogens wil be decreased in complete DGS but is normal in partial DGS.  
           Very low naïve (CD45RA) T cell numbers can be a useful clue for lack of thymic output.
           TRECs (T cell receptor excision circles) are loops of DNA excised during TCR rearrangement.  Because TRECs are not replicated with cell                     division, they

           are gradually diluted as T cells become activated and expand.  Thus, naïve T cells that are recent thymic emigrants have high TREC                              numbers.  Complete

           DGS patients lacking thymic tissue have very low TREC numbers.

 

STEP 4:  Atypical Complete DGS Immune Evaluation - Patients who present with DGS, severe rash, and lymphadenopathy should be suspected of having this condition.  


          Naïve (CD45RA) and memory (CD45RO) T cell enumeration by flow cytometry
          T-cell proliferation to Mitogens (PHA, ConA, PWM)
          TREC Analysis
          TCR Gene Rearrangement PCR (TCR Spectratyping)
          Maternal Engraftment Study

 

          Despite having normal or high total T cell numbers, atypical complete DGS patients have low naïve T cell numbers.  The clonally                                   expanded T cells

           primarily have an activated (HLA-DR+) memory (CD45RO+) phenotype. 
          Oligoclonally expanded T cells typically have low in vitro proliferation in response to mitogens and specific antigens. 
          Low TREC numbers are helpful for confirming athymia despite normal T cell numbers.
          TCR Gene rearrangement is helpful for confirming the presence of oligoclonal T cells.
          Oligoclonal expansion of maternally derived T cells could also create a similar clinical picture (rash, lymphadenopathy)

 

 

MANAGEMENT

 

1. Thymic Tissue is Present (partial DGS) - most patients fall into this categoryThe majority of 22q11.2 deletion patients have some thymic tissue that is able to support T cell development (although total T cell numbers may be decreased). T-cell function is intact despite decreased T-cell numbers. Patients typically are not at risk for opportunistic infections and do not require prophylaxis for opportunistic organisms such as Pneumocystis. For rare patients with defects of antibody production or function, IVIG or prophylactic antibiotics may be considered. The risk of using live viral vaccines (ex. MMR, Varicella, Rotavirus) is quite low except for patients who have very low T-cells. Patients who fulfill the following criteria may safely receive live viral vaccines: - CD8+ T-cell count > 300 cells/mm3- Normal specific antibody responses to non-live viral vaccines- Normal T-cell proliferation to mitogens

 

2. Absent Thymic Tissue (complete DGS) - less than 1% of patients Patients with absent thymic tissue and absent T-cells have a form of severe combined immune deficiency. This is an immunologic emergency and the following management steps should be taken: a. Avoid all live viral vaccinesb. Start prophylaxis for pneumocystis jiroveci with trimethoprim-sulfamethoxazolec. All blood products should be irradiated and CMV negatived. IVIG therapy should be startede. Duke University thymic transplant team should be contactedf. Alternatively, a HLA identical sibling or unrelated donor should be identified for a non T-cell depleted Hematopoietic stem cell transplantation.

 

                                                                           

RESOURCES

 

 

Diagnostic Resources  

 

1.  FISH for 22q11.2 Deletion
   -This test is readily available at many academic centers and commercial laboratories.

 

2.  GENOME WIDE ARRAY (SNP ARRAY)
   -This detects the common 22q11.2 deletion as well as other deletions causing DGS including 10p13-14 and CHD7 deletions.

   -Cincinnati Childrens Cytogenetics Laboratory
   -Childrens Hospital of Philadelphia
   -GeneDx 

 

3.   Lymphocyte Subset enumeration by Flow Cytometry for T-cell (CD3, CD4, CD8), B-cell (CD19), and NK cell (CD16/56).   
   -This test is commonly available at many academic centers as well as commercial laboratories including those listed below:

   -Childrens Hospital of Philadelphia  Basic Lymphocyte Panel (click here for pdf)

   -Cincinnati Childrens Diagnostic Immunology Laboratory (click here for pdf)

   -ARUP Laboratories (Lymphocyte Subset Panel 5)
    www.aruplab.com/guides/ug/tests/0095892.jsp

   -Quest Diagnostics (Lymphocyte Subset Panel 1)
    www.questdiagnostics.com

 

4.  Naïve (CD45 RA) and Memory (CD45 RO) T cells enumeration by Flow Cytometry

     - Childrens Hospital of Philadelphia - DiGeorge Panel 
     - The DiGeorge Panel is an advanced lymphocyte flow cytometry panel which     
       includes CD45RA and CD45RO (this test is NOT a FISH for 22q11.2 deletion 
       or a SNP array) (click here for pdf)

    - Cincinnati Childrens Diagnostic Immunology Laboratory (click here for pdf)

    - ARUP Laboratories (Lymphocyte Subset Panel 7)
      http://www.aruplab.com/guides/ug/tests/0095899.jsp

 

5.  T-cell proliferation to Mitogens and Specific Antigens (candida, tetanus)
    -This test is available at both academic centers and commercial laboratories.

    -Childrens Hospital of Philadelphia 
    -Cincinnati Childrens Hospital 
    -ARUP laboratories

6.  TREC (T-cell receptor excision circle) Analysis

    -Mayo Clinic TRECs 
    -10ml lavender (EDTA) tube
    -www.mayomedicallaboratories.com/test-catalog/Specimen/87959

 

7.  T-cell Recepror Gene Rearrangement (TCR Spectratyping)

    -Mayo Clinic TCR Gene Rearrangement 

 

8.  Thymic Transplant:  Duke University Medical Center  Durham, NC 

     Contacts:  
     Louise Markert, MD, PhD 
             919-684-6263  marke001@mc.duke.edu
     Elizabeth A. McCarthy, RN, MSN 
             919-684-6828  mccar006@mc.duke.e

 

 

 

Literature Resources

 

1.  Sullivan 2008

     22q11.2 Deletion Syndrome Review

 

2.  Kobrynski 2007

     22q11.2 Deletion Syndrome Review

 

3.  Jawad 2001

     Immunologic Features of 22q11.2 Deletion Syndrome

 

4.  Sullivan 1999

     Longitudinal Analysis of T-Cells in 22q11.2 Deletion Syndrome

 

5.  Markert 2010

     Thymus Transplantation Review

 

6.  Land 2008

     Long Term Results of BMT for DGS